Outdated version of the documentation. Find the latest one here.

` `

Creando capaa

Layers can be created in many ways, including:

  • empty layers from scratch;
  • layers from existing layers;
  • layers from the clipboard;
  • layers as a result of an SQL-like query based on one or many layers: the virtual layer.

QGIS also provides tools to import/export different formats.

Creating new vector layers

QGIS allows you to create new Shapefile layers, new SpatiaLite layers, new GPX layers and new Temporary Scratch layers. Creation of a new GRASS layer is supported within the GRASS plugin. (Please refer to section Crear una nueva capa vectorial GRASS for more information on creating GRASS vector layers.)

Crear una nueva capa de archivo shape

To create a new Shapefile layer, choose Create Layer ‣ newVectorLayer New Shapefile Layer... from the Layer menu. The New Shapefile Layer dialog will be displayed as shown in figure_create_shapefile. Choose the type of layer (point, line or polygon) and the CRS (coordinate reference system).

Note that QGIS does not yet support creation of 2.5D features (i.e., features with X,Y,Z coordinates).

../../../_images/editNewVector.png

Creating a new Shapefile layer dialog

To complete the creation of the new Shapefile layer, add the desired attributes by specifying a name and type for each attribute and clicking on the [Add to fields list] button. A first ‘id’ column is added by default but can be removed, if not wanted. Only Decimal number selectString, Whole number selectString, Text data selectString and Date selectString attributes are supported. Additionally, depending on the attribute type, you can also define the length and precision of the new attribute column. Once you are happy with the attributes, click [OK] and provide a name for the Shapefile. QGIS will automatically add the .shp extension to the name you specify. Once the Shapefile has been created, it will be added to the map as a new layer, and you can edit it in the same way as described in section Digitalizando una capa existente.

Crear una nueva capa SpatiaLite

To create a new SpatiaLite layer for editing, choose New ‣ newSpatiaLiteLayer New SpatiaLite Layer... from the Layer menu. The New SpatiaLite Layer dialog will be displayed as shown in Figure_create_spatialite.

../../../_images/editNewSpatialite.png

Creating a New SpatiaLite layer dialog

The first step is to select an existing SpatiaLite database or to create a new SpatiaLite database. This can be done with the browse button browseButton to the right of the database field. Then, add a name for the new layer, define the layer type, and specify the coordinate reference system with [Specify CRS]. If desired, you can select checkbox Create an autoincrementing primary key.

To define an attribute table for the new SpatiaLite layer, add the names of the attribute columns you want to create with the corresponding column type, and click on the [Add to attribute list] button. Once you are happy with the attributes, click [OK]. QGIS will automatically add the new layer to the legend, and you can edit it in the same way as described in section Digitalizando una capa existente.

Further management of SpatiaLite layers can be done with the DB Manager. See Complemento administrador de BBDD.

Creating a new GeoPackage layer

To create a new GeoPackage layer go to Layer ‣ New ‣ newGeoPackageLayer New GeoPackage Layer.... The New GeoPackage Layer dialog will be displayed as shown in figure_create_geopackage.

../../../_images/editNewGeoPackage.png

Creating a New GeoPackage layer dialog

The first step is to select an existing GeoPackage or create a new one. This can be done by pressing the ellipses [...] button at the right of the Database field. Then, give a name for the new layer, define the layer type and specify the coordinate reference system with [Specify CRS].

To define an attribute table for the new GeoPackage layer, add the names of the attribute columns you want to create with the corresponding column type, and click on the [Add to fields list] button. Once you are happy with the attributes, click [OK]. QGIS will automatically add the new layer to the legend, and you can edit it in the same way as described in section Digitalizando una capa existente.

Crear una nueva capa GPX

To create a new GPX file, you need to load the GPS plugin first. Plugins ‣ showPluginManager Plugin Manager... opens the Plugin Manager Dialog. Activate the checkbox GPS Tools checkbox.

When this plugin is loaded, choose New ‣ createGPX Create new GPX Layer... from the Layer menu. In the Save new GPX file as dialog, choose where to save the new file and press [Save]. Three new layers are added to the Layers Panel: waypoints, routes and tracks with predefined structure.

Crear una nueva capa temporal

Temporary Scratch Layers are in-memory layers, meaning that they are not saved on disk and will be discarded when QGIS is closed. They can be handy to store features you temporarily need or as intermediate layers during geoprocessing operations.

Empty, editable temporary scratch layers can be defined using Layer ‣ Create Layer ‣ New Temporary Scratch Layer. Here you can create radioButtonOffMultipoint, radioButtonOffMultiline and radioButtonOffMultipolygon Layers beneath radioButtonOnPoint, radioButtonOffLine and radioButtonOffPolygon layers.

