16.3. Editing

QGIS has various capabilities for editing OGR, SpatiaLite, PostGIS, MS SQL Server and Oracle Spatial vector layers and tables. They can be of 2D or 3D geometry type.

Note

The procedure for editing GRASS layers is different - see section Digitizing and editing a GRASS vector layer for details.

Attention

Concurrent Edits

QGIS does not track if somebody else is editing the same feature at the same time as you are. The last person to save the edits wins.

Tip

Validating Edits

Continuous validation can be activated on a layer basis in the Layer Properties ► Digitizing tab. More at Digitizing Properties.

16.3.1. Setting the snapping tolerance and search radius

Under the Settings ► Options… ► Digitizing menu, QGIS provides a number of parameters to configure default behaviour of editing tools. More information at Digitizing settings.

For optimal and accurate editing of vector layer geometries, we need to set an appropriate value of snapping tolerance and search radius for features vertices. The Snapping group provides related options, namely handling of the snapping tolerance and the search radius.

• Snapping tolerance: When you add a new vertex or move an existing one, the snapping tolerance is the distance QGIS uses to search for the closest vertex or segment you are trying to connect to. If you are not within the snapping tolerance, QGIS will leave the vertex where you release the mouse button, instead of snapping it to an existing vertex or segment.

  The tolerance setting affects all tools that work with snapping and applies by default to new layers and projets. It can however be overridden at layer level (see Snapping and Digitizing Options).

• Search radius: Search radius for vertex edits is the distance QGIS uses to search for the vertex to select when you click on the map. If you are not within the search radius, QGIS will not find and select any vertex for editing.

Snap tolerance and search radius are set in map units or pixels. You may need to experiment to get them right. If you specify a too big tolerance, QGIS may snap to the wrong vertex, especially if you are dealing with a large number of vertices in close proximity. The smaller the search radius, the more difficult it will be to hit what you want to move.

16.3.2. Snapping and Digitizing Options

Global snapping and digitizing settings (snapping mode, tolerance value, and units…) can be overridden in the project from the Project ► Snapping Options… menu. In the Snapping and Digitizing Options, you can also configure some other properties (snapping layers, scale limit, topology…) The Snapping Toolbar gives access to most of these features.

By default, snapping is disabled in a project until you press the ^{Enable snapping} button or press S. The snapping mode, tolerance value, and units can also be configured in this toolbar.

16.3.2.1. Snapping properties

There are three options to select the layer(s) to snap to:

• All layers: quick setting for all visible layers in the project so that the pointer snaps to all vertices and/or segments. In most cases, it is sufficient to use this snapping mode, but beware when using it for projects with many vector layers, as it may affect performance.

• Current layer: only the active layer is used, a convenient way to ensure topological consistency within the layer being edited.

• Advanced Configuration: allows you to enable and adjust snapping mode, tolerance and units, overlaps and scales of snapping on a layer basis (see Fig. 16.78). If you need to edit a layer and snap its vertices to another, make sure that the target layer is checked and increase the snapping tolerance to a higher value. Snapping will not occur to a layer that is not checked in the snapping options dialog.

When moving or creating vertex, you can opt for the following snapping modes:

• Vertex

• Segment: snaps along a line or a polygon perimeter. If topological editing is enabled, then a new vertex is added at the snapping location.

• Area: guarantees that the snap point lies anywhere on a polygon’s area, not necessarily on its boundary

• Centroid: snaps to the centroid of the geometry of a feature. In case of a multipart geometry, the target point may be distinct from the existing parts.

• Middle of Segments on line or polygon feature

• Line Endpoints: snaps to the first or last vertex of every part of a line or polygon feature.

QGIS will show different snap icons depending on the kind of snap:

Table 16.2 Snapping icons

Snapping to a vertex: box icon	Snapping to a segment: hourglass icon	Snapping to an intersection: cross icon

Note that it is possible to change the color of these icons in the Digitizing part of the global settings.

The tolerance values can be set either in the project’s map units or in pixels. The advantage of choosing pixels is that it keeps the snapping constant at different map scales. 10 to 12 pixels is normally a good value, but it depends on the DPI of your screen. Using map units allows the tolerance to be related to real ground distances. For example, if you have a minimum distance between elements, this option can be useful to ensure that you don’t add vertices too close to each other.

Fig. 16.78 Snapping options (Advanced Configuration mode)

Note

By default, only visible features (the features whose style is displayed, except for layers where the symbology is “No symbols”) can be snapped. You can enable the snapping on invisible features by checking Enable snapping on invisible features under the Settings ► Options ► Digitizing tab.

Tip

Enable snapping by default

You can set snapping to be enabled by default on all new projects in the Settings ► Options ► Digitizing tab. You can also set the default snapping mode, tolerance value, and units, which will populate the Snapping Options dialog.

16.3.2.2. Enable snapping on intersections

Another available option is to use snapping on intersection, which allows you to snap to geometry intersections of snapping enabled layers, even if there are no vertices at the intersections.

16.3.2.3. Limit snapping to a scale range

In some cases snapping can become very slow. This is often caused by the amount of features in some layers that require a heavy index to compute and maintain. Some parameters exist to enable snapping only when the map view is inside a relevant scale range. This allows to only do the costly index computation related to snapping at a scale where drawing is relevant.

Scale limit to snapping is configured in Project ► Snapping Options…. Limiting snapping to scale is only available in Advanced Configuration mode.

To limit snapping to a scale range you have three modes available:

• Disabled: Snapping is enabled whatever the current map scale is. This is the default mode.

• Global: Snapping is limited and only enabled when the current scale of the map is between a global minimum and a global maximum value. When selecting this mode two widgets become available to configure the range of scales in which snapping is enabled.

• Per layer: The snapping scale range limit is defined for each layer. When selecting this mode two columns become available to configure the minimum and maximum scales for each layer.

Please note that the minimum and maximum scales follow the QGIS convention: minimum scale is the most “zoomed out” scale while maximum scale is the most “zoomed in”. A minimum or maximum scale that is set to “0” or “not set” is considered not limiting.

16.3.2.4. Self-snapping

The ^{Self-snapping} option allows you to snap to the geometry that is being edited. Combined with the advanced digitizing panel, this provides a handy way to digitize new edges relative to the previous edges or vertices. Self-snapping can cause invalid geometries, use with caution.

Fig. 16.79 Drawing features with self-snapping

16.3.2.5. Snapping on custom grid

A snapping distance can also be customized on a layer basis in the Digitizing tab of the layer properties dialog. With setting the Geometry precision distance, you enable a dotted grid visible when the map canvas is at a coherent scale for display. Snapping can then be performed on the dots of the grid: an added or modified geometry will have all of its vertices snapped automatically to the closest node of the grid. More information at Digitizing Properties.

16.3.3. Topological editing

In addition to these snapping options, the Snapping options… dialog (Project ► Snapping options) and the Snapping toolbar allow you to enable / disable some other topological functionalities.

16.3.3.1. Enable topological editing

The ^{Topological editing} button helps when editing and maintaining features with common boundaries. With this option enabled, QGIS ‘detects’ shared boundaries. When you move common vertices/segments, QGIS will also move them in the geometries of the neighboring features.

Topological editing works with features from different layers, as long as the layers are visible and in editing mode.

In layer with Z or M values, topological editing will interpolate the Z or M value of the vertex based on the value of the edge used for the connection.

