11. 常用工具

11.1. 上下文帮助

每当您需要具体主题帮助时，可以通过多数对话框中的 Help 按钮访问当前用户手册中的相应页面 --- 请注意，第三方插件可以指向专用网页。

11.2. 面板

By default, QGIS provides many panels to work with. Some of these panels are described below while others may be found in different parts of the document. A complete list of default panels provided by QGIS is available via the View ► Panels ► menu and mentioned at 面板.

11.2.1. Layers Panel

The Layers panel (also called the map legend) lists all the layers in the project and helps you manage their visibility. You can show or hide it by pressing Ctrl+1. A layer can be selected and dragged up or down in the legend to change the Z-ordering. Z-ordering means that layers listed nearer the top of the legend are drawn over layers listed lower down in the legend.

备注

The Z-ordering behavior can be overridden by the Layer Order panel.

At the top of the Layers panel, a toolbar allows you to:

• ^{Open the layer styling dock (F7)}: toggle the layer styling panel on and off.

• ^{Add new group}

• ^{Manage Map Themes}: control visibility of layers and arrange them in different map themes.

• ^{Filter Legend by Map Content}: only the layers that are set visible and whose features intersect the current map canvas have their style rendered in the layers panel. Otherwise, a generic NULL symbol is applied to the layer. Based on the layer symbology, this is a convenient way to identify which kind of features from which layers cover your area of interest.

• ^{Filter Legend by Expression}: apply an expression to remove styles from the selected layer tree that have no feature satisfying the condition. This can be used to highlight features that are within a given area/feature of another layer. From the drop-down list, you can edit and clear the expression currently applied.

• ^{Expand All} or ^{Collapse All} layers and groups in the layers panel.

• ^{Remove Layer/Group} currently selected.

图 11.1 Layer Toolbar in Layers Panel

备注

Tools to manage the layers panel are also available for map and legend items in print layouts

11.2.1.1. Configuring map themes

The ^{Manage Map Themes} drop-down button provides access to convenient shortcuts to manipulate visibility of the layers in the Layers panel:

• Show All Layers

• Hide All Layers

• Show Selected Layers

• Hide Selected Layers

• Toggle Selected Layers: changes the visibility of the first selected layer in the panel, and applies that state to the other selected layers. Also accesible through Space shortcut.

• Toggle Selected Layers Independently: changes the visibility status of each selected layer

• Hide Deselected Layers

Beyond the simple control of layer visibility, the ^{Manage Map Themes} menu allows you to configure Map Themes in the legend and switch from one map theme to another. A map theme is a snapshot of the current map legend that records:

• the layers set as visible in the Layers panel

• and for each visible layer:

  • the reference to the style applied to the layer

  • the visible classes of the style, ie the layer checked node items in the Layers panel. This applies to symbologies other than the single symbol rendering

  • the collapsed/expanded state of the layer node(s) and the group(s) it's placed inside

To create a map theme:

1. Check a layer you want to show

2. Configure the layer properties (symbology, diagram, labels...) as usual

3. Expand the Style ► menu at the bottom and click on Add... to store the settings as a new style embedded in the project

   备注

   A map theme does not remember the current details of the properties: only a reference to the style name is saved, so whenever you apply modifications to the layer while this style is enabled (eg change the symbology rendering), the map theme is updated with new information.

4. Repeat the previous steps as necessary for the other layers

5. If applicable, expand or collapse groups or visible layer nodes in the Layers panel

6. Click on the ^{Manage Map Themes} button on top of the panel, and Add Theme...

7. Enter the map theme's name and click OK

The new theme is listed in the lower part of the drop-down menu.

You can create as many map themes as you need: whenever the current combination in the map legend (visible layers, their active style, the map legend nodes) does not match any existing map theme contents as defined above, click on Add Theme... to create a new map theme, or use Replace Theme ► to update a map theme. You can rename the active map theme with Rename Current Theme... or use the Remove Current Theme button to delete it.

Map themes are helpful to switch quickly between different preconfigured combinations: select a map theme in the list to restore its combination. All configured themes are also accessible in the print layout, allowing you to create different map items based on specific themes and independent of the current main canvas rendering (see Map item layers).

11.2.1.2. Overview of the context menu of the Layers panel

At the bottom of the toolbar, the main component of the Layers panel is the frame listing vector or raster layers added to the project, optionally organized in groups. Depending on the item selected in the panel, a right-click shows a dedicated set of options presented below.

Option	Vector Layer	Raster Layer	Group
Zoom to Layer/Group
Zoom to Selection
Show in Overview
Show Feature Count
Copy Layer/Group
Rename Layer/Group
Zoom to Native Resolution (100%)
Stretch Using Current Extent
Update SQL Layer...
Add Group
Duplicate Layer
Remove Layer/Group...
Move Out of Group
Move to Top
Move to Bottom
Check and all its Parents
Group Selected
Open Attribute Table
Toggle Editing
Current Edits ►
Filter...
Change Data Source...
Repair Data Source...
Actions on selections ► (in edit mode)
► Duplicate Feature
► Duplicate Feature and Digitize
Set Layer Scale Visibility...
Zoom to Visible Scale
Set CRS ►
► Set Layer/Group CRS...
► Set Project CRS from Layer
Set Group WMS Data...
Mutually Exclusive Group
Check and all its children (Ctrl-click)
Uncheck and all its children (Ctrl-click)
Make Permanent
Export ►
► Save As...
► Save Features As...
► Save Selected Features As...
► Save As Layer Definition File...
► Save As QGIS Layer Style File...
Styles ►
► Copy Style
► Paste Style
► Add...
► Rename Current...
► Edit symbol...
► Copy Symbol
► Paste Symbol
Properties...

Table: Context menu from Layers Panel items

For GRASS vector layers, ^{Toggle editing} is not available. See section Digitizing and editing a GRASS vector layer for information on editing GRASS vector layers.

11.2.1.3. Interact with groups and layers

Layers in the legend window can be organized into groups. There are two ways to do this:

1. Press the icon to add a new group. Type in a name for the group and press Enter. Now click on an existing layer and drag it onto the group.

2. Select some layers, right-click in the legend window and choose Group Selected. The selected layers will automatically be placed in a new group.

To move a layer out of a group, drag it out, or right-click on it and choose Move Out of Group: the layer is moved from the group and placed above it. Groups can also be nested inside other groups. If a layer is placed in a nested group, Move Out of Group will move the layer out of all nested groups.

To move a group or layer to the top of the layer panel, either drag it to the top, or choose Move to Top. If you use this option on a layer nested in a group, the layer is moved to the top in its current group. The Move to Bottom option follows the same logic to move layers and groups down.

The checkbox for a group will show or hide the checked layers in the group with one click. With Ctrl pressed, the checkbox will also turn on or off all the layers in the group and its sub-groups.

Ctrl-click on a checked / unchecked layer will uncheck / check the layer and all its parents.

Enabling the Mutually Exclusive Group option means you can make a group have only one layer visible at the same time. Whenever a layer within the group is set visible the others will be toggled not visible.

It is possible to select more than one layer or group at the same time by holding down the Ctrl key while clicking additional layers. You can then move all selected layers to a new group at the same time.

You may also delete more than one layer or group at once by selecting several items with the Ctrl key and then pressing Ctrl+D: all selected layers or groups will be removed from the layers list.

More information on layers and groups using indicator icon

In some circumstances, icons appears next to the layer or group in the Layers panel to give more information about the layer/group. These symbols are:

• to indicate that the layer is in edit mode and you can modify the data

• to indicate that the layer being edited has some unsaved changes

• to indicate a filter applied to the layer. Hover over the icon to see the filter expression and double-click to update the setting

• to identify layers that are required in the project, hence non removable

• to identify an embedded group or layer and the path to their original project file

• to identify a layer whose data source was not available at the project file opening (see 处理损坏的文件路径). Click the icon to update the source path or select Repair Data Source... entry from the layer contextual menu.

• to remind you that the layer is a temporary scratch layer and its content will be discarded when you close this project. To avoid data loss and make the layer permanent, click the icon to store the layer in any of the OGR vector formats supported by QGIS.

