23.1.17. Vector selection

23.1.17.1. Extract by attribute

Creates two vector layers from an input layer: one will contain only matching features while the second will contain all the non-matching features.

The criteria for adding features to the resulting layer is based on the values of an attribute from the input layer.

23.1.17.1.1. Parameters

Label

Name

Type

Description

Input layer

INPUT

[vector: any]

Layer to extract features from.

Selection attribute

FIELD

[tablefield: any]

Filtering field of the layer

Operator

OPERATOR

[enumeration]

Default: 0

Many different operators are available:

  • 0 — =

  • 1 — ≠

  • 2 — >

  • 3 — >=

  • 4 — <

  • 5 — <=

  • 6 — begins with

  • 7 — contains

  • 8 — is null

  • 9 — is not null

  • 10 — does not contain

Value

Optional

VALUE

[string]

Value to be evaluated

Extracted (attribute)

OUTPUT

[same as input]

Default: [Create Temporary Layer]

Specify the output vector layer for matching features. One of:

  • Create Temporary Layer (TEMPORARY_OUTPUT)

  • Save to File…

  • Save to Geopackage…

  • Save to PostGIS Table…

The file encoding can also be changed here.

Extracted (non-matching)

FAIL_OUTPUT

[same as input]

Default: [Skip output]

Specify the output vector layer for non-matching features. One of:

  • Skip Output

  • Create Temporary Layer (TEMPORARY_OUTPUT)

  • Save to File…

  • Save to Geopackage…

  • Save to PostGIS Table…

23.1.17.1.2. Outputs

Label

Name

Type

Description

Extracted (attribute)

OUTPUT

[same as input]

Vector layer with matching features from the input layer

Extracted (non-matching)

FAIL_OUTPUT

[same as input]

Vector layer with non-matching features from the input layer

23.1.17.1.3. Python code

Algorithm ID: qgis:extractbyattribute

import processing
processing.run("algorithm_id", {parameter_dictionary})

The algorithm id is displayed when you hover over the algorithm in the Processing Toolbox. The parameter dictionary provides the parameter NAMEs and values. See Using processing algorithms from the console for details on how to run processing algorithms from the Python console.

23.1.17.2. Extract by expression

Creates two vector layers from an input layer: one will contain only matching features while the second will contain all the non-matching features.

The criteria for adding features to the resulting layer is based on a QGIS expression. For more information about expressions see the Expressions.

23.1.17.2.1. Parameters

Label

Name

Type

Description

Input layer

INPUT

[vector: any]

Input vector layer

Expression

EXPRESSION

[expression]

Expression to filter the vector layer

Matching features

OUTPUT

[same as input]

Default: [Create Temporary Layer]

Specify the output vector layer for matching features. One of:

  • Create Temporary Layer (TEMPORARY_OUTPUT)

  • Save to File…

  • Save to Geopackage…

  • Save to PostGIS Table…

The file encoding can also be changed here.

Non-matching

FAIL_OUTPUT

[same as input]

Default: [Skip output]

Specify the output vector layer for non-matching features. One of:

  • Skip Output

  • Create Temporary Layer (TEMPORARY_OUTPUT)

  • Save to File…

  • Save to Geopackage…

  • Save to PostGIS Table…

23.1.17.2.2. Outputs

Label

Name

Type

Description

Matching features

OUTPUT

[same as input]

Vector layer with matching features from the input layer

Non-matching

FAIL_OUTPUT

[same as input]

Vector layer with non-matching features from the input layer

23.1.17.2.3. Python code

Algorithm ID: qgis:extractbyexpression

import processing
processing.run("algorithm_id", {parameter_dictionary})

The algorithm id is displayed when you hover over the algorithm in the Processing Toolbox. The parameter dictionary provides the parameter NAMEs and values. See Using processing algorithms from the console for details on how to run processing algorithms from the Python console.

23.1.17.3. Extract by location

Creates a new vector layer that only contains matching features from an input layer.

The criteria for adding features to the resulting layer is based on the spatial relationship between each feature and the features in an additional layer.

../../../../_images/selectbylocation.png

Fig. 23.93 In this example, the dataset from which we want to select (the source vector layer) consists of the green circles, the orange rectangle is the dataset that it is being compared to (the intersection vector layer).

Available geometric predicates are:

Intersect

Tests whether a geometry intersects another. Returns 1 (true) if the geometries spatially intersect (share any portion of space - overlap or touch) and 0 if they don’t. In the picture above, this will select circles 1, 2 and 3.