16.3.3.2. Overlapping control

Overlapping prevents you from drawing new features that overlap existing ones in the selected layer, speeding up digitizing of adjacent polygons. It can be controlled by the overlap tool. Three modes are available:

1. Allow Overlap (default)

2. Avoid Overlap on Active Layer: prevents any overlap with other features from the layer being edited. Digitize the new geometries so that they overlap their neighbours and QGIS will cut the overlapping part(s) of the new geometries and snap them to the boundary of the existing features. The advantage is that you don’t have to digitize the common vertices on boundary.

3. Follow Advanced Configuration: allows the overlapping setting to be set on a layer basis in the Advanced configuration view mode.

Note

If the new geometry is totally covered by existing ones, it gets cleared, and QGIS will show an error message.

Warning

Use cautiously the Avoid overlap option

Since this option will cut new overlapping geometries of any polygon layer, you can get unexpected geometries if you forget to uncheck it when no longer needed.

16.3.3.3. Automatic Tracing

Usually, when using capturing map tools (add feature, add part, add ring, reshape and split), you need to click each vertex of the feature. With the automatic tracing mode, you can speed up the digitization process as you no longer need to manually place all the vertices during digitization:

1. Enable the ^Tracing tool (in the Snapping toolbar) by pushing the icon or pressing T key.

2. Snap to a vertex or segment of a feature you want to trace along.

3. Move the mouse over another vertex or segment you’d like to snap and, instead of the usual straight line, the digitizing rubber band represents a path from the last point you snapped to the current position. The tool also works with curved geometries.

   QGIS actually uses the underlying features topology to build the shortest path between the two points. Tracing requires snapping to be activated in traceable layers to build the path. You should also snap to an existing vertex or segment while digitizing and ensure that the two nodes are topologically connectable through existing features edges, otherwise QGIS is unable to connect them and thus traces a single straight line.

4. Click and QGIS places the intermediate vertices following the displayed path.

Unfold the ^{Enable Tracing} icon and set the Offset option to digitize a path parallel to the features instead of tracing along them. A positive value shifts the new drawing to the left side of the tracing direction and a negative value does the opposite.

Note

Adjust map scale or snapping settings for an optimal tracing

If there are too many features in map display, tracing is disabled to avoid potentially long tracing structure preparation and large memory overhead. After zooming in or disabling some layers the tracing is enabled again.

Note

Does not add topological points

This tool does not add points to existing polygon geometries even if Topological editing is enabled. If geometry precision is activated on the edited layer, the resulting geometry might not exactly follow an existing geometry.

Tip

Quickly enable or disable automatic tracing by pressing the T key

By pressing the T key, tracing can be enabled/disabled anytime (even while digitizing a feature), so it is possible to digitize parts of the feature with tracing enabled and other parts with tracing disabled. Tools behave as usual when tracing is disabled.

Tip

Convert tracing to curved geometries

By using Settings ► Options ► Digitizing ► Tracing you can create curved geometries while digitizing. See digitizing options.

16.3.4. Digitizing an existing layer

By default, QGIS loads layers read-only. This is a safeguard to avoid accidentally editing a layer if there is a slip of the mouse. However, you can choose to edit any layer as long as the data provider supports it (see Exploring Data Formats and Fields), and the underlying data source is writable (i.e., its files are not read-only).

Tip

Restrict edit permission on layers within a project

From the Project ► Properties… ► Data Sources ► Layers Capabilities table, you can choose to set any layer read-only regardless the provider permission. This can be a handy way, in a multi-users environment to avoid unauthorized users to mistakenly edit layers (e.g., Shapefile), hence potentially corrupt data. Note that this setting only applies inside the current project.

In general, tools for editing vector layers are divided into a digitizing and an advanced digitizing toolbar, described in section Advanced digitizing. You can select and unselect both under View ► Toolbars ►.

Using the basic digitizing tools, you can perform the following functions:

Table 16.3 Vector layer basic editing toolbar

Tool	Purpose	Tool	Purpose
	Access to save, rollback or cancel changes in all or selected layers simultaneously		Turn on or off edit status of selected layer(s) based on the active layer status
	Save edits to the active layer
	Digitize using straight segments		Digitize using curve lines
	Enable freehand digitizing		Digitize polygon of regular shape
	Add new record		Add Feature: Capture Point
	Add Feature: Capture Line		Add Feature: Capture Polygon
	Vertex Tool (All Layers)		Vertex Tool (Current Layer)
Show Vertex Editor	Set whether the vertex editor panel should auto-open		Modify the attributes of all selected features simultaneously
	Delete Selected features from the active layer		Cut Features from the active layer
	Copy selected Features from the active layer		Paste Features into the active layer
	Undo changes in the active layer		Redo changes in active layer

Note that while using any of the digitizing tools, you can still zoom or pan in the map canvas without losing the focus on the tool.

All editing sessions start by choosing the ^{Toggle editing} option found in the context menu of a given layer, from the attribute table dialog, the digitizing toolbar or the Layer menu.

Once the layer is in edit mode, additional tool buttons on the editing toolbar will become available and markers will appear at the vertices of all features unless Show markers only for selected features option under Settings ► Options… ► Digitizing menu is checked.

Tip

Save Regularly

Remember to ^{Save Layer Edits} regularly. This will also check that your data source can accept all the changes.

16.3.4.1. Geometry editing techniques

When a geometry drawing tool (mainly the ones that add, split, reshape features) is enabled for a line or polygon based layer, you can select the technique for adding new vertices:

• The ^{Digitize with Segment}: draws straight segment whose start and end points are defined by left clicks.

• The ^{Digitize with Curve}: draws curve line based on three consecutive nodes defined by left clicks (start, point along the arc, end). If the geometry type does not support curves, then consecutive smaller segments are used to approximate the curvature.

• The ^{Stream Digitizing}: draws lines in freehand mode, i.e. nodes are added following cursor movement in the map canvas and a Streaming Tolerance. The streaming tolerance defines the spacing between consecutive vertices. Currently, the only supported unit is pixels (px). Only the starting left click and the ending right click are necessary in this mode.

• The ^{Digitize Shape}: triggers tools on the Shape Digitizing Toolbar to draw a polygon of a regular shape.

The selected technique remains while switching among the digitizing tools. You can combine any of the first three methods while drawing the same geometry.

16.3.4.2. Adding Features

Depending on the layer type, you can use the ^{Add Record}, ^{Add Point Feature}, ^{Add Line Feature} or ^{Add Polygon Feature} icons on the toolbar to add new features into the current layer.

To add a geometryless feature, click on the ^{Add Record} button and you can enter attributes in the feature form that opens.

To create features with the spatially enabled tools, you first digitize the geometry then enter its attributes. To digitize the geometry:

1. (Optional as it is the default) Select the ^{Digitize With Segment} geometry drawing method

2. Left-click on the map area to create the first point of your new feature. For point features, this should be enough and trigger, if required, the feature form to fill in their attributes.

3. For line or polygon geometries, keep on left-clicking for each additional point you wish to capture. You can rely on the snapping to features options, the snap-to-grid or the advanced digitizing panel to accurately position each vertex.

   Along with drawing straight segments between nodes you click one by one, lines and polygons can be:

   • traced automatically, accelerating the digitization. This will create consecutive straight lines between the vertices you place, following existing features.

   • free-hand digitized, pressing R or activating ^{Stream Digitizing}.

   • drawn as curve, pressing Ctrl+Shift+G or activating ^{Digitize with Curve}.

   Note

   While digitizing line or polygon geometries, you can switch back and forth between the geometry drawing methods, allowing you to create features mixing straight segments, free-hand ones and curved parts.