• to identify a layer that has no/unknown CRS

• to identify a temporal layer controlled by canvas animation

11.2.1.4. Editing vector layer style

From the Layers panel, you have shortcuts to change the layer rendering quickly and easily. Right-click on a vector layer and select Styles ► in the list in order to:

• see the styles currently applied to the layer. If you defined many styles for the layer, you can switch from one to another and your layer rendering will automatically be updated on the map canvas.

• copy part or all of the current style, and when applicable, paste a copied style from another layer

  小技巧

  Quickly share a layer style

  From the context menu, copy the style of a layer and paste it to a group or a selection of layers: the style is applied to all the layers that are of the same type (vector/raster) as the original layer and, for vector layers, have the same geometry type (point, line or polygon).

• rename the current style, add a new style (which is actually a copy of the current one) or delete the current style (when multiple styles are available).

备注

The previous options are also available for raster or mesh layers.

• update the symbol color using a Color Wheel. For convenience, the recently used colors are also available at the bottom of the color wheel.

• Edit Symbol...: open the Symbol Selector dialog and change feature symbol (symbol, size, color...).

When using a classification symbology type (based on categorized, graduated or rule-based), the aforementioned symbol-level options are available from the class entry context menu. Also provided are the Toggle Items, Show All Items and Hide All Items entries to switch the visibility of all the classes of features. These avoid (un)checking items one by one.

小技巧

Double-clicking a class leaf entry also opens the Symbol Selector dialog.

11.2.2. Layer Styling Panel

The Layer Styling panel (also enabled with Ctrl+3) is a shortcut to some of the functionalities of the Layer Properties dialog. It provides a quick and easy way to define the rendering and the behavior of a layer, and to visualize its effects without having to open the layer properties dialog.

In addition to avoiding the blocking (or "modal") layer properties dialog, the layer styling panel also avoids cluttering the screen with dialogs, and contains most style functions (color selector, effects properties, rule edit, label substitution...): e.g., clicking color buttons inside the layer style panel causes the color selector dialog to be opened inside the layer style panel itself rather than as a separate dialog.

From a drop-down list of current layers in the layer panel, select an item and:

• Depending on the layer type, set:

  • Symbology, Transparency, and Histogram properties for raster layer. These options are the same as in the Raster Properties Dialog.

  • Symbology, Labels, Mask and 3D View properties for vector layer. These options are the same as in the The Vector Properties Dialog and can be extended by custom properties introduced by third-party plugins.

  • Symbology and 3D View properties for mesh layer. These options are the same as in the Mesh Dataset Properties.

• Manage the associated style(s) in the Style Manager (more details at Managing Custom Styles).

• See the History of changes you applied to the layer style in the current project: you can therefore cancel or restore to any state by selecting it in the list and clicking Apply.

For Vector Tile layers there is an option to show Visible rules only. This is very useful if you just want to work with rules that fall inside the current map canvas zoom level.

Another powerful feature of this panel is the Live update checkbox. Tick it to render your changes immediately on the map canvas: you no longer need to click the Apply button.

图 11.2 Defining a layer's symbology from the layer styling panel

11.2.3. Layer Order Panel

By default, layers shown on the QGIS map canvas are drawn following their order in the Layers panel: the higher a layer is in the panel, the higher (hence, more visible) it'll be in the map view.

You can define a drawing order for the layers independent of the order in the layers panel with the Layer Order panel enabled in View ► Panels ► menu or with Ctrl+9. Check Control rendering order underneath the list of layers and reorganize the layers in the panel as you want. This order becomes the one applied to the map canvas. For example, in 图 11.3, you can see that the airports features are displayed over the alaska polygon despite those layers' respective placement in the Layers panel.

Unchecking Control rendering order will revert to default behavior.

图 11.3 Define a layer order independent of the legend

11.2.4. Overview Panel

The Overview panel (Ctrl+8) displays a map with a full extent view of some of the layers. The Overview map is filled with layers using the Show in Overview option from the Layer menu or in the layer contextual menu. Within the view, a red rectangle shows the current map canvas extent, helping you quickly to determine which area of the whole map you are currently viewing. If you click-and-drag the red rectangle in the overview frame, the main map view extent will update accordingly.

Note that labels are not rendered to the map overview even if the layers used in the map overview have been set up for labeling.

11.2.5. Log Messages Panel

When loading or processing some operations, you can track and follow messages that appear in different tabs using the Log Messages Panel. It can be activated using the most right icon in the bottom status bar.

11.2.6. Undo/Redo Panel

For each layer being edited, the Undo/Redo (Ctrl+5) panel shows the list of actions carried out, allowing you quickly to undo a set of actions by selecting the action listed above. More details at Undo and Redo edits.

11.2.7. Statistical Summary Panel

The Statistics panel (Ctrl+6) provides summarized information on any vector layer. This panel allows you to select:

• the vector layer to compute the statistics on

• the column to use, or an expression

• the statistics to return using the drop-down button at the bottom-right of the dialog. Depending on the field's (or expression's values) type, available statistics are:

Statistics	String	Integer	Float	Date
Count
Count Distinct Value
Count Missing value
Sum
Mean
Standard Deviation
Standard Deviation on Sample
Minimal value
Maximal value
范围（最大值和最小值之差）
Minority
Majority
Variety
First Quartile
Third Quartile
Inter Quartile Range
Minimum Length
Maximum Length
Mean Length

Table: Statistics available for each field type

The statistical summary can be:

• returned for the whole layer or selected features only

• recalculated using the button when the underlying data source changes (eg, new or removed features/fields, attribute modification)

• copied to the clipboard and pasted as a table in another application

图 11.4 Show statistics on a field

11.3. Nesting Projects

Sometimes, you'd like to keep some layers in different projects, but with the same style. You can either create a default style for these layers or embed them from another project to save time and effort.

Embed layers and groups from an existing project has some advantages over styling:

• All types of layers (vector or raster, local or online...) can be added

• Fetching groups and layers, you can keep the same tree structure of the "background" layers in your different projects

• While the embedded layers are editable, you can't change their properties such as symbology, labels, forms, default values and actions, ensuring consistency across projects

• Modify the items in the original project and changes are propagated to all the other projects

If you want to embed content from other project files into your project, select Layer ► Embed Layers and Groups:

1. Click the ... button to look for a project: you can see the content of the project (see 图 11.5)

2. Hold down Ctrl ( or Cmd) and click on the layers and groups you wish to retrieve

3. Click OK

The selected layers and groups are embedded in the Layers panel and displayed on the map canvas. An icon is added next to their name for recognition and hovering over displays a tooltip with the original project file path.

图 11.5 Select layers and groups to embed

Like any other layer, an embedded layer can be removed from the project by right-clicking on the layer and clicking ^Remove.

小技巧

Change rendering of an embedded layer

It's not possible to change the rendering of an embedded layer, unless you make the changes in the original project file. However, right-clicking on a layer and selecting Duplicate creates a layer which is fully-featured and not dependent on the original project. You can then safely remove the linked layer.

11.4. Working with the map canvas

11.4.1. Rendering

By default, QGIS renders all visible layers whenever the map canvas is refreshed. The events that trigger a refresh of the map canvas include:

• adding a layer

• panning or zooming

• resizing the QGIS window

• changing the visibility of a layer or layers

QGIS allows you to control the rendering process in a number of ways.

11.4.1.1. Scale Dependent Rendering

Scale-dependent rendering allows you to specify the minimum and maximum scales at which a layer (raster or vector) will be visible. To set scale-dependent rendering, open the Properties dialog by double-clicking on the layer in the legend. On the Rendering tab, tick Scale dependent visibility and enter the Minimum (exclusive) and Maximum (inclusive) scale values.

You can also activate scale dependent visibility on a layer from the Layers panel. Right-click on the layer and in the context menu, select Set Layer Scale Visibility.

The ^{Set to current canvas scale} button allow you to use the current map canvas scale as boundary of the range visibility.

备注

When a layer is not rendered in the map canvas because the map scale is out of its visibility scale range, the layer is greyed in the Layers panel and a new option Zoom to Visible Scale appears in the layer context menu. Select it and the map is zoomed to the layer's nearest visibility scale.