You can also create Temporary Scratch Layers from the clipboard. See Creating new layers from the clipboard.

Creating new layers from an existing layer

Both raster and vector layers can be saved in a different format and/or reprojected to a different coordinate reference system (CRS) using the Save As... function in the layer context menu (by right-clicking in the layer in the layer tree) or in the Layer ‣ Save As... menu.

Common parameters

The Save As dialog shows several parameters to change the behavior when saving the layer. Common parameters for raster and vector are:

  • Format
  • File name
  • CRS can be changed to reproject the data
  • Add saved file to map to add the new layer to the canvas
  • Extent (possible values are layer, Map view or user-defined extent)

However, some parameters are specific to raster and vector formats:

Raster specific parameters

  • Output mode (it can be raw data or rendered image)
  • Resolution
  • Create Options: advanced options (file compression, block sizes, colorimetry...) to fine tune the output file. See the gdal-ogr driver documentation.
  • Pyramids creation
  • VRT Tiles
  • No data values
../../../_images/saveasraster.png

Saving as a new raster layer

Vector specific parameters

Depending on the format of export, some of these options are available or not:

  • Encoding
  • Save only selected features
  • Select fields to export and their export options. In case you set your fields behavior with some Edit widgets, e.g. value map, you can keep the displayed values in the layer by checking checkbox Replace all selected raw fields values by displayed values.
  • Symbology export: can be used mainly for DXF export and for all file formats who manage OGR feature styles (see note below) as DXF, KML, tab file formats:
    • No symbology: default style of the application that reads the data
    • Feature symbology: save style with OGR Feature Styles (see note below)
    • Symbol Layer symbology: save with OGR Feature Styles (see note below) but export the same geometry multiple times if there are multiple symbology symbol layers used
    • A Scale value can be applied to the latest options.

Nota

OGR Feature Styles are a way to store style directly in the data as a hidden attribute. Only some formats can handle this kind of information. KML, DXF and TAB file formats are such formats. For advanced users, you can read the OGR Feature Styles specification document.

  • Geometry: you can configure the geometry capabilities of the output layer
    • geometry type: keep the original geometry of the features when set to Automatic, otherwise removes or overrides it with any type. You can add an empty geometry column to an attribute table, remove the geometry column of a spatial layer.
    • Force multi-type: force creation of multi-geometry features in the layer
    • Include z-dimension to geometries.

Truco

Overriding layer geometry type makes it possible to do things like save a geometryless table (e.g. .csv file) into a shapefile WITH any type of geometry (point, line, polygon), so that geometries can then be manually added to rows with the addPart Add Part tool .

  • Datasources Options, Layer Options or Custom Options which allow you to configure some advanced parameters. See the gdal-ogr driver documentation.
../../../_images/saveasvector.png

Saving as a new vector layer

When saving a vector layer into an existing file, depending on the capabilities of the output format (Geopackage, SpatiaLite, FileGDB...), the user can decide whether to:

  • overwrite the whole file
  • overwrite only the target layer (the layer name is configurable)
  • append features to the existing target layer
  • append features, add new fields if there are any.

For formats like ESRI Shapefile, MapInfo .tab, feature append is also available.

Creating new DXF files

Besides the Save As... dialog which provides options to export a single layer to another format, including *.DXF, QGIS provides another tool to export multiple layers as a single DXF layers. It’s accessible in the Project ‣ DXF Export... menu.

The DXF Export dialog allows the user to:

  • indicate the destination layer file;

  • choose the symbology mode and scale (see the OGR Feature Styles note);

  • select the encoding and CRS;

  • check the loaded layers to include in the DXF files or pick them from an existing visibility preset.

    For each layer, you can choose a field whose values are used to split features in generated destination layers in the DXF output. You can also choose to checkbox Use the layer title as name if set and keep features grouped.

  • choose to only Export features intersecting the current map extent.

Creating new layers from the clipboard

Features that are on the clipboard can be pasted into a new layer. To do this, Select some features, copy them to the clipboard, and then paste them into a new layer using Edit ‣ Paste Features as ‣ and choosing:

  • New Vector Layer...: you need to select the layer CRS, poping up the Save vector layer as... dialog from which you can select any supported data format (see Creating new layers from an existing layer for parameters);
  • or Temporary Scratch Layer...: you need to select the layer CRS and give a name.

A new layer, filled with selected features and their attributes is created and added to map canvas if asked.

Nota

Creating layers from clipboard applies to features selected and copied within QGIS and also to features from another source defined using well-known text (WKT).