4. Press Delete or Backspace key to revert the last node(s) you may wrongly add.

5. When you have finished adding points, right-click anywhere on the map area to confirm you have finished entering the geometry of that feature.

   Tip

   Customize the digitizing rubber band

   While capturing polygon, the by-default red rubber band can hide underlying features or places you’d like to capture a point. This can be fixed by setting a lower opacity (or alpha channel) to the rubber band’s Fill Color in Settings ► Options ► Digitizing menu. You can also avoid the use of the rubber band by checking Don’t update rubber band during node editing.

6. For line feature pressing Shift + right-click will close the line automatically.

7. The attribute window will appear, allowing you to enter the information for the new feature. Fig. 16.80 shows setting attributes for a fictitious new river. However, in the Digitizing menu under the Settings ► Options menu, you can also:

   • Suppress attributes pop-up windows after each created feature to avoid the form opening;

   • or Reuse last entered attribute values to have fields automatically filled at the opening of the form and just have to type changing values.

Fig. 16.80 Enter Attribute Values Dialog after digitizing a new vector feature

16.3.4.3. Vertex tool

QGIS provides two tools to interact with vector features vertices:

• ^{Vertex Tool (Current Layer)}: only overlaid features in the active layer (in the Layers panel) are affected

• ^{Vertex Tool (All Layers)}: any overlaid features in all editable layers are affected. This allows you to edit features without switching the active layer or edit multiple layers at once (e.g., country and their regions boundaries)

For any editable vector layer, the vertex tools provide manipulation capabilities of feature vertices similar to CAD programs. It is possible to select multiple vertices at once and to move, add or delete them altogether. The vertex tools also support the topological editing feature. They are selection persistent, so when some operation is done, selection stays active for this feature and tool.

It is important to set the property Settings ► Options ► Digitizing ► Search Radius: to a number greater than zero. Otherwise, QGIS will not be able to tell which vertex is being edited and will display a warning.

Tip

Vertex Markers

QGIS supports different kinds of vertex markers: ‘Semi-transparent circle’, ‘Cross’ and ‘None’. To change the marker style, choose Options from the Settings menu, click on the Digitizing tab and select the appropriate entry.

Basic operations

Given a layer in edit mode, start by activating the vertex tool. Red circles will appear when hovering vertices.

• Selecting vertices: You can select vertices by:

  • Clicking on them one at a time holding Shift key pressed

  • Click-and-dragging a rectangle surrounding the target vertices

  • Drawing a polygon surrounding the target vertices: Hold Alt and click using the vertex tool to start digitizing a polygon. Each subsequent click adds a new vertex to the rubberband polygon. Backspace or Delete removes last added rubberband vertex. Esc cancels the polygon selection mode, as also does backspacing/deleting all of the rubberband’s vertices. Right click finalizes the polygon digitizing and selects all vertices within the rubberband polygon.

  When a vertex is selected, its color changes to blue. To add more vertices to the current selection, hold down the Shift key while proceeding as above. To remove vertices from the selection, hold down Ctrl.

  Tip

  Feature selection bounds vertex tool

  Vertices can be selected accross different features (or layers). If you are looking for vertices of a specific feature in a crowded place, first select that feature. Then draw the rectangle or polygon selector with the vertex tool around the vertices: only the selected feature’s vertices are selected.

  This is also the case if you display the feature in the vertex editor panel.

• Batch vertex selection mode: The batch selection mode can be activated by pressing Shift+R. Select a first node with one single click, and then hover without clicking another vertex. This will dynamically select all the nodes in between using the shortest path (for polygons).

  

  Fig. 16.81 Batch vertex selection using Shift+R

  Press Ctrl will invert the selection, selecting the longest path along the feature boundary. Ending your node selection with a second click, or pressing Esc will escape the batch mode.

• Adding vertices: To add a vertex to a line or polygon geometry, hold Shift and double-click the place on the segment.

  When hovering a segment, a virtual new node appears on the center. Click on it, move the cursor to a new location and click again to add a new vertex. For lines, a virtual node is also proposed at both extremities: click on it, do subsequent clicks and finish with a right-click; this allows to easily extend an existing line.

  

  Fig. 16.82 Virtual nodes for adding vertices

• Deleting vertices: Select the vertices and click the Delete key. Deleting all the vertices of a feature generates, if compatible with the datasource, a geometryless feature. Note that this doesn’t delete the complete feature, just the geometry part. To delete a complete feature use the ^{Delete Selected} tool.

• Moving vertices: Select all the vertices you want to move, click on a selected vertex or edge, and click on the desired new location. You can use the snapping to feature capabilities and the Advanced Digitizing Panel constraints for distance, angles, exact X and Y location before the second click. All the selected vertices will be translated.

  However, if the snap-to-grid option is enabled, selected vertices are snapped to the closest grid intersection to their translated position. Unselected vertices are also moved to their closest grid intersection. There is no simple translation.

  

  Fig. 16.83 Moving the top vertex snaps all the vertices to the grid

• Converting adjacent segments to/from curve: Select the center vertex of the segment you want to convert, hit the O letter key. If the vertex was in a curve, the curve is converted into straight lines. If the vertex was between two straight lines, they are converted into a curve. A first or a last vertex of a line can’t be converted to a center vertex curve. The layer must be compatible with curve geometry type.

  

  Fig. 16.84 Switch from curve to straight lines with O letter

Each change made with the vertex tool is stored as a separate entry in the Undo dialog. Remember that all operations support topological editing when this is turned on. On-the-fly projection is also supported.

The Vertex Editor Panel

With enabling a vertex tool, you also open the Vertex Editor panel. Right-clicking over a feature fills the panel with the list of all the vertices of the feature with their x, y (z, m if applicable) coordinates and r (for the radius, in case of circular geometry). The feature is also made exclusive for editing, meaning that the edit of any other features is disabled:

• Selecting a row in the table does select the corresponding vertex in the map canvas, and vice versa.

• Clicking or dragging over the map canvas will only select or move vertices and segments of that feature

• Change a coordinate in the table and the vertex position is updated. This is a convenient way to edit Z coordinate or M value on vertices.

• You can also select multiple rows and delete them altogether.

• New vertices can only be added to the bound feature

If you do not want the Vertex Editor panel to immediately show each time you interact with vertex tools (and potentially hide other panels or disturb panels placement), uncheck the Auto-open table entry in the ^Options menu at the top of the panel. You can then also close the panel. To reopen the panel, you would need to right-click over a panel or toolbar and select it in the list or tick the Show vertex editor entry in the Digitizing toolbar.

Fig. 16.85 Vertex editor panel showing selected nodes

16.3.4.4. Rules of Z coordinate or M value assignment

Digitizing 3D vector features or features with M value is not that different from (X,Y) 2D layers’. Tools and options described in this chapter are still available and help you place the vertex or point in a planar environment. Then you may need to handle the Z coordinate (or M value) assignment:

• By default, QGIS will assign to new vertices the Default Z value (respectively Default M value) set in the Settings ► Options ► Digitizing tab. If the Advanced Digitizing Panel is in use, then the value is taken from its z (respectively m) widget.

• When snapping to a vertex, the new or moved vertex takes the snapped one’s Z or M value.

• When snapping to a segment while the topological editing is on, then the new vertex Z or M value is interpolated along the segment.

• If the z (respectively m) widget of the Advanced Digitizing Panel is locked, then its value is applied to the vertex, taking precedence over any snapped vertex or segment Z or M value.