11.4.1.2. Controlling Map Rendering

Map rendering can be controlled in various ways, as described below.

Suspending Rendering

To suspend rendering, click the Render checkbox in the bottom-right corner of the status bar. When Render is not checked, QGIS does not redraw the canvas in response to any of the events described in the section Rendering. Examples of when you might want to suspend rendering include:

• adding many layers and symbolizing them prior to drawing

• adding one or more large layers and setting scale dependency before drawing

• adding one or more large layers and zooming to a specific view before drawing

• any combination of the above

Checking the Render checkbox enables rendering and causes an immediate refresh of the map canvas.

Setting Layer Add Option

You can set an option to always load new layers without drawing them. This means the layer will be added to the map, but its visibility checkbox in the legend will be unchecked by default. To set this option, choose menu option Settings ► Options and click on the Rendering tab. Uncheck By default new layers added to the map should be displayed. Any layer subsequently added to the map will be off (invisible) by default.

Stopping Rendering

To stop the map drawing, press the Esc key. This will halt the refresh of the map canvas and leave the map partially drawn. It may take a bit of time between pressing Esc for the map drawing to halt.

Influence Rendering Quality

QGIS has an option to influence the rendering quality of the map. Choose menu option Settings ► Options, click on the Rendering tab and select or deselect Make lines appear less jagged at the expense of some drawing performance.

Speed-up rendering

There are some settings that allow you to improve rendering speed. Open the QGIS options dialog using Settings ► Options, go to the Rendering tab and select or deselect the following checkboxes:

• Use render caching where possible to speed up redraws.

• Render layers in parallel using many CPU cores and then set the Max cores to use.

• The map renders in the background onto a separate image and each Map Update interval, the content from this (off-screen) image will be taken to update the visible screen representation. However, if rendering finishes faster than this duration, it will be shown instantaneously.

• With Enable Feature simplification by default for newly added layers, you simplify features' geometry (fewer nodes) and as a result, they display more quickly. Be aware that this can cause rendering inconsistencies.

11.4.2. Zooming and Panning

There are multiple ways to zoom and pan to an area of interest. You can use the Map Navigation toolbar, the mouse and keyboard on the map canvas and also the menu actions from the View menu and the layers' contextual menu in the Layers panel.

Icon	Label	Usage	View menu	Map Navigation Toolbar	Layer Contextual Menu
	Pan Map	When activated, left click anywhere on the map canvas to pan the map at the cursor position. You can also pan the map by holding down the left mouse button and dragging the map canvas.
	Zoom In	When activated, left click anywhere on the map canvas to zoom in one level. The mouse cursor position will be the center of the zoomed area of interest. You can also zoom in to an area by dragging a rectangle on the map canvas with the left mouse button.
	Zoom Out	When activated, left click anywhere on the map canvas to zoom out one level. The mouse cursor position will be the center of the zoomed area of interest. You can also zoom out from an area by dragging a rectangle on the map canvas with the left mouse button.
	Pan Map to Selection	Pan the map to the active layer's selected features.
	Zoom To Selection	Zoom to the active layer's selected features.
	Zoom To Layer	Zoom to the active layer's extent.
	Zoom Full	Zoom to the extent of all the layers in the project.
	Zoom Last	Zoom the map to the previous extent in history.
	Zoom Next	Zoom the map to the next extent in history.
	Zoom to Native Resolution	Zoom the map to a level where one pixel of the active raster layer covers one screen pixel.

A Zoom factor can be set under the Settings ► Options ► Map tools menu to define the scale behavior while zooming. There, you can also set a list of Predefined Scales that will be available at the bottom of the map canvas.

11.4.2.1. With the Mouse on the Map Canvas

In addition to using the ^{Pan Zoom In} and ^{Zoom Out} tools described above, you can hold the mouse wheel inside of the map canvas and drag the mouse cursor (on macOS, you may need to hold down the cmd key). You can also roll the mouse wheel to zoom in and out on the map. The mouse cursor position will be the center of the zoomed area of interest. Holding down Ctrl while rolling the mouse wheel results in a finer zoom.

11.4.2.2. With the Keyboard on the Map Canvas

Holding down spacebar on the keyboard and moving the mouse cursor will pan the map the same way dragging the map canvas with ^Pan does.

Panning the map is possible with the arrow keys. Place the mouse cursor inside the map area, and press on the arrow keys to pan up, down, left and right.

The PgUp and PgDown keys on the keyboard will cause the map display to zoom in or out following the zoom factor set. Pressing Ctrl++ or Ctrl+- also performs an immediate zoom in/out on the map canvas.

When certain map tools are active (Identify, Measure...), you can perform a zoom by holding down Shift and dragging a rectangle on the map to zoom to that area. This is not enabled for selection tools (since they use Shift for adding to selection) or edit tools.

11.4.3. Spatial Bookmarks

Spatial Bookmarks allow you to "bookmark" a geographic location and return to it later. By default, bookmarks are saved in the user's profile (as User Bookmarks), meaning that they are available from any project the user opens. They can also be saved for a single project (named Project Bookmarks) and stored within the project file, which can be helpful if the project is to be shared with other users.

11.4.3.1. Creating a Bookmark

To create a bookmark:

1. Zoom and pan to the area of interest.

2. Select the menu option View ► New Spatial Bookmark..., press Ctrl+B or right-click the Spatial Bookmarks entry in the Browser panel and select New Spatial Bookmark. The Bookmark Editor dialog opens.

   

   图 11.6 The Bookmark Editor Dialog

3. Enter a descriptive name for the bookmark

4. Enter or select a group name in which to store related bookmarks

5. Select the extent of the area you wish to save, using the extent selector; the extent can be calculated from a loaded layer extent, the current map canvas or drawn over the current map canvas.

6. Indicate the CRS to use for the extent

7. Select whether the bookmark will be Saved in User Bookmarks or Project Bookmarks

8. Press Save to add the bookmark to the list

Note that you can have multiple bookmarks with the same name.

11.4.3.2. Working with Bookmarks

To use and manage bookmarks, you can either use the Spatial Bookmarks panel or Browser.

Select View ► Show Spatial Bookmark Manager or press Ctrl+7 to open the Spatial Bookmarks Manager panel. Select View ► Show Bookmarks or Ctrl+Shift+B to show the Spatial Bookmarks entry in the Browser panel.

You can perform the following tasks:

Task	Spatial Bookmark Manager	Browser
Zoom to a Bookmark	Double-click on it, or select the bookmark and press the Zoom to bookmark button.	Double-click on it, drag and drop it to the map canvas, or right-click the bookmark and select Zoom to Bookmark.
Delete a bookmark	Select the bookmark and click the Delete bookmark button. Confirm your choice.	Right-click the bookmark and select Delete Spatial Bookmark. Confirm your choice.
Export bookmarks to XML	Click the Import/Export Bookmarks button and select Export. All the bookmarks (user or project) are saved in an xml file.	Select one or more folders (user or project) or subfolders (groups), then right-click and select Export Spatial Bookmarks.... The selected bookmark subset is saved.
Import bookmarks from XML	Click the Import/Export Bookmarks button and select Import. All bookmarks in the XML file are imported as user bookmarks.	Right-click the Spatial Bookmarks entry or one of its folders (user or project) or subfolders (groups) to determine where to import the bookmarks, then select Import Spatial Bookmarks. If performed on the Spatial Bookmarks entry, the bookmarks are added to User Bookmarks.
Edit bookmark	You can change a bookmark by changing the values in the table. You can edit the name, the group, the extent and if it is stored in the project or not.	Right-click the desired bookmark and select Edit Spatial Bookmark.... The Bookmark Editor will open, allowing you to redefine every aspect of the bookmark as if you were creating it for the first time. You can also drag and drop the bookmark between folders (user and project) and subfolders (groups).

You can also zoom to bookmarks by typing the bookmark name in the locator.

11.4.4. Decorations

Decorations include Grid, Title Label, Copyright Label, Image, North Arrow, Scale Bar and Layout Extents. They are used to 'decorate' the map by adding cartographic elements.

11.4.4.1. Grid

Grid allows you to add a coordinate grid and coordinate annotations to the map canvas.