Contain

Returns 1 (true) if and only if no points of b lie in the exterior of a, and at least one point of the interior of b lies in the interior of a. In the picture, no circle is selected, but the rectangle would be if you would select it the other way around, as it contains a circle completely. This is the opposite of are within.

Disjoint

Returns 1 (true) if the geometries do not share any portion of space (no overlap, not touching). Only circle 4 is selected.

Equal

Returns 1 (true) if and only if geometries are exactly the same. No circles will be selected.

Touch

Tests whether a geometry touches another. Returns 1 (true) if the geometries have at least one point in common, but their interiors do not intersect. Only circle 3 is selected.

Overlap

Tests whether a geometry overlaps another. Returns 1 (true) if the geometries share space, are of the same dimension, but are not completely contained by each other. Only circle 2 is selected.

Are within

Tests whether a geometry is within another. Returns 1 (true) if geometry a is completely inside geometry b. Only circle 1 is selected.

Cross

Returns 1 (true) if the supplied geometries have some, but not all, interior points in common and the actual crossing is of a lower dimension than the highest supplied geometry. For example, a line crossing a polygon will cross as a line (selected). Two lines crossing will cross as a point (selected). Two polygons cross as a polygon (not selected).

23.1.17.3.1. Parameters

Label

Name

Type

Description

Extract features from

INPUT

[vector: any]

Input vector layer

Where the features (geometric predicate)

PREDICATE

[enumeration] [list]

Default: [0]

Spatial condition for the selection. One or more of:

  • 0 — intersect

  • 1 — contain

  • 2 — disjoint

  • 3 — equal

  • 4 — touch

  • 5 — overlap

  • 6 — are within

  • 7 — cross

If more than one condition is chosen, at least one of them (OR operation) has to be met for a feature to be extracted.

By comparing to the features from

INTERSECT

[vector: any]

Intersection vector layer

Extracted (location)

OUTPUT

[same as input]

Default: [Create temporary layer]

Specify the output vector layer for the features that have the chosen spatial relationship(s) with one or more features in the comparison layer. One of:

  • Create Temporary Layer (TEMPORARY_OUTPUT)

  • Save to File…

  • Save to Geopackage…

  • Save to PostGIS Table…

23.1.17.3.2. Outputs

Label

Name

Type

Description

Extracted (location)

OUTPUT

[same as input]

Vector layer with features from the input layer that have the chosen spatial relationship(s) with one or more features in the comparison layer.

23.1.17.3.3. Python code

Algorithm ID: qgis:extractbylocation

import processing
processing.run("algorithm_id", {parameter_dictionary})

The algorithm id is displayed when you hover over the algorithm in the Processing Toolbox. The parameter dictionary provides the parameter NAMEs and values. See Using processing algorithms from the console for details on how to run processing algorithms from the Python console.

23.1.17.4. Random extract

Takes a vector layer and generates a new one that contains only a subset of the features in the input layer.

The subset is defined randomly, based on feature IDs, using a percentage or count value to define the total number of features in the subset.

See also

Random selection

23.1.17.4.1. Parameters

Label

Name

Type

Description

Input layer

INPUT

[vector: any]

Source vector layer to select the features from

Method

METHOD

[enumeration]

Default: 0

Random selection methods. One of:

  • 0 — Number of selected features

  • 1 — Percentage of selected features

Number/percentage of selected features

NUMBER

[number]

Default: 10

Number or percentage of features to select

Extracted (random)

OUTPUT

[vector: any]

Default: [Create temporary layer]

Specify the output vector layer for the randomly selected features. One of:

  • Create Temporary Layer (TEMPORARY_OUTPUT)

  • Save to File…

  • Save to Geopackage…

  • Save to PostGIS Table…

Vector layer containing randomly selected features

23.1.17.4.2. Outputs

Label

Name

Type

Description

Extracted (random)

OUTPUT

[same as input]

Vector layer containing randomly selected features from the input layer

23.1.17.4.3. Python code

Algorithm ID: qgis:randomextract

import processing
processing.run("algorithm_id", {parameter_dictionary})

The algorithm id is displayed when you hover over the algorithm in the Processing Toolbox. The parameter dictionary provides the parameter NAMEs and values. See Using processing algorithms from the console for details on how to run processing algorithms from the Python console.

23.1.17.5. Random extract within subsets

Takes a vector layer and generates a new one that contains only a subset of the features in the input layer.

The subset is defined randomly, based on feature IDs, using a percentage or count value to define the total number of features in the subset. The percentage/count value is not applied to the whole layer, but instead to each category. Categories are defined according to a given attribute.