To edit Z or M values of an existing feature, you can use the Vertex editor panel. To create features with custom Z or M values you may want to rely on the Advanced Digitizing Panel.

16.3.4.5. Cutting, Copying and Pasting Features

Selected features can be cut, copied and pasted between layers in the same QGIS project, as long as destination layers are set to ^{Toggle editing} beforehand.

Tip

Transform polygon into line and vice-versa using copy/paste

Copy a line feature and paste it in a polygon layer: QGIS pastes in the target layer a polygon whose boundary corresponds to the closed geometry of the line feature. This is a quick way to generate different geometries of the same data.

Features can also be pasted to external applications as text. That is, the features are represented in CSV format, with the geometry data appearing in the OGC Well-Known Text (WKT) format. WKT and GeoJSON features from outside QGIS can also be pasted to a layer within QGIS.

When would the copy and paste function come in handy? Well, it turns out that you can edit more than one layer at a time and copy/paste features between layers. Why would we want to do this? Say we need to do some work on a new layer but only need one or two lakes, not the 5,000 on our big_lakes layer. We can create a new layer and use copy/paste to plop the needed lakes into it.

As an example, we will copy some lakes to a new layer:

1. Load the layer you want to copy from (source layer)

2. Load or create the layer you want to copy to (target layer)

3. Start editing for target layer

4. Make the source layer active by clicking on it in the legend

5. Use the ^{Select Features by area or single click} tool to select the feature(s) on the source layer

6. Click on the ^{Copy Features} tool

7. Make the destination layer active by clicking on it in the legend

8. Click on the ^{Paste Features} tool

9. Stop editing and save the changes

What happens if the source and target layers have different schemas (field names and types are not the same)? QGIS populates what matches and ignores the rest. If you don’t care about the attributes being copied to the target layer, it doesn’t matter how you design the fields and data types. If you want to make sure everything - the feature and its attributes - gets copied, make sure the schemas match.

Note

Congruency of Pasted Features

If your source and destination layers use the same projection, then the pasted features will have geometry identical to the source layer. However, if the destination layer is a different projection, then QGIS cannot guarantee the geometry is identical. This is simply because there are small rounding-off errors involved when converting between projections.

Tip

Copy string attribute into another

If you have created a new column in your attribute table with type ‘string’ and want to paste values from another attribute column that has a greater length the length of the column size will be extended to the same amount. This is because the GDAL Shapefile driver knows to auto-extend string and integer fields to dynamically accommodate for the length of the data to be inserted.

16.3.4.6. Deleting Selected Features

If we want to delete an entire feature (attribute and geometry), we can do that by first selecting the geometry using the regular ^{Select Features by area or single click} tool. Selection can also be done from the attribute table. Once you have the selection set, press Delete or Backspace key or use the ^{Delete Selected} tool to delete the features. Multiple selected features can be deleted at once.

The ^{Cut Features} tool on the digitizing toolbar can also be used to delete features. This effectively deletes the feature but also places it on a “spatial clipboard”. So, we cut the feature to delete. We could then use the ^{Paste Features} tool to put it back, giving us a one-level undo capability. Cut, copy, and paste work on the currently selected features, meaning we can operate on more than one at a time.

16.3.4.7. Undo and Redo

The ^Undo and ^Redo tools allows you to undo or redo vector editing operations. There is also a dockable widget, which shows all operations in the undo/redo history (see Fig. 16.86). This widget is not displayed by default; it can be displayed by right-clicking on the toolbar and activating the Undo/Redo Panel checkbox. The Undo/Redo capability is however active, even if the widget is not displayed.

Fig. 16.86 Redo and Undo digitizing steps

When Undo is hit or Ctrl+Z (or Cmd+Z) pressed, the state of all features and attributes are reverted to the state before the reverted operation happened. Changes other than normal vector editing operations (for example, changes done by a plugin) may or may not be reverted, depending on how the changes were performed.

To use the undo/redo history widget, simply click to select an operation in the history list. All features will be reverted to the state they were in after the selected operation.

16.3.4.8. Saving Edited Layers

When a layer is in editing mode, any changes remain in the memory of QGIS. Therefore, they are not committed/saved immediately to the data source or disk. If you want to save edits to the current layer but want to continue editing without leaving the editing mode, you can click the ^{Save Layer Edits} button. When you turn editing mode off with ^{Toggle editing} (or quit QGIS for that matter), you are also asked if you want to save your changes or discard them.

If the changes cannot be saved (e.g., disk full, or the attributes have values that are out of range), the QGIS in-memory state is preserved. This allows you to adjust your edits and try again.

Tip

Data Integrity

It is always a good idea to back up your data source before you start editing. While the authors of QGIS have made every effort to preserve the integrity of your data, we offer no warranty in this regard.

Saving multiple layers at once

This feature allows the digitization of multiple layers. Choose Save for Selected Layers to save all changes you made in multiple layers. You also have the opportunity to Rollback for Selected Layers, so that the digitization may be withdrawn for all selected layers. If you want to stop editing the selected layers, Cancel for Selected Layer(s) is an easy way.

The same functions are available for editing all layers of the project.

Tip

Use transaction group to edit, save or rollback multiple layers changes at once

When working with layers from the same PostGreSQL database, activate the Automatically create transaction groups where possible option in Project ► Properties… ► Data Sources to sync their behavior (enter or exit the edit mode, save or rollback changes at the same time).

16.3.5. Advanced digitizing

Table 16.4 Vector layer advanced editing toolbar

Icon	Purpose	Icon	Purpose
	Enable Advanced Digitizing Tools
	Move Feature(s)		Copy and Move Feature(s)
	Rotate Feature(s)		Simplify Feature
	Scale Feature
	Add Ring		Add Part
	Fill Ring		Swap direction
	Delete Ring		Delete Part
	Offset Curve		Reshape Features
	Split Parts		Split Features
	Merge Attributes of Selected Features		Merge Selected Features
	Rotate Point Symbols		Offset Point Symbols
	Trim or Extend Feature

16.3.5.1. Move Feature(s)

The ^{Move Feature(s)} tool allows you to move existing features:

1. Select the feature(s) to move.

2. Click on the map canvas to indicate the origin point of the displacement; you can rely on snapping capabilities to select an accurate point.

   You can also take advantages of the advanced digitizing constraints to accurately set the origin point coordinates. In that case:

   1. First click on the button to enable the panel.

   2. Type x and enter the corresponding value for the origin point you’d like to use. Then press the button next to the option to lock the value.

   3. Do the same for the y coordinate.

   4. Click on the map canvas and your origin point is placed at the indicated coordinates.

3. Move over the map canvas to indicate the destination point of the displacement, still using snapping mode or, as above, use the advanced digitizing panel which would provide complementary distance and angle placement constraints to place the end point of the translation.

4. Click on the map canvas: the whole features are moved to new location.

Likewise, you can create a translated copy of the feature(s) using the ^{Copy and Move Feature(s)} tool.

Note

If no feature is selected when you first click on the map canvas with any of the Move Feature(s) or Copy and Move Feature(s) tools, then only the feature under the mouse is affected by the action. So, if you want to move several features, they should be selected first.

16.3.5.2. Rotate Feature(s)

Use the ^{Rotate Feature(s)} tool to rotate one or multiple features in the map canvas:

1. Press the ^{Rotate Feature(s)} icon

2. Then click on the feature to rotate. The feature’s centroid is referenced as rotation center, a preview of the rotated feature is displayed and a widget opens showing the current Rotation angle.