1. Select menu option View ► Decorations ► Grid... to open the dialog.

   

   图 11.7 The Grid Dialog

2. Tick Enable grid and set grid definitions according to the layers loaded in the map canvas:

   • The Grid type: it can be Line or Marker

   • The associated Line symbol or marker symbol used to represent the grid marks

   • The Interval X and Interval Y between the grid marks, in map units

   • An Offset X and Offset Y distance of the grid marks from the bottom left corner of the map canvas, in map units

   • The interval and offset parameters can be set based on the:

     • Canvas Extents: generates a grid with an interval that is approximatively 1/5 of the canvas width

     • Active Raster Layer resolution

3. Tick Draw annotations to display the coordinates of the grid marks and set:

   • The Annotation direction, ie how the labels would be placed relative to their grid line. It can be:

     • Horizontal or Vertical for all the labels

     • Horizontal and Vertical, ie each label is parallel to the grid mark it refers to

     • Boundary direction, ie each label follows the canvas boundary, and is perpendicular to the grid mark it refers to

   • The Annotation font (text formatting, buffer, shadow...) using the font selector widget

   • The Distance to map frame, margin between annotations and map canvas limits. Convenient when exporting the map canvas eg to an image format or PDF, and avoid annotations to be on the "paper" limits.

   • The Coordinate precision

4. Click Apply to verify that it looks as expected or OK if you're satisfied.

11.4.4.2. Title Label

Title Label allows you to decorate your map with a Title.

To add a Title Label decoration:

1. Select menu option View ► Decorations ► Title Label... to open the dialog.

   

   图 11.8 The Title Decoration Dialog

2. Make sure Enable Title Label is checked

3. Enter the title text you want to place on the map. You can make it dynamic using the Insert or Edit an Expression... button.

4. Choose the Font for the label using the font selector widget with full access to QGIS text formatting options. Quickly set the font color and opacity by clicking the black arrow to the right of the font combo box.

5. Select the color to apply to the title's Background bar color.

6. Choose the Placement of the label in the canvas: options are Top left, Top Center (default), Top Right, Bottom left, Bottom Center and Bottom Right.

7. Refine the placement of the item by setting a horizontal and/or vertical Margin from Edge. These values can be in Millimeters or Pixels or set as a Percentage of the width or height of the map canvas.

8. Click Apply to verify that it looks as expected or OK if you're satisfied.

11.4.4.3. Copyright Label

Copyright Label can be used to decorate your map with a Copyright label.

To add this decoration:

1. Select menu option View ► Decorations ► Copyright Label... to open the dialog.

   

   图 11.9 The Copyright Decoration Dialog

2. Make sure Enable Copyright Label is checked

3. Enter the copyright text you want to place on the map. You can make it dynamic using the Insert or Edit an Expression... button.

4. Choose the Font for the label using the font selector widget with full access to QGIS text formatting options. Quickly set the font color and opacity by clicking the black arrow to the right of the font combo box.

5. Choose the Placement of the label in the canvas: options are Top left, Top Center, Top Right, Bottom left, Bottom Center, and Bottom Right (default for Copyright decoration)

6. Refine the placement of the item by setting a horizontal and/or vertical Margin from Edge. These values can be in Millimeters or Pixels or set as a Percentage of the width or height of the map canvas.

7. Click Apply to verify that it looks as expected or OK if you're satisfied.

11.4.4.4. Image Decoration

Image allows you to add an image (logo, legend, ..) on the map canvas.

To add an image:

1. Select menu option View ► Decorations ► Image... to open the dialog.

   

   图 11.10 The Image Decoration Dialog

2. Make sure Enable Image is checked

3. Select a bitmap (e.g. png or jpg) or SVG image using the ... ^Browse button

4. If you have chosen a parameter enabled SVG then you can also set a Fill or Stroke (outline) color. For bitmap images, the color settings are disabled.

5. Set a Size of the image in mm. The width of selected image is used to resize it to given Size.

6. Choose where you want to place the image on the map canvas with the Placement combo box. The default position is Top Left.

7. Set the Horizontal and Vertical Margin from (Canvas) Edge. These values can be set in Millimeters, Pixels or as a Percentage of the width or height of the map canvas.

8. Click Apply to verify that it looks as expected and OK if you're satisfied.

11.4.4.5. North Arrow

North Arrow allows you to add a north arrow on the map canvas.

To add a north arrow:

1. Select menu option View ► Decorations ► North Arrow... to open the dialog.

   

   图 11.11 The North Arrow Dialog

2. Make sure Enable north arrow is checked

3. Optionally change the color and size, or choose a custom SVG

4. Optionally change the angle or choose Automatic to let QGIS determine the direction

5. Optionally choose the placement from the Placement combo box

6. Optionally refine the placement of the arrow by setting a horizontal and/or vertical Margin from (Canvas) Edge. These values can be in Millimeters or Pixels or set as a Percentage of the width or height of the map canvas.

7. Click Apply to verify that it looks as expected and OK if you're satisfied.

11.4.4.6. Scale Bar

Scale Bar adds a simple scale bar to the map canvas. You can control the style and placement, as well as the labelling of the bar.

QGIS only supports displaying the scale in the same units as your map frame. So, if the units of your project's CRS are meters, you can't create a scale bar in feet. Likewise, if you are using decimal degrees, you can't create a scale bar to display distance in meters.

To add a scale bar:

1. Select menu option View ► Decorations ► Scale Bar... to open the dialog

   

   图 11.12 The Scale Bar Dialog

2. Make sure Enable scale bar is checked

3. Choose a style from the Scale bar style combo box

4. Select the Color of bar by choosing a fill color (default: black) and an outline color (default: white). The scale bar fill and outline can be made opaque by clicking on the down arrow to the right of the color input.

5. Select the font for the scale bar from the Font of bar combo box

6. Set the Size of bar

7. Optionally check Automatically snap to round number on resize to display easy-to-read values

8. Choose the placement from the Placement combo box

9. You can refine the placement of the item by setting a horizontal and/or vertical Margin from (Canvas) Edge. These values can be in Millimeters or Pixels or set as a Percentage of the width or height of the map canvas.

10. Click Apply to verify that it looks as expected or OK if you're satisfied.

11.4.4.7. Layout Extents

Layout Extents adds the extents of map item(s) in print layout(s) to the canvas. When enabled, the extents of all map items within all print layouts are shown using a lightly dotted border labeled with the name of the print layout and map item. You can control the style and labeling of the displayed layout extents. This decoration is useful when you are tweaking the positioning of map elements such as labels, and need to know the actual visible region of print layouts.

图 11.13 Example of layout extents displayed in a QGIS project with two print layouts. The print layout named 'Sights' contains two map items, while the other print layout contains one map item.

To add layout extent(s):

1. Select View ► Decorations ► Layout Extents to open the dialog

   

   图 11.14 The Layout Extents Dialog

2. Make sure Show layout extents is checked.

3. Optionally change the symbol and labeling of the extents.

4. Click Apply to verify that it looks as expected and OK if you're satisfied.

小技巧

Decorations Settings

When you save a QGIS project file, any changes you have made to Grid, North Arrow, Scale Bar, Copyright and Layout Extents will be saved in the project and restored the next time you load the project.

11.4.5. Annotation Tools

Annotations are information added to the map canvas and shown within a balloon. This information can be of different types and annotations are added using the corresponding tools in the Attributes Toolbar:

• ^{Text Annotation} for custom formatted text

• ^{HTML Annotation} to place the content of an html file

• ^{SVG Annotation} to add an SVG symbol

• ^{Form Annotation}: useful to display attributes of a vector layer in a customized ui file (see 图 11.15). This is similar to the custom attribute forms, but displayed in an annotation item. Also see this video https://www.youtube.com/watch?v=0pDBuSbQ02o&feature=youtu.be&t=2m25s from Tim Sutton for more information.

图 11.15 Customized QT Designer annotation form

To add an annotation, select the corresponding tool and click on the map canvas. An empty balloon is added. Double-click on it and a dialog opens with various options. This dialog is almost the same for all the annotation types:

• At the top, a file selector to fill with the path to an html, svg or ui file depending on the type of annotation. For text annotation, you can enter your message in a text box and set its rendering with the normal font tools.