23.1.17.5.1. Parameters

Label

Name

Type

Description

Input layer

INPUT

[vector: any]

Vector layer to select the features from

ID field

FIELD

[tablefield: any]

Category of the source vector layer to select the features from

Method

METHOD

[enumeration]

Default: 0

Random selection method. One of:

  • 0 — Number of selected features

  • 1 — Percentage of selected features

Number/percentage of selected features

NUMBER

[number]

Default: 10

Number or percentage of features to select

Extracted (random stratified)

OUTPUT

[same as input]

Default: [Create temporary layer]

Specify the output vector layer for the randomly selected features. One of:

  • Create Temporary Layer (TEMPORARY_OUTPUT)

  • Save to File…

  • Save to Geopackage…

  • Save to PostGIS Table…

The file encoding can also be changed here.

23.1.17.5.2. Outputs

Label

Name

Type

Description

Extracted (random stratified)

OUTPUT

[same as input]

Vector layer containing randomly selected features from the input layer

23.1.17.5.3. Python code

Algorithm ID: qgis:randomextractwithinsubsets

import processing
processing.run("algorithm_id", {parameter_dictionary})

The algorithm id is displayed when you hover over the algorithm in the Processing Toolbox. The parameter dictionary provides the parameter NAMEs and values. See Using processing algorithms from the console for details on how to run processing algorithms from the Python console.

23.1.17.6. Random selection

Takes a vector layer and selects a subset of its features. No new layer is generated by this algorithm.

The subset is defined randomly, based on feature IDs, using a percentage or count value to define the total number of features in the subset.

Default menu: Vector ► Research Tools

See also

Random extract

23.1.17.6.1. Parameters

Label

Name

Type

Description

Input layer

INPUT

[vector: any]

Vector layer for the selection

Method

METHOD

[enumeration]

Default: 0

Random selection method. One of:

  • 0 — Number of selected features

  • 1 — Percentage of selected features

Number/percentage of selected features

NUMBER

[number]

Default: 10

Number or percentage of features to select

23.1.17.6.2. Outputs

Label

Name

Type

Description

Input layer

INPUT

[same as input]

The input layer with features selected

23.1.17.6.3. Python code

Algorithm ID: qgis:randomselection

import processing
processing.run("algorithm_id", {parameter_dictionary})

The algorithm id is displayed when you hover over the algorithm in the Processing Toolbox. The parameter dictionary provides the parameter NAMEs and values. See Using processing algorithms from the console for details on how to run processing algorithms from the Python console.

23.1.17.7. Random selection within subsets

Takes a vector layer and selects a subset of its features. No new layer is generated by this algorithm.

The subset is defined randomly, based on feature IDs, using a percentage or count value to define the total number of features in the subset.

The percentage/count value is not applied to the whole layer, but instead to each category.

Categories are defined according to a given attribute, which is also specified as an input parameter for the algorithm.

No new outputs are created.

Default menu: Vector ► Research Tools

23.1.17.7.1. Parameters

Label

Name

Type

Description

Input layer

INPUT

[vector: any]

Vector layer to select features in

ID field

FIELD

[tablefield: any]

Category of the input layer to select the features from

Method

METHOD

[enumeration]

Default: 0

Random selection method. One of:

  • 0 — Number of selected features

  • 1 — Percentage of selected features

Number/percentage of selected features

NUMBER

[number]

Default: 10

Number or percentage of features to select

23.1.17.7.2. Outputs

Label

Name

Type

Description

Input layer

INPUT

[same as input]

The input layer with features selected

23.1.17.7.3. Python code

Algorithm ID: qgis:randomselectionwithinsubsets

import processing
processing.run("algorithm_id", {parameter_dictionary})

The algorithm id is displayed when you hover over the algorithm in the Processing Toolbox. The parameter dictionary provides the parameter NAMEs and values. See Using processing algorithms from the console for details on how to run processing algorithms from the Python console.

23.1.17.8. Select by attribute

Creates a selection in a vector layer.

The criteria for selecting features is based on the values of an attribute from the input layer.