3. Click on the map canvas when you are satisfied with the new placement or manually enter the rotation angle in the text box. You can also use the Snap to ° box to constrain the rotation values.

4. If you want to rotate several features at once, they shall be selected first, and the rotation is by default around the centroid of their combined geometries.

You can also use an anchor point different from the default feature centroid: press the Ctrl button, click on the map canvas and that point will be used as the new rotation center.

If you hold Shift before clicking on the map, the rotation will be done in 45 degree steps, which can be modified afterwards in the user input widget.

To abort feature rotation, press the ESC button or click on the ^{Rotate Feature(s)} icon.

16.3.5.3. Scale Feature

The ^{Scale Feature} tool is similar to the Rotate feature. Though instead of performing a rotation of selected features, it rescales their geometry. The change is performed in relation to the anchor point and the scale ratio can be manually specified in the widget that appears in the upper corner of the canvas.

16.3.5.4. Simplify Feature

The ^{Simplify Feature} tool allows you to interactively reshape a line or polygon geometry by reducing or densifying the number of vertices, as long as the geometry remains valid:

1. Select the ^{Simplify Feature} tool.

2. Click on the feature or drag a rectangle over the features.

3. A dialog pops up allowing you to define the Method to apply, ie whether you would like to:

   • simplify the geometry, meaning less vertices than the original. Available methods are Simplify by distance, Simplify by snapping to grid or simplify by area (Visvalingam). You’d then need to indicate the value of Tolerance in Layer units, Pixels or map units to use for simplification. The higher the tolerance is the more vertices can be deleted.

   • or densify the geometries with new vertices thanks to the Smooth option: for each existing vertex, two vertices are placed on each of the segments originated from it, at an Offset distance representing the percentage of the segment length. You can also set the number of Iterations the placement would be processed: the more iterations, the more vertices and smoother is the feature.

   Settings that you used will be saved when leaving a project or an edit session. So you can go back to the same parameters the next time you simplify a feature.

4. A summary of the modifications that would apply is shown at the bottom of the dialog, listing number of features and number of vertices (before and after the operation and the ratio the change represents). Also, in the map canvas, the expected geometry is displayed over the existing one, using the rubberband color.

5. When the expected geometry fits your needs, click OK to apply the modification. Otherwise, to abort the operation, you can either press Cancel or right-click in the map canvas.

Note

Unlike the feature simplification option in Settings ► Options ► Rendering menu which simplifies the geometry just for rendering, the ^{Simplify Feature} tool permanently modifies feature’s geometry in data source.

16.3.5.5. Add Part

You can ^{Add Part} to a selected feature generating a multipoint, multiline or multipolygon feature. The new part must be digitized outside the existing one which should be selected beforehand.

The ^{Add Part} can also be used to add a geometry to a geometryless feature. First, select the feature in the attribute table and digitize the new geometry with the ^{Add Part} tool.

16.3.5.6. Delete Part

The ^{Delete Part} tool allows you to delete parts from multifeatures (e.g., to delete polygons from a multi-polygon feature). This tool works with all multi-part geometries: point, line and polygon. Furthermore, it can be used to totally remove the geometric component of a feature. To delete a part, simply click within the target part.

16.3.5.7. Add Ring

You can create ring polygons using the ^{Add Ring} icon in the toolbar. This means that inside an existing area, it is possible to digitize further polygons that will occur as a ‘hole’, so only the area between the boundaries of the outer and inner polygons remains as a ring polygon.

16.3.5.8. Fill Ring

The ^{Fill Ring} tool helps you create polygon feature that totally falls within another one without any overlapping area; that is the new feature covers a hole within the existing one. To create such a feature:

1. Select the ^{Fill Ring} tool.

2. Draw a new polygon over the existing feature: QGIS adds a ring to its geometry (like if you used the ^{Add Ring} tool) and creates a new feature whose geometry matches the ring (like if you traced over the interior boundaries with the ^{Add polygon feature} tool).

3. Or alternatively, if the ring already exists on the feature, place the mouse over the ring and left-click while pressing Shift: a new feature filling the hole is drawn at that place.

   The Feature Attributes form of the new feature opens, pre-filled with values of the “parent” feature and/or fields constraints.

16.3.5.9. Delete Ring

The ^{Delete Ring} tool allows you to delete rings within an existing polygon, by clicking inside the hole. This tool only works with polygon and multi-polygon features. It doesn’t change anything when it is used on the outer ring of the polygon.

16.3.5.10. Reshape Features

You can reshape line and polygon features using the ^{Reshape Features} tool on the toolbar. For lines, it replaces the line part from the first to the last intersection with the original line.

Fig. 16.87 Reshape line

Tip

Extend linestring geometries with reshape tool

Use the ^{Reshape Features} tool to extend existing linestring geometries: snap to the first or last vertex of the line and draw a new one. Validate and the feature’s geometry becomes the combination of the two lines.

For polygons, it will reshape the polygon’s boundary. For it to work, the reshape tool’s line must cross the polygon’s boundary at least twice. To draw the line, click on the map canvas to add vertexes. To finish it, just right-click. Like with the lines, only the segment between the first and the last intersections is considered. The reshape line’s segments that are inside the polygon will result in cropping it, where the ones outside the polygon will extend it.

Fig. 16.88 Reshape polygon

With polygons, reshaping can sometimes lead to unintended results. It is mainly useful to replace smaller parts of a polygon, not for major overhauls, and the reshape line is not allowed to cross several polygon rings, as this would generate an invalid polygon.

Note

The reshape tool may alter the starting position of a polygon ring or a closed line. So, the point that is represented ‘twice’ will not be the same any more. This may not be a problem for most applications, but it is something to consider.

16.3.5.11. Offset Curves

The ^{Offset Curve} tool creates parallel shifts of line layers. The tool can be applied to the edited layer (the geometries are modified) or also to background layers (in which case it creates copies of the lines / rings and adds them to the edited layer). It is thus ideally suited for the creation of distance line layers. The User Input dialog pops-up, showing the displacement distance and other settings.

To create a shift of a line or polygon layer, you must first go into editing mode and activate the ^{Offset Curve} tool. Then click on a feature to shift it. Move the mouse and click where wanted or enter the desired distance in the user input widget. Holding Ctrl during the 2nd click will make an offset copy. Your changes may then be saved with the ^{Save Layer Edits} tool.

For geometries on background layers make sure that snapping is on and hold Ctrl to select the geometry from the background. Also hold Ctrl when doing the second click. Geometries will be converted to the target layer geometry type.

QGIS options dialog (Digitizing tab then Curve offset tools section) or the icon in the User Input dialog allows you to configure some parameters like Join style, Quadrant segments, Miter limit and End cap style.

16.3.5.12. Reverse Line

Changing the direction of a line geometry can be useful for cartographical purposes or when preparing for network analysis.

To change a line direction:

1. Activate the reverse line tool by clicking ^{Reverse line}.

2. Click on the line. The direction of the line is reversed.

16.3.5.13. Split Features

Use the ^{Split Features} tool to split a feature into two or more new and independent features, ie. each geometry corresponding to a new row in the attribute table.

To split line or polygon features:

1. Select the ^{Split Features} tool.

2. Draw a line across the feature(s) you want to split. If a selection is active, only selected features are split. Fields of resulting features are filled according to their splitting policy.

3. You can then as usual modify any of the attributes of any resulting feature.

Tip

Split a polyline into new features in one-click

Using the ^{Split Features} tool, snap and click on an existing vertex of a polyline feature to split that feature into two new features.