• Fixed map position: when unchecked, the balloon placement is based on a screen position (instead of the map), meaning that it's always shown regardless the map canvas extent.

• Linked layer: associates the annotation with a map layer, making it visible only when that layer is visible.

• Map marker: using QGIS symbols, sets the symbol to display at the balloon anchor position (shown only when Fixed map position is checked).

• Frame style: sets the frame background color, transparency, stroke color or width of the balloon using QGIS symbols.

• Contents margins: sets interior margins of the annotation frame.

图 11.16 Annotation text dialog

Annotations can be selected when an annotation tool is enabled. They can then be moved by map position (by dragging the map marker) or by moving only the balloon. The ^{Move Annotation} tool also allows you to move the balloon on the map canvas.

To delete an annotation, select it and either press the Del or Backspace button, or double-click it and press the Delete button in the properties dialog.

备注

If you press Ctrl+T while an Annotation tool (move annotation, text annotation, form annotation) is active, the visibility states of the items are inverted.

小技巧

Layout the map with annotations

You can print or export annotations with your map to various formats using:

• map canvas export tools available in the Project menu

• print layout, in which case you need to check Draw map canvas items in the corresponding map item properties

11.4.6. Measuring

11.4.6.1. General information

QGIS provides four means of measuring geometries:

• interactive measurement tools

• measuring in the ^{Field Calculator}

• derived measurements in the Identifying Features tool

• the vector analysis tool: Vector ► Geometry Tools ► Export/Add Geometry Columns

Measuring works within projected coordinate systems (e.g., UTM) and unprojected data. The first three measuring tools behave equally to global project settings:

• Unlike most other GIS, the default measurement metric is ellipsoidal, using the ellipsoid defined in Project ► Properties... ► General. This is true both when geographic and projected coordinate systems are defined for the project.

• If you want to calculate the projected/planimetric area or distance using cartesian maths, the measurement ellipsoid has to be set to "None/Planimetric" (Project ► Properties... ► General). However, with a geographic (ie unprojected) CRS defined for the data and project, area and distance measurement will be ellipsoidal.

However, neither the identify tool nor the field calculator will transform your data to the project CRS before measuring. If you want to achieve this, you have to use the vector analysis tool: Vector ► Geometry Tools ► Add Geometry Attributes.... Here, measurement is planimetric, unless you choose the ellipsoidal measurement.

11.4.6.2. Measure length, areas and angles interactively

Click the icon in the Attribute toolbar to begin measurements. The down arrow near the icon switches between length, area or angle. The default unit used in the dialog is the one set in Project ► Properties... ► General menu.

备注

Configuring the measure tool

While measuring length or area, clicking the Configuration button at the bottom of the widget opens the Settings ► Options ► Map Tools menu, where you can select the rubberband color, the precision of the measurements and the unit behavior. You can also choose your preferred measurement or angle units, but keep in mind that those values are overridden in the current project by the selection made in the Project ► Properties... ► General menu, and by the selection made in the measurement widget.

All measuring modules use the snapping settings from the digitizing module (see section Setting the Snapping Tolerance and Search Radius). So, if you want to measure exactly along a line feature, or around a polygon feature, first set its layer snapping tolerance. Now, when using the measuring tools, each mouse click (within the tolerance setting) will snap to that layer.

By default, ^{Measure Line} measures real distances between given points according to a defined ellipsoid. The tool then allows you to click points on the map. Each segment length, as well as the total, shows up in the measure window. To stop measuring, click the right mouse button. Now it is possible to copy all your line measurements at once to the clipboard using the Copy All button.

Note that you can use the drop-down list near the total to change the measurement units interactively while working with the measure tool ('Meters', 'Kilometers', 'Feet', 'Yards', 'Miles', 'Nautical miles', 'Centimeters', 'Millimeters', 'Degrees', 'Map units'). This unit is retained for the widget until a new project is created or another project is opened.

The Info section in the dialog explains how calculations are made according to the CRS settings available.

图 11.17 Measure Distance

^{Measure Area}: Areas can also be measured. In the measure window, the accumulated area size appears. Right-click to stop drawing. The Info section is also available as well as the ability to switch between different area units ('Square meters', 'Square kilometers', 'Square feet', 'Square yards', 'Square miles', 'Hectares', 'Acres', 'Square centimeters', 'Square millimeters', 'Square nautical miles', 'Square degrees', 'Map units').

图 11.18 Measure Area

^{Measure Angle}: You can also measure angles. The cursor becomes cross-shaped. Click to draw the first segment of the angle you wish to measure, then move the cursor to draw the desired angle. The measurement is displayed in a pop-up dialog.

图 11.19 Measure Angle

11.5. Interacting with features

11.5.1. Selecting features

QGIS provides several tools to select features on the map canvas. Selection tools are available in the Edit ► Select menu or in the Selection Toolbar.

备注

Selection tools work with the currently active layer.

11.5.1.1. Selecting manually on the map canvas

To select one or more features with the mouse, you can use one of the following tools:

• ^{Select Features by area or single click}

• ^{Select Features by Polygon}

• ^{Select Features by Freehand}

• ^{Select Features by Radius}

备注

Other than ^{Select Features by Polygon}, these manual selection tools allow you to select feature(s) on the map canvas with a single click.

备注

Use the ^{Select Features by Polygon} tool to use an existing polygon feature (from any layer) to select overlapping features in the active layer. Right-click in the polygon and choose it from the context menu that shows a list of all the polygons that contain the clicked point. All the overlapping features from the active layer are selected.

小技巧

Use the Edit ► Select ► Reselect Features tool to redo your latest selection. Very useful when you have painstakingly made a selection, and then click somewhere else accidentally and clear your selection.

While using the Select Feature(s) tool, holding Shift or Ctrl toggles whether a feature is selected (ie either adds to the current selection or remove from it).

For the other tools, different behaviors can be performed by holding down:

• Shift: add features to the current selection

• Ctrl: substract features from the current selection

• Ctrl+Shift: intersect with current selection, ie only keep overlapping features from the current selection

• Alt: select features that are totally within the selection shape. Combined with Shift or Ctrl keys, you can add or substract features to/from the current selection.

11.5.1.2. Automatic selection

The other selection tools, most of them available from the Attribute table, perform a selection based on a feature's attribute or its selection state (note that attribute table and map canvas show the same information, so if you select one feature in the attribute table, it will be selected on the map canvas too):

• ^{Select By Expression...} select features using expression dialog

• ^{Select Features By Value...} or press F3

• ^{Deselect Features from All Layers} or press Ctrl+Alt+A to deselect all selected features in all layers

• ^{Deselect Features from the Current Active Layer} or press Ctrl+Shift+A

• ^{Select All Features} or press Ctrl+A to select all features in the current layer

• ^{Invert Feature Selection} to invert the selection in the current layer

• ^{Select by Location} to select the features based on their spatial relationship with other features (in the same or another layer - see Select by location)

For example, if you want to find regions that are boroughs from regions.shp of the QGIS sample data, you can:

1. Use the ^{Select features using an Expression} icon

2. Expand the Fields and Values group

3. Double-click the field that you want to query ("TYPE_2")

4. Click All Unique in the panel that shows up on the right

5. From the list, double-click 'Borough'. In the Expression editor field, write the following query:

   "TYPE_2"  =  'Borough'
   

6. Click Select Features

From the expression builder dialog, you can also use Function list ► Recent (Selection) to make a selection that you have used before. The dialog remembers the last 20 expressions used. See Expressions for more information and examples.

小技巧

Save your selection into a new file

Users can save selected features into a New Temporary Scratch Layer or a New Vector Layer using Edit ► Copy Features and Edit ► Paste Features as in the desired format.

11.5.1.3. Select Features By Value

This selection tool opens the layer's feature form allowing the user to choose which value to look for for each field, whether the search should be case-sensitive, and the operation that should be used. The tool has also autocompletes, automatically filling the search box with existing values.

图 11.20 Filter/Select features using form dialog

Alongside each field, there is a drop-down list with options to control the search behaviour:

Field search option	String	Numeric	Date
Exclude Field from the search
Equal to (=)
Not equal to (≠)
Greater than (>)
Less than (<)
Greater than or equal to (≥)
Less than or equal to (≤)
Between (inclusive)
Not between (inclusive)
Contains
Does not contain
Is missing (null)
Is not missing (not null)
Starts with
Ends with

For string comparisons, it is also possible to use the Case sensitive option.