Creating virtual layers

Virtual layers are a special kind of vector layer. They allow you to define a layer as the result of an SQL query involving any number of other vector layers that QGIS is able to open. Virtual layers do not carry data by themselves and can be seen as views to other layers.

To create a virtual layer, open the virtual layer creation dialog by clicking on Add Virtual Layer in the Layer menu or from the corresponding toolbar.

The dialog allows you to specify a Layer name and an SQL Query. The query can use the name (or id) of loaded vector layers as tables, as well as their field names as columns.

For example, if you have a layer called airports, you can create a new virtual layer called public_airports with an SQL query like:

SELECT *
FROM airports
WHERE USE = "Civilian/Public"

The SQL query will be executed, regardless of the underlying provider of the airports layer, even if this provider does not directly support SQL queries.

../../../_images/create_virtual_layers.png

Create virtual layers dialog

Joins and complex queries can also be created, for example, to join airports and country information:

SELECT airports.*, country.population
FROM airports
JOIN country
ON airports.country = country.name

Nota

It’s also possible to create virtual layers using the SQL window of Complemento administrador de BBDD.

Embedding layers for use in queries

Besides the vector layers available in the map canvas, the user can add layers to the Embedded layers list, which he can use in queries without the need to have them showing in the map canvas or Layers panel.

To embed a layer, click Add and provide the Local name, Provider, Encoding and the path to the Source.

The Import button allows adding layers loaded in the map canvas into the Embedded layers list. This allows to later remove those layers from the Layers panel without breaking any existent query.

Supported query language

The underlying engine uses SQLite and SpatiaLite to operate.

It means you can use all of the SQL your local installation of SQLite understands.

Functions from SQLite and spatial functions from SpatiaLite can also be used in a virtual layer query. For instance, creating a point layer out of an attribute-only layer can be done with a query similar to:

SELECT id, MakePoint(x, y, 4326) as geometry
FROM coordinates

Functions of QGIS expressions can also be used in a virtual layer query.

To refer the geometry column of a layer, use the name geometry.

Contrary to a pure SQL query, all the fields of a virtual layer query must be named. Don’t forget to use the as keyword to name your columns if they are the result of a computation or function call.

Problemas de Rendimiento

With default parameters set, the virtual layer engine will try its best to detect the type of the different columns of the query, including the type of the geometry column if one is present.

This is done by introspecting the query when possible or by fetching the first row of the query (LIMIT 1) at last resort. Fetching the first row of the result just to create the layer may be undesirable for performance reasons.

The creation dialog allows to specify different parameters:

  • Unique identifier column: this option allows specifying which field of the query represents unique integer values that QGIS can use as row identifiers. By default, an autoincrementing integer value is used. Defining a unique identifier column allows to speed up the selection of rows by id.
  • No geometry: this option forces the virtual layer to ignore any geometry field. The resulting layer is an attribute-only layer.
  • Geometry Column: this option allows to specify the name of the column that is to be used as the geometry of the layer.
  • Geometry Type: this option allows to specify the type of the geometry of the virtual layer.
  • Geometry CRS: this option allows to specify the coordinate reference system of the virtual layer.

Special comments

The virtual layer engine tries to determine the type of each column of the query. If it fails, the first row of the query is fetched to determine column types.

The type of a particular column can be specified directly in the query by using some special comments.

The syntax is the following: /*:type*/. It has to be placed just after the name of a column. type can be either int for integers, real for floating point numbers or text.

For instance:

SELECT id+1 as nid /*:int*/
FROM table

The type and coordinate reference system of the geometry column can also be set thanks to special comments with the following syntax /*:gtype:srid*/ where gtype is the geometry type (point, linestring, polygon, multipoint, multilinestring or multipolygon) and srid an integer representing the EPSG code of a coordinate reference system.

Use of indexes

When requesting a layer through a virtual layer, indexes of this source layer will be used in the following ways:

  • if an = predicate is used on the primary key column of the layer, the underlying data provider will be asked for a particular id (FilterFid)
  • for any other predicates (>, <=, !=, etc.) or on a column without a primary key, a request built from an expression will be used to request the underlying vector data provider. It means indexes may be used on database providers if they exist.

A specific syntax exists to handle spatial predicates in requests and triggers the use of a spatial index: a hidden column named _search_frame_ exists for each virtual layer. This column can be compared for equality to a bounding box. Example:

SELECT *
FROM vtab
WHERE _search_frame_=BuildMbr(-2.10,49.38,-1.3,49.99,4326)

Spatial binary predicates like ST_Intersects are significantly sped up when used in conjunction with this spatial index syntax.