16.3.5.14. Split parts

In QGIS it is possible to split the parts of a multi part feature so that the number of parts is increased. Just draw a line across the part you want to split using the ^{Split Parts} icon.

Tip

Split a polyline into new parts in one-click

Using the ^{Split Parts} tool, snap and click on an existing vertex of a polyline feature to split the feature into two new polylines belonging to the same feature.

16.3.5.15. Merge selected features

The ^{Merge Selected Features} tool allows you to create a new feature by merging existing ones: their geometries are merged to generate a new one. If features don’t have common boundaries, a multipolygon/multipolyline/multipoint feature is created.

1. First, select the features you’d like to combine.

2. Then press the ^{Merge Selected Features} button.

3. In the new dialog, the Merge line at the bottom of the table shows the attributes of the resulting feature. You can alter any of these values either by:

   • manually replacing the value in the corresponding cell;

   • selecting a row in the table and pressing Take attributes from selected feature to use the values of this initial feature;

   • pressing the Take attributes from the largest geometry to use the attributes from the longest line feature, the largest polygon, or the multipoints with the most parts;

   • pressing Skip all fields to use empty attributes;

   • expanding the drop down menu at the top of the table, select any of the above options to apply to the corresponding field only. There, you can also choose to aggregate the initial features attributes (Minimum, Maximum, Median, Sum, Count, Concatenation… depending on the type of the field. see Statistical Summary Panel for the full list of functions).

   Note

   If the layer has default values or clauses present on fields, these are used as the initial value for the merged feature.

4. Press OK to apply the modifications. A single (multi)feature is created in the layer, replacing the previously selected ones.

16.3.5.16. Merge attributes of selected features

The ^{Merge Attributes of Selected Features} tool allows you to apply same attributes to features without merging their boundaries. The dialog is the same as the Merge Selected Features tool’s except that unlike that tool, selected objects are kept with their geometry while some of their attributes are made identical.

16.3.5.17. Rotate Point Symbols

The ^{Rotate Point Symbols} allows you to individually change the rotation of point symbols in the map canvas.

1. First, you need to indicate the field to store the rotation value in. This is made by assigning a field to the symbol data-defined rotation property:

   1. In the Layer Properties ► Symbology dialog, browse to the symbol editor dialog.

   2. Click the Data-defined override widget near the Rotation option of the top Marker level (preferably) of the symbol layers.

   3. Choose a field in the Field Type combobox. Values of this field are hence used to rotate each feature’s symbol accordingly.

      You can also check the ^{Store data in project} entry to generate an auxiliary data storage field to control the rotation value.

   Note

   Make sure that the same field is assigned to all the symbol layers

   Setting the data-defined rotation field at the topmost level of the symbol tree automatically propagates it to all the symbol layers, a prerequisite to perform graphical symbol rotation with the Rotate Point Symbols tool. Indeed, if a symbol layer does not have the same field attached to its rotation property, the tool will not work.

   

   Fig. 16.89 Rotating a point symbol

2. Then click on a point symbol in the map canvas with the ^{Rotate Point Symbols} tool

3. Move the mouse around. A red arrow with the rotation value will be visualized (see Fig. 16.89). If you hold the Ctrl key while moving, the rotation will be done in 15 degree steps.

4. When you get the expected angle value, click again. The symbol is rendered with this new rotation and the associated field is updated accordingly.

   You can right-click to abort symbol rotation.

16.3.5.18. Offset Point Symbols

The ^{Offset Point Symbols} allows you to interactively change the rendered position of point symbols in the map canvas. This tool behaves like the ^{Rotate Point Symbols} tool except that it requires you to connect a field to the data-defined Offset (X,Y) property of each layer of the symbol. The field will then be populated with the offset coordinates for the features whose symbol is moved in the map canvas.

1. Associate a field to the data-defined widget of the Offset (X,Y) property of the symbol. If the symbol is made with many layers, you may want to assign the field to each of them

2. Select the ^{Offset Point Symbols} tool

3. Click a point symbol

4. Move to a new location

5. Click again. The symbol is moved to the new place. Offset values from the original position are stored in the linked field.

   You can right-click to abort symbol offset.

Note

The ^{Offset Point Symbols} tool doesn’t move the point feature itself; you should use the ^{Vertex Tool (Current Layer)} or ^{Move Feature} tool for this purpose.

16.3.5.19. Trim/Extend Feature

The ^Trim/Extend tool allows you to shorten or lengthen segments of a (multi)line or (multi)polygon geometry to converge with a selected segment (the cutting line). This results in a modified geometry with a vertex snapped to the target segment or in its prolongation. Depending on how the selected geometries are placed in relation to each other, the tool will either:

• Trim: removes parts of the line segment or polygon boundary, beyond the cutting line

• Extend: extends polygon boundaries or line segments so that they can snap to the cutting line.

In order to trim or extend existing geometries:

1. Enable appropriate snapping settings on segment for the involved layer(s)

2. Select the ^Trim/Extend tool

3. Click the target limit segment, i.e. the segment with respect to which you want to extend or trim another segment. It appears highlighted.

4. Move to the segment you want to trim or extend. It does not need to be the last segment of the geometry, but has to be on the active layer.

5. Hover over the segment, and QGIS displays a preview of what the feature’s geometry would be. If OK, click the segment. In the case of a trim, you must select the part that should be shortened.

6. When both segments are in 3D, the tool performs an interpolation on the limit segment to get the Z value.

Attention

Pay attention to the modified geometry while using the ^Trim/Extend tool. Depending on the inputs, it can create invalid geometries, potentially resulting in failure at layer saving.

16.3.6. Shape digitizing

The Shape Digitizing toolbar offers a set of tools to draw lines or polygons features of regular shape. It is synchronized with the ^{Digitize Shape} geometry drawing method you can select on the Digitizing Toolbar. To use it:

1. Display the toolbar: View ► Toolbars ► Shape Digitizing

2. Select a tool that creates or modifies the shape of a geometry, e.g. ^{Add line feature}, ^{Add polygon feature}, ^{Add part}, ^{Add ring}, ^{Reshape Features}, …

3. The ^{Digitize with segment} button on the Digitizing Toolbar is enabled. The first time, you may need to switch it to the ^{Digitize Shape} in order to enable tools on the Shape Digitizing toolbar.

4. Pick a shape digitizing tool and draw.

16.3.6.1. Circular string by radius

The ^{Circular string by radius} button allows to add line or polygon features with a circular geometry, given two nodes on the curve and a radius:

1. Left click twice to place the two points on the geometry.

2. A Radius widget in the top right corner of the map canvas displays current radius (corresponding to distance between the points). Edit that field to the value you want.

3. An overview of the arcs matching these constraints is displayed while moving around the cursor. Right-click to validate when the expected arc is shown.

4. Add a new point to start shaping another arc.

Note

Curved geometries are stored as such only in compatible data provider

Although QGIS allows to digitize curved geometries within any editable data format, you need to be using a data provider (e.g. PostGIS, memory layer, GML or WFS) that supports curves to have features stored as curved, otherwise QGIS segmentizes the circular arcs.

16.3.6.2. Draw Circles

There is a set of tools for drawing circles. The tools are described below.

Circles are converted into circular strings. Therefore, as explained in Circular string by radius, if allowed by the data provider, it will be saved as a curved geometry, if not, QGIS will segmentize the circular arcs.