After setting all search options, click Select features to select the matching features. The drop-down options are:

• Select features

• Add to current selection

• Remove from current selection

• Filter current selection

You can also clear all search options using the Reset form button.

Once the conditions are set, you can also either:

• Zoom to features on the map canvas without the need of a preselection

• Flash features, highlighting the matching features. This is a handy way to identify a feature without selection or using the Identify tool. Note that the flash does not alter the map canvas extent and would be visible only if the feature is within the bounds of the current map canvas.

11.5.2. Identifying Features

The Identify tool allows you to interact with the map canvas and get information on features in a pop-up window. To identify features, use:

• View ► Identify Features

• Ctrl+Shift+I (or Cmd+Shift+I),

• ^{Identify Features} icon on the Attributes toolbar

11.5.2.1. Using the Identify Features tool

QGIS offers several ways to identify features with the ^{Identify Features} tool:

• left click identifies features according to the selection mode and the selection mask set in the Identify Results panel

• right click with Identify Feature(s) as selection mode set in the Identify Results panel fetches all snapped features from all visible layers. This opens a context menu, allowing the user to choose more precisely the features to identify or the action to execute on them.

• right click with Identify Features by Polygon as selection mode in the Identify Results panel identifies the features that overlap with the chosen existing polygon, according to the selection mask set in the Identify Results panel

小技巧

Filter the layers to query with the Identify Features tool

Under Layer Capabilities in Project ► Properties... ► Data Sources, uncheck the Identifiable column next to a layer to avoid it being queried when using the ^{Identify Features} tool in a mode other than Current Layer. This is a handy way to return features from only layers that are of interest for you.

If you click on feature(s), the Identify Results dialog will list information about the feature(s) clicked. The default view is a tree view in which the first item is the name of the layer and its children are its identified feature(s). Each feature is described by the name of a field along with its value. This field is the one set in Layer Properties ► Display. All the other information about the feature follows.

11.5.2.2. Feature information

The Identify Results dialog can be customized to display custom fields, but by default it will display the following information:

• The feature display name;

• Actions: Actions can be added to the identify feature windows. The action is run by clicking on the action label. By default, only one action is added, namely View feature form for editing. You can define more actions in the layer's properties dialog (see Actions Properties).

• Derived: This information is calculated or derived from other information. It includes:

  • general information about the feature's geometry:

    • depending on the geometry type, the cartesian measurements of length, perimeter or area in the layer's CRS units. For 3D line vectors the cartesian line length is available.

    • depending on the geometry type and if an ellipsoid is set in the project properties dialog for Measurements, the ellipsoidal values of length, perimeter or area using the specified units

    • the count of geometry parts in the feature and the number of the part clicked

    • the count of vertices in the feature

  • coordinate information, using the project properties Coordinates display settings:

    • X and Y coordinate values of the point clicked

    • the number of the closest vertex to the point clicked

    • X and Y coordinate values of the closest vertex (and Z/M if applicable)

    • if you click on a curved segment, the radius of that section is also displayed.

• Data attributes: This is the list of attribute fields and values for the feature that has been clicked.

• information about the related child feature if you defined a relation:

  • the name of the relation

  • the entry in reference field, e.g. the name of the related child feature

  • Actions: lists actions defined in the layer's properties dialog (see Actions Properties) and the default action is View feature form.

  • Data attributes: This is the list of attributes fields and values of the related child feature.

备注

Links in the feature's attributes are clickable from the Identify Results panel and will open in your default web browser.

图 11.21 Identify Results dialog

11.5.2.3. The Identify Results dialog

At the top of the window, you have a handful of tools:

• ^{Open Form} of the current feature

• ^{Expand tree}

• ^{Collapse tree}

• ^{Expand New Results by Default} to define whether the next identified feature's information should be collapsed or expanded

• ^{Clear Results}

• ^{Copy selected feature to clipboard}

• ^{Print selected HTML response}

• selection mode to use to fetch features to identify:

  • ^{Identify Features by area or single click}

  • ^{Identify Features by Polygon}

  • ^{Identify Features by Freehand}

  • ^{Identify Features by Radius}

  备注

  When using ^{Identify Features by Polygon}, you can right-click any existing polygon and use it to identify overlapping features in another layer.

At the bottom of the window are the Mode and View combo boxes. Mode defines from which layers features should be identified:

• Current layer: only features from the selected layer are identified. The layer need not be visible in the canvas.

• Top down, stop at first: only features from the upper visible layer.

• Top down: all features from the visible layers. The results are shown in the panel.

• Layer selection: opens a context menu where the user selects the layer to identify features from, similar to a right-click. Only the chosen features will be shown in the result panel.

The View can be set as Tree, Table or Graph. 'Table' and 'Graph' views can only be set for raster layers.

The identify tool allows you to Auto open form for single feature results, found under ^{Identify Settings}. If checked, each time a single feature is identified, a form opens showing its attributes. This is a handy way to quickly edit a feature's attributes.

Other functions can be found in the context menu of the identified item. For example, from the context menu you can:

• View the feature form

• Zoom to feature

• Copy feature: Copy all feature geometry and attributes

• Toggle feature selection: Add identified feature to selection

• Copy attribute value: Copy only the value of the attribute that you click on

• Copy feature attributes: Copy the attributes of the feature

• Clear result: Remove results in the window

• Clear highlights: Remove features highlighted on the map

• Highlight all

• Highlight layer

• Activate layer: Choose a layer to be activated

• Layer properties: Open layer properties window

• Expand all

• Collapse all

11.6. Save and Share Layer Properties

11.6.1. Managing Custom Styles

When a vector layer is added to the map canvas, QGIS by default uses a random symbol/color to render its features. However, you can set a default symbol in Project ► Properties... ► Default styles that will be applied to each newly added layer according to its geometry type.

Most of the time, though, you'd rather have a custom and more complex style that can be applied automatically or manually to the layers (with less effort). You can achieve this by using the Style menu at the bottom of the Layer Properties dialog. This menu provides you with functions to create, load and manage styles.

A style stores any information set in the layer properties dialog to render or interact with the layer (including symbology, labeling, fields and form definitions, actions, diagrams...) for vector layers, or the pixels (band or color rendering, transparency, pyramids, histogram ...) for raster.

图 11.22 Vector layer style combo box options

By default, the style applied to a loaded layer is named default. Once you have got the ideal and appropriate rendering for your layer, you can save it by clicking the Style combo box and choosing:

• Rename Current: The active style is renamed and updated with the current options

• Add: A new style is created using the current options. By default, it will be saved in the QGIS project file. See below to save the style in another file or a database

• Remove: Delete unwanted style, in case you have more than one style defined for the layer.

At the bottom of the Style drop-down list, you can see the styles set for the layer with the active one checked.

Note that each time you validate the layer properties dialog, the active style is updated with the changes you've made.

You can create as many styles as you wish for a layer but only one can be active at a time. In combination with Map Themes, this offers a quick and powerful way to manage complex projects without the need to duplicate any layer in the map legend.

备注

Given that whenever you apply modifications to the layer properties, changes are stored in the active style, always ensure you are editing the right style to avoid mistakenly altering a style used in a map theme.

小技巧

Manage styles from layer context menu

Right-click on the layer in the Layers panel to copy, paste, add or rename layer styles.

11.6.2. Storing Styles in a File or a Database

While styles created from the Style combo box are by default saved inside the project and can be copied and pasted from layer to layer in the project, it's also possible to save them outside the project so that they can be loaded in another project.

11.6.2.1. Save as text file

Clicking the Style ► Save Style, you can save the style as a:

• QGIS layer style file (.qml)

• SLD file (.sld), only available for vector layers

Used on file-based format layers (.shp, .tab...), Save as Default generates a .qml file for the layer (with the same name). SLDs can be exported from any type of renderer -- single symbol, categorized, graduated or rule-based -- but when importing an SLD, either a single symbol or rule-based renderer is created. This means that categorized or graduated styles are converted to rule-based. If you want to preserve those renderers, you have to use the QML format. On the other hand, it can be very handy sometimes to have this easy way of converting styles to rule-based.