23.1.17.8.1. Parameters

Label

Name

Type

Description

Input layer

INPUT

[vector: any]

Vector layer to select features in

Selection attribute

FIELD

[tablefield: any]

Filtering field of the layer

Operator

OPERATOR

[enumeration]

Default: 0

Many different operators are available:

  • 0 — =

  • 1 — ≠

  • 2 — >

  • 3 — >=

  • 4 — <

  • 5 — <=

  • 6 — begins with

  • 7 — contains

  • 8 — is null

  • 9 — is not null

  • 10 — does not contain

Value

Optional

VALUE

[string]

Value to be evaluated

Modify current selection by

METHOD

[enumeration]

Default: 0

How the selection of the algorithm should be managed. One of:

  • 0 — creating new selection

  • 1 — adding to current selection

  • 2 — selecting within current selection

  • 3 — removing from current selection

23.1.17.8.2. Outputs

Label

Name

Type

Description

Input layer

INPUT

[same as input]

The input layer with features selected

23.1.17.8.3. Python code

Algorithm ID: qgis:selectbyattribute

import processing
processing.run("algorithm_id", {parameter_dictionary})

The algorithm id is displayed when you hover over the algorithm in the Processing Toolbox. The parameter dictionary provides the parameter NAMEs and values. See Using processing algorithms from the console for details on how to run processing algorithms from the Python console.

23.1.17.9. Select by expression

Creates a selection in a vector layer.

The criteria for selecting features is based on a QGIS expression. For more information about expressions see the Expressions.

23.1.17.9.1. Parameters

Label

Name

Type

Description

Input layer

INPUT

[vector: any]

Input vector layer

Expression

EXPRESSION

[expression]

Expression to filter the input layer

Modify current selection by

METHOD

[enumeration]

Default: 0

How the selection of the algorithm should be managed. One of:

  • 0 — creating new selection

  • 1 — adding to current selection

  • 2 — selecting within current selection

  • 3 — removing from current selection

23.1.17.9.2. Outputs

Label

Name

Type

Description

Input layer

INPUT

[same as input]

The input layer with features selected

23.1.17.9.3. Python code

Algorithm ID: qgis:selectbyexpression

import processing
processing.run("algorithm_id", {parameter_dictionary})

The algorithm id is displayed when you hover over the algorithm in the Processing Toolbox. The parameter dictionary provides the parameter NAMEs and values. See Using processing algorithms from the console for details on how to run processing algorithms from the Python console.

23.1.17.10. Select by location

Creates a selection in a vector layer.

The criteria for selecting features is based on the spatial relationship between each feature and the features in an additional layer.

../../../../_images/selectbylocation.png

Fig. 23.94 In this example, the dataset from which we want to select (the source vector layer) consists of the green circles, the orange rectangle is the dataset that it is being compared to (the intersection vector layer).

Available geometric predicates are:

Intersect

Tests whether a geometry intersects another. Returns 1 (true) if the geometries spatially intersect (share any portion of space - overlap or touch) and 0 if they don’t. In the picture above, this will select circles 1, 2 and 3.

Contain

Returns 1 (true) if and only if no points of b lie in the exterior of a, and at least one point of the interior of b lies in the interior of a. In the picture, no circle is selected, but the rectangle would be if you would select it the other way around, as it contains a circle completely. This is the opposite of are within.

Disjoint

Returns 1 (true) if the geometries do not share any portion of space (no overlap, not touching). Only circle 4 is selected.

Equal

Returns 1 (true) if and only if geometries are exactly the same. No circles will be selected.

Touch

Tests whether a geometry touches another. Returns 1 (true) if the geometries have at least one point in common, but their interiors do not intersect. Only circle 3 is selected.

Overlap

Tests whether a geometry overlaps another. Returns 1 (true) if the geometries share space, are of the same dimension, but are not completely contained by each other. Only circle 2 is selected.

Are within

Tests whether a geometry is within another. Returns 1 (true) if geometry a is completely inside geometry b. Only circle 1 is selected.

Cross

Returns 1 (true) if the supplied geometries have some, but not all, interior points in common and the actual crossing is of a lower dimension than the highest supplied geometry. For example, a line crossing a polygon will cross as a line (selected). Two lines crossing will cross as a point (selected). Two polygons cross as a polygon (not selected).

Default menu: Vector ► Research Tools

23.1.17.10.1. Parameters

Label

Name

Type

Description

Select features from

INPUT

[vector: any]

Input vector layer

Where the features (geometric predicate)

PREDICATE

[enumeration] [list]

Default: [0]

Spatial condition for the selection. One or more of:

  • 0 — intersect

  • 1 — contain

  • 2 — disjoint

  • 3 — equal

  • 4 — touch

  • 5 — overlap

  • 6 — are within

  • 7 — cross

If more than one condition is chosen, at least one of them (OR operation) has to be met for a feature to be extracted.

By comparing to the features from

INTERSECT

[vector: any]

Intersection vector layer

Modify current selection by

METHOD

[enumeration]