• ^{Circle from 2 points}: The two points define the diameter and the orientation of the circle. (Left-click, right-click)

• ^{Circle from 3 points}: Draws a circle from three known points on the circle. (Left-click, left-click, right-click)

• ^{Circle by a center point and another point}: Draws a circle with a given center and a point on the circle (Left-click, right-click). When used with the The Advanced Digitizing panel this tool can become a “Add circle from center and radius” tool by setting and locking the distance value after first click.

• ^{Circle from 3 tangents}: Draws a circle that is tangential to three segments. Note that you must activate snapping to segments (See Setting the snapping tolerance and search radius). Click on a segment to add a tangent. If two tangents are parallel, the coordinates of the click on the first parallel tangent are used to determine the positioning of the circle. If three tangents are parallel, an error message appears and the input is cleared. (Left-click, left-click, right-click)

• ^{Circle from 2 tangents and a point}: Similar to circle from 3 tangents, except that you have to select two tangents, enter a radius and select the desired center.

16.3.6.3. Draw Ellipses

There is a set of tools for drawing ellipses. The tools are described below.

Ellipses cannot be converted as circular strings, so they will always be segmented.

• ^{Ellipse from center and two points}: Draws an ellipse with a given center, major axis and minor axis. (Left-click, left-click, right-click)

• ^{Ellipse from center and a point}: Draws an ellipse into a bounding box with the center and a corner. (Left-click, right-click)

• ^{Ellipse from extent}: Draws an ellipse into a bounding box with two opposite corners. (Left-click, right-click)

• ^{Ellipse from foci}: Draws an ellipse by 2 points for foci and a point on the ellipse. (Left-click, left-click, right-click)

16.3.6.4. Draw Rectangles

There is a set of tools for drawing rectangles. The tools are described below.

• ^{Rectangle from center and a point}: Draws a rectangle from the center and a corner. (Left-click, right-click)

• ^{Rectangle from extent}: Draws a rectangle from two opposite corners. (Left-click, right-click)

• ^{Rectangle from 3 points (distance)}: Draws an oriented rectangle from three points. The first and second points determine the length and angle of the first edge. The third point determines the length of the other edge. One can use The Advanced Digitizing panel to set the length of the edges. (Left-click, left-click, right-click)

• ^{Rectangle from 3 points (projected)}: Same as the preceding tool, but the length of the second edge is computed from the projection of the third point on the first edge. (Left-click, left-click, right-click)

  

  Fig. 16.90 Draw rectangle from 3 points using distance (right) and projected (left)

16.3.6.5. Draw Regular Polygons

There is a set of tools for drawing regular polygons. The tools are described below. Left-click to place the first point. A dialog appears, where you can set the number of polygon edges. Right-click to finish the regular polygon.

• ^{Regular polygon from two points}: Draws a regular polygon where the two points determine the length and angle of the first edge.

• ^{Regular polygon from center and a point}: Draws a regular polygon from the provided center point. The second point determines the angle and distance to the middle of an edge.

• ^{Regular polygon from center and a corner}: Same as the preceding tool, but the second point determines the angle and distante to a vertex.

16.3.7. The Advanced Digitizing panel

When capturing, reshaping, splitting new or existing geometries you also have the possibility to use the Advanced Digitizing panel. You can digitize lines exactly parallel or perpendicular to a particular angle or lock lines to specific angles. Furthermore, you can make a precise definition of your new geometry by entering X and Y coordinates as well as Z for 3D features, or M values.

Fig. 16.91 The Advanced Digitizing panel

The Advanced Digitizing panel can be opened either with a right-click on the toolbar, from View ► Panels ► menu or pressing Ctrl+4. Once the panel is visible, click the ^{Enable advanced digitizing tools} button to activate the set of tools.

Note

The tools are not enabled if the map view is in geographic coordinates.

The aim of the Advanced Digitizing tool is to lock coordinates, lengths, and angles when moving the mouse during the digitalizing in the map canvas.

You can also create constraints with relative or absolute reference. Relative reference means that the next vertex constraints’ values will be relative to the previous vertex or segment.

16.3.7.1. The toolbar

At the top of the Digitizing panel, you find the following buttons:

• ^{Enable advanced digitizing tools}

• ^{Construction mode}: allows to capture the clicks’ positions to reuse as reference points to lock distance, angle, X, Y, Z or M relative values. More details at Construction mode.

• ^Parallel to draw a line parallel to an existing one (more at Parallel and perpendicular lines)

• ^{Perpendicular} to draw a line perpendicular to an existing one (more at Parallel and perpendicular lines)

• ^{Snap to common angles}: when moving the cursor, displays a virtual line that you can snap to to add the next vertex. The snapping line is defined by the last added vertex and an (absolute or relative to previous segment) angle from a preset list (following steps of 0.1°, 0.5°, 1°, 5°, 10°, 15°, 18°, 22.5°, 30°, 45° or 90°). Choose Do not snap to common angles to disable this feature.

  Snapping to features can be used along with snapping to common angles for accurate digitizing. For a fine-grained control on how the target element to snap to is retained, you can indicate whether to prioritize snapping to features over common angles, and vice-versa under the Snapping priority entry. You can switch from one method to the other during the digitizing operation, and this avoids disabling any of the snapping options in the meantime. Press N (or Shift+N) during a digitizing operation to cycle through the angles list.

• ^{Floater settings}: if the Show floater item is checked, a contextual menu with digitizing information follows the cursor during digitizing. The values can be accessed using the panel’s shortcuts, edited and ^Locked after validation (pressing Enter). The type of information to display can be selected in the bottom part of the menu:

  • Show distance

  • Show angle

  • Show XY coordinates

  • Show Z value

  • Show M value

  • Show bearing/azimuth

  • Show common snapping angle

• ^{Construction Tools} provides a couple of options that constrain the vertices placement based on extrapolated coordinates of existing elements:

  • Line Extension: hover over a segment and you get a purple dotted line extending the segment across the map canvas. You can snap the vertex anywhere on this virtual line.

  • X/Y Point: hover over a vertex and you get a purple dotted line along its X or Y coordinate, across the map canvas. You can snap the vertex anywhere on this virtual line. It is even possible to hover over two different vertices, generating virtual coordinate lines for both, and snap to their intersection.

Below the toolbar, you will find a number of text boxes whose value reflects by default the position or movement of the cursor in the map canvas. Editing these values helps you constrain the position of the items you edit:

• d for the distance from a reference position, usually the last edited vertex

• a for the angle (absolute or relative) from a reference position, usually the last edited segment

• x for the X coordinate of the pointer

• y for the Y coordinate of the pointer

• z for the default Z value or the Z coordinate of the vertex or segment under the pointer

• m for the default M value or the M value of the vertex or segment under the pointer

16.3.7.2. Keyboard shortcuts

To speed up the use of Advanced Digitizing Panel, there are a couple of keyboard shortcuts available:

Table 16.5 Keyboard shortcuts of the Advanced Digitizing Panel tools

Key	Simple	Ctrl+ or Alt+	Shift+
D	Set distance	Lock distance
A	Set angle	Lock angle	Toggle relative angle to last segment
X	Set X coordinate	Lock X coordinate	Toggle relative X to last vertex
Y	Set Y coordinate	Lock Y coordinate	Toggle relative Y to last vertex
Z	Set Z coordinate	Lock Z coordinate	Toggle relative Z to last vertex
M	Set M value	Lock M value	Toggle relative M to last vertex
C	Toggle construction mode
P	Toggle perpendicular and parallel modes

Note

Z coordinate and M value options are available only if compatible with the layer geometry dimension.

16.3.7.3. Absolute reference digitizing

When drawing a new geometry from scratch, it is very useful to have the possibility to start digitizing vertexes at given coordinates.

For example, to add a new feature to a polygonal layer, click the button. You can enter the exact coordinates where you want to start editing the feature, i.e.:

1. Click the x text box (or use the X keyboard shortcut).

2. Type the X coordinate value you want and press Enter or click the button to their right to lock the mouse to the X axis on the map canvas.

3. Click the y text box (or use the Y keyboard shortcut).

4. Type the Y coordinate value you want and press Enter or click the button to their right to lock the mouse to the Y axis on the map canvas.

5. If the layer has Z coordinate or M values, the corresponding z or m widget is enabled and displays its default value, as set in Settings ► Options ► Digitizing tab.

   1. Click the z or m text box (or use respectively the Z or M keyboard shortcut).

   2. Type the coordinate value you want and press Enter or click the button to their right to lock the value in the widget.

   Note

   Read Rules of Z coordinate or M value assignment for details on how Z coordinate and M values are automatically determined from existing features.

6. Two blue dotted lines and a green cross identify the exact coordinates you entered. Click on the map canvas to add a vertex at the green cross position.

   

   Fig. 16.92 Start drawing at given coordinates

7. You can proceed as above, adding a new set of coordinates for the next vertex, or switch to another mode of digitizing (e.g. segment, curve or stream).

8. If you want to draw a segment of a given length:

   1. Click the d (distance) text box (keyboard shortcut D)

   2. Type the distance value (in map units)

   3. Press Enter or click the button on the right to lock the mouse in the map canvas to the length of the segment. In the map canvas, the latest vertex is surrounded by a circle whose radius is the value entered in the distance text box. A cross on the circle shows the position of the next vertex if you click.

   

   Fig. 16.93 Fixed length segment

9. You can also constrain the vertex position, setting the angle of the segment. As described before:

   1. Click the a (angle) text box (keyboard shortcut A)

   2. Type the angle value (in degrees)

   3. Press Enter or click the button on the right to lock it. A line going through the latest vertex and rotated based on the set angle appears in the map canvas and a cross on it shows the next vertex position if you click.

   

   Fig. 16.94 Fixed angle segment

Hint

Pressing Ctrl+<key> or Alt+<key> automatically locks the target property and puts its value into edit. Modify, press Enter and you are done. Combined with the ^{Toggle floater}, this can be a real time saver, with keyboard digitizing.

16.3.7.4. Relative reference digitizing

Instead of using absolute values of angles or coordinates, you can also use values relative to the last digitized vertex or segment.

For angles, you can click the button on the left of the a text box (or press Shift+A) to toggle relative angles to the previous segment. With that option on, angles are measured between the last segment and the mouse pointer.

For coordinates, click the buttons to the left of the x, y, z or m text boxes (or press Shift+<key>) to toggle relative coordinates to the previous vertex. With these options on, coordinates measurement will consider the last vertex to be the origin of the set coordinates.

16.3.7.5. Continuous lock

Both in absolute or relative reference digitizing, angle, distance, X, Y, Z and M constraints can be locked continuously by clicking the Continuous lock buttons. Using continuous lock allows you to digitize several points or vertexes using the same constraints.

16.3.7.6. Parallel and perpendicular lines

All the tools described above can be combined with the ^{Perpendicular} and ^Parallel tools. These two tools allow drawing segments perfectly perpendicular or parallel to another segment. The target segment can be on another layer, another feature within the layer or the feature being digitized (requires self-snapping option).

To draw a perpendicular segment:

1. First add one of the segment vertices.

2. Click the ^{Perpendicular} icon (keyboard shortcut P) to activate it.

3. Click on the segment that you want to be perpendicular to.

4. A virtual dotted line perpendicular to the segment through the previous vertex appears. The angle property is locked, constraining the next vertex on that line and, a cross indicates the projected position of the cursor on the line. Click to place the new vertex.

   

   Fig. 16.95 Perpendicular digitizing

To draw a parallel segment, the steps are the same except that you need to click on the ^Parallel icon (keyboard shortcut P twice).

Fig. 16.96 Parallel digitizing

These two tools just find the right angle of the perpendicular and parallel angle and lock this parameter during your editing. Unlock the angle parameter to cancel their use in the middle of the process.

16.3.7.7. Construction mode

You can enable and disable construction mode by clicking on the ^{Construction mode} icon or with the C keyboard shortcut. While in construction mode, clicking the map canvas won’t add new vertexes, but will capture the clicks’ positions so that you can use them as reference points to then lock distance, angle or X, Y, Z, M relative values.

As an example, the construction mode can be used to draw some point at an exact distance from an existing point.

With an existing point in the map canvas and the snapping mode correctly activated, you can easily draw other points at given distances and angles from it. In addition to the button, you have to activate also the construction mode by clicking the ^{Construction mode} icon or with the C keyboard shortcut.

Click next to the point from which you want to calculate the distance and click on the d box (D shortcut) type the desired distance and press Enter to lock the mouse position in the map canvas:

Fig. 16.97 Distance from point

Before adding the new point, press C to exit the construction mode. Now, you can click on the map canvas, and the point will be placed at the distance entered.

You can also use the angle constraint to, for example, create another point at the same distance of the original one, but at a particular angle from the newly added point. Click the ^{Construction mode} icon or with the C keyboard shortcut to enter construction mode. Click the recently added point, and then the other one to set a direction segment. Then, click on the d text box (D shortcut) type the desired distance and press Enter. Click the a text box (A shortcut) type the angle you want and press Enter. The mouse position will be locked both in distance and angle.

Fig. 16.98 Distance and angle from points

Before adding the new point, press C to exit the construction mode. Now, you can click on the map canvas, and the point will be placed at the distance and angle entered. Repeating the process, several points can be added.

Fig. 16.99 Points at given distance and angle

16.3.8. The Processing in-place layer modifier

The Processing menu provides access to a large set of tools to analyze and create new features based on the properties of the input features or their relations with other features (within the same layer or not). While the common behavior is to create new layers as outputs, some algorithms also allow modifications to the input layer. This is a handy way to automate multiple features modification using advanced and complex operations.

To edit features in-place:

1. Select the layer to edit in the Layers panel.

2. Select the concerned features. You can skip this step, in which case the modification will apply to the whole layer.

3. Press the ^{Edit Features In-Place} button at the top of the Processing toolbox. The list of algorithms is filtered, showing only those compatible with in-place modifications, i.e.:

   • They work at the feature source and not at the layer level.

   • They do not change the layer structure, e.g. adding or removing fields.

   • They do not change the geometry type, e.g. from line to point layer.

   

   Fig. 16.100 Processing algorithms: all (left) vs polygon in-place editors (right)

4. Find the algorithm you’d like to run and double-click it.

   Note

   If the algorithm does not need any additional user-set parameters (excluding the usual input and output layer parameters), then the algorithm is run immediately without any dialog popup.

   1. If parameters other than the usual input or output layers are needed, the algorithm dialog pops up. Fill in the required information.

   2. Click Modify Selected Features or Modify All Features depending on whether there’s an active selection.

   Changes are applied to the layer and placed in the edit buffer: the layer is indeed toggled to editing mode with unsaved modification as indicated by the icon next to the layer name.

5. As usual, press ^{Save layer edits} to commit the changes in the layer. You can also press ^Undo to rollback the whole modification.