11.6.2.2. Save in database

Vector layer styles can also be stored in a database if the layer datasource is a database provider. Supported formats are PostGIS, GeoPackage, SpatiaLite, MSSQL and Oracle. The layer style is saved inside a table (named layer_styles) in the database. Click on Save Style... ► Save in database then fill in the dialog to define a style name, add a description, a .ui file if applicable and to check if the style should be the default style.

You can save several styles for a single table in the database. However, each table can have only one default style. Default styles can be saved in the layer database or in qgis.db, a local SQLite database in the active user profile directory.

图 11.23 Save Style in database Dialog

小技巧

Sharing style files between databases

You can only save your style in a database if the layer comes from such a database. You can't mix databases (layer in Oracle and style in MSSQL for instance). Use instead a plain text file if you want the style to be shared among databases.

备注

You may encounter issues restoring the layer_styles table from a PostgreSQL database backup. Follow QGIS layer_style table and database backup to fix that.

11.6.2.3. Load style

When loading a layer in QGIS, if a default style already exists for this layer, QGIS loads the layer with this style. Also Style ► Restore Default looks for and loads that file, replacing the layer's current style.

Style ► Load Style helps you apply any saved style to a layer. While text-file styles (.sld or .qml) can be applied to any layer whatever its format, loading styles stored in a database is only possible if the layer is from the same database or the style is stored in the QGIS local database.

The Database Styles Manager dialog displays a list of styles related to the layer found in the database and all the other styles saved in it, with name and description.

小技巧

Quickly share a layer style within the project

You can also share layer styles within a project without importing a file or database style: right-click on the layer in the Layers Panel and, from the Styles combo box , copy the style of a layer and paste it to a group or a selection of layers: the style is applied to all the layers that are of the same type (vector vs raster) as the original layer and, in the case of vector layers, have the same geometry type (point, line or polygon).

11.6.3. Layer definition file

Layer definitions can be saved as a Layer Definition File (.qlr) using Export ► Save As Layer Definition File... in the active layers' context menu. A layer definition file (.qlr) includes references to the data source of the layers and their styles. .qlr files are shown in the Browser Panel and can be used to add the layers (with the saved style) to the Layers Panel. You can also drag and drop .qlr files from the system file manager into the map canvas.

11.7. Storing values in Variables

In QGIS, you can use variables to store useful recurrent values (e.g. the project's title, or the user's full name) that can be used in expressions. Variables can be defined at the application's global level, project level, layer level, layout level, and layout item's level. Just like CSS cascading rules, variables can be overwritten - e.g., a project level variable will overwrite any application global level variables set with the same name. You can use these variables to build text strings or other custom expressions using the @ character before the variable name. For example in print layout creating a label with this content:

This map was made using QGIS [% @qgis_version %]. The project file for this
map is: [% @project_path %]

Will render the label like this:

This map was made using QGIS 3.4.4-Madeira. The project file for this map is:
/gis/qgis-user-conference-2019.qgs

Besides the preset read-only variables, you can define your own custom variables for any of the levels mentioned above. You can manage:

• global variables from the Settings ► Options menu

• project variables from the Project Properties dialog (see 工程属性)

• vector layer variables from the Layer Properties dialog (see The Vector Properties Dialog);

• layout variables from the Layout panel in the Print layout (see The Layout Panel);

• and layout item variables from the Item Properties panel in the Print layout (see Layout Items Common Options).

To differentiate from editable variables, read-only variable names and values are displayed in italic. On the other hand, higher level variables overwritten by lower level ones are strike through.

图 11.24 Variables editor at the project level

备注

You can read more about variables and find some examples in Nyall Dawson's Exploring variables in QGIS 2.12, part 1, part 2 and part 3 blog posts.

11.8. Authentication

QGIS has the facility to store/retrieve authentication credentials in a secure manner. Users can securely save credentials into authentication configurations, which are stored in a portable database, can be applied to server or database connections, and are safely referenced by their ID tokens in project or settings files. For more information see 认证系统.

A master password needs to be set up when initializing the authentication system and its portable database.

11.9. Common widgets

In QGIS, there are some options you'll often have to work with. For convenience, QGIS provides you with special widgets that are presented below.

11.9.1. Color Selector

11.9.1.1. The color dialog

The Select Color dialog will appear whenever you click the icon to choose a color. The features of this dialog depend on the state of the Use native color chooser dialogs parameter checkbox in Settings ► Options... ► General. When checked, the color dialog used is the native one of the OS on which QGIS is running. Otherwise, the QGIS custom color chooser is used.

The custom color chooser dialog has four different tabs which allow you to select colors by ^{Color ramp}, ^{Color wheel}, ^{Color swatches} or ^{Color picker}. With the first two tabs, you can browse to all possible color combinations and apply your choice to the item.

图 11.25 Color selector ramp tab

In the ^{Color swatches} tab, you can choose from a list of color palettes (see 颜色设置 for details). All but the Recent colors palette can be modified with the ^{Add current color} and ^{Remove selected color} buttons at the bottom of the frame.

The ... button next to the palette combo box also offers several options to:

• copy, paste, import or export colors

• create, import or remove color palettes

• add the custom palette to the color selector widget with the Show in Color Buttons item (see 图 11.27)

图 11.26 Color selector swatches tab

Another option is to use the ^{Color picker} which allows you to sample a color from under your mouse cursor at any part of the QGIS UI or even from another application: press the space bar while the tab is active, move the mouse over the desired color and click on it or press the space bar again. You can also click the Sample Color button to activate the picker.

Whatever method you use, the selected color is always described through color sliders for HSV (Hue, Saturation, Value) and RGB (Red, Green, Blue) values. The color is also identifiable in HTML notation.

Modifying a color is as simple as clicking on the color wheel or ramp or on any of the color parameters sliders. You can adjust such parameters with the spinbox beside or by scrolling the mouse wheel over the corresponding slider. You can also type the color in HTML notation. Finally, there is an Opacity slider to set transparency level.

The dialog also provides a visual comparison between the Old color (applied to object) and the Current one (being selected). Using drag-and-drop or pressing the ^{Add color to swatch} button, any of these colors can be saved in a slot for easy access.

小技巧

Quick color modification

Drag-and-drop a color selector widget onto another one to apply its color.

11.9.1.2. The color drop-down shortcut

Click the drop-down arrow to the right of the color button to display a widget for quick color selection. This shortcut provides access to:

• a color wheel to pick a color from

• an alpha slider to change color opacity

• the color palettes previously set to Show in Color Buttons

• copy the current color and paste it into another widget

• pick a color from anywhere on your computer display

• choose a color from the color selector dialog

• drag-and-drop the color from one widget to another for quick modification

备注

When the color widget is set to a project color through the data-defined override properties, the above functions for changing the color are unavailable. You'd first need to Unlink color or Clear the definition.

图 11.27 Quick color selector menu

11.9.1.3. The color ramp drop-down shortcut

Color ramps are a practical way to apply a set of colors to one or many features. Their creation is described in the Setting a Color Ramp section. As for the colors, pressing the color ramp button opens the corresponding color ramp type dialog allowing you to change its properties.

图 11.28 Customizing a colorbrewer ramp

The drop-down menu to the right of the button gives quick access to a wider set of color ramps and options:

• Invert Color Ramp

• a preview of the gradient or catalog: cpt-city color ramps flagged as Favorites in the Style Manager dialog

• All Color Ramps to access the compatible color ramps database

• Create New Color Ramp... of any supported type that could be used in the current widget (note that this color ramp will not be available elsewhere unless you save it in the library)

• Edit Color Ramp..., the same as clicking the whole color ramp button

• Save Color Ramp..., to save the current color ramp with its customizations in the style library

图 11.29 Quick color ramp selection widget

11.9.2. Symbol Widget

The Symbol selector widget is a convenient shortcut when you want to set symbol properties of a feature. Clicking the drop-down arrow shows the following symbol options, together with the features of the color drop-down widget:

• Configure Symbol...: the same as pressing the symbol selector widget. It opens a dialog to set the symbol parameters.

• Copy Symbol from the current item

• Paste Symbol to the current item, speeding configuration

11.9.3. Font Selector

The Font selector widget is a convenient shortcut when you want to set font properties for textual information (feature labels, decoration labels, map legend text, ...). Clicking the drop-down arrow shows some or all of the following options:

图 11.30 Font selector drop-down menu

• Font Size in the associated unit

• Recent Fonts ► menu with the active font checked (at the top)

• Configure Format...: same as pressing the font selector widget. It opens a dialog to set text format parameters. Depending on the context, it can be the OS default Text format dialog or the QGIS custom dialog with advanced formatting options (opacity, orientation, buffer, background, shadow, ...) as described in section Formatting the label text.

• Copy Format of the text

• Paste Format to the text, speeding configuration

• the color widget for quick color setting

11.9.4. Unit Selector

Size properties of the items (labels, symbols, layout elements, ...) in QGIS are not necessarily bound to either the project units or the units of a particular layer. For a large set of properties, the Unit selector drop-down menu allows you to tweak their values according to the rendering you want (based on screen resolution, paper size, or the terrain). Available units are:

• Millimeters

• Points

• Pixels

• Inches

• Meters at Scale: This allows you to always set the size in meters, regardless of what the underlying map units are (e.g. they can be in inches, feet, geographic degrees, ...). The size in meters is calculated based on the current project ellipsoid setting and a projection of the distances in meters at the center of the current map extent.

• and Map Units: The size is scaled according to the map view scale. Because this can lead to too big or too small values, use the button next to the entry to constrain the size to a range of values based on:

  • The Minimum scale and the Maximum scale: The value is scaled based on the map view scale until you reach any of these scale limits. Out of the range of scale, the value at the nearest scale limit is kept.

  • and/or The Minimum size and the Maximum size in mm: The value is scaled based on the map view scale until it reaches any of these limits; Then the limit size is kept.

  

  图 11.31 Adjust scaling range dialog

11.9.5. Number Formatting

Numeric formatters allow formatting of numeric values for display, using a variety of different formatting techniques (for instance scientific notation, currency values, percentage values, etc). One use of this is to set text in a layout scale bar or fixed table.

Different categories of formats are supported. For most of them, you can set part or all of the following numeric options:

• Show thousands separator

• Show plus sign

• Show trailing zeros

But they can also have their custom settings. Provided categories are:

• General, the default category: has no setting and displays values as set in the parent widget properties or using the global settings.

• Number

  • The value can be Round to a self defined number of Decimal places or their Significant figures

  • customize the Thousands separator and Decimal separator

• Bearing for a text representation of a direction/bearing using:

  • Format: possible ranges of values are 0 to 180°, with E/W suffix, -180 to +180° and 0 to 360°

  • number of Decimal places

• Currency for a text representation of a currency value.

  • Prefix

  • Suffix

  • number of Decimal places

• Fraction for a vulgar fractional representation of a decimal value (e.g. 1/2 instead of 0.5)

  • Use unicode super/subscript to show. For example ^1/2 instead of 1/2

  • Use dedicated Unicode characters

  • customize the Thousands separator

• Percentage - appends % to the values, with setting of:

  • number of Decimal places

  • Scaling to indicate whether the actual values already represent percentages (then they will be kept as is) or fractions (then they are converted)

• Scientific notation in the form 2.56e+03. The number of Decimal places can be set.

A live preview of the settings is displayed under the Sample section.

11.9.6. Blending Modes

QGIS offers different options for special rendering effects with these tools that you may previously only know from graphics programs. Blending modes can be applied on layers and features, and also on print layout items:

• Normal: This is the standard blend mode, which uses the alpha channel of the top pixel to blend with the pixel beneath it. The colors aren't mixed.

• Lighten: This selects the maximum of each component from the foreground and background pixels. Be aware that the results tend to be jagged and harsh.

• Screen: Light pixels from the source are painted over the destination, while dark pixels are not. This mode is most useful for mixing the texture of one item with another item (such as using a hillshade to texture another layer).

• Dodge: Brighten and saturate underlying pixels based on the lightness of the top pixel. Brighter top pixels cause the saturation and brightness of the underlying pixels to increase. This works best if the top pixels aren't too bright. Otherwise the effect is too extreme.

• Addition: Adds pixel values of one item to the other. In case of values above the maximum value (in the case of RGB), white is displayed. This mode is suitable for highlighting features.

• Darken: Retains the lowest values of each component of the foreground and background pixels. Like lighten, the results tend to be jagged and harsh.

• Multiply: Pixel values of the top item are multiplied with the corresponding values for the bottom item. The results are darker.

• Burn: Darker colors in the top item cause the underlying items to darken. Burn can be used to tweak and colorize underlying layers.

• Overlay: Combines multiply and screen blending modes. Light parts become lighter and dark parts become darker.

• Soft light: Very similar to overlay, but instead of using multiply/screen it uses color burn/dodge. This is supposed to emulate shining a soft light onto an image.

• Hard light: Hard light is also very similar to the overlay mode. It's supposed to emulate projecting a very intense light onto an image.

• Difference: Subtracts the top pixel from the bottom pixel, or the other way around, in order always to get a positive value. Blending with black produces no change, as the difference with all colors is zero.

• Subtract: Subtracts pixel values of one item from the other. In the case of negative values, black is displayed.

11.9.7. Data defined override setup

Next to many options in the vector layer properties dialog or settings in the print layout, you will find a ^{Data defined override} icon. Using expressions based on layer attributes or item settings, prebuilt or custom functions and variables, this tool allows you to set dynamic values for parameters. When enabled, the value returned by this widget is applied to the parameter regardless of its normal value (checkbox, textbox, slider...).

11.9.7.1. The data defined override widget

Clicking the ^{Data defined override} icon shows the following entries:

• Description... that indicates if the option is enabled, which input is expected, the valid input type and the current definition. Hovering over the widget also pops up this information.

• Store data in the project: a button allowing the property to be stored using to the Auxiliary Storage Properties mechanism.

• Field type: an entry to select from the layer's fields that match the valid input type.

• Color: when the widget is linked to a color property, this menu gives access to the colors defined as part of the current project's colors scheme.

• Variable: a menu to access the available user-defined variables

• Edit... button to create or edit the expression to apply, using the Expression String Builder dialog. To help you correctly fill in the expression, a reminder of the expected output's format is provided in the dialog.

• Paste and Copy buttons.

• Clear button to remove the setup.

• For numeric and color properties, Assistant... to rescale how the feature data is applied to the property (more details below)

小技巧

Use right-click to (de)activate the data override

When the data-defined override option is set up correctly the icon is yellow or . If it is broken, the icon is red or .

You can enable or disable a configured ^{data-defined override} button by simply clicking the widget with the right mouse button.

11.9.7.2. Using the data-defined assistant interface

When the ^{Data-defined override} button is associated with a numeric or color parameter, it has an Assistant... option that allows you to change how the data is applied to the parameter for each feature. The assistant allows you to:

• Define the Input data, ie:

  • the attribute to represent, using the Field listbox or the ^{Set column expression} function (see Expressions)

  • the range of values to represent: you can manually enter the values or use the ^{Fetch value range from layer} button to fill these fields automatically with the minimum and maximum values returned by the chosen attribute or the expression applied to your data

• Apply transform curve: by default, output values (see below for setting) are applied to input features following a linear scale. You can override this logic: enable the transform option, click on the graphic to add break point(s) and drag the point(s) to apply a custom distribution.

• Define the Output values: the options vary according to the parameter to define. You can globally set:

  • the minimum and maximum values to apply to the selected property (n case of a color setting, you'll need to provide a color ramp)

  • the Scale method of representation which can be Flannery, Exponential, Surface or Radius

  • the Exponent to use for data scaling

  • the output value or color to represent features with NULL values

When compatible with the property, a live-update preview is displayed in the right-hand side of the dialog to help you control the value scaling.

图 11.32 The data-defined size assistant

The values presented in the varying size assistant above will set the size 'Data-defined override' with:

coalesce(scale_exp(Importance, 1, 20, 2, 10, 0.57), 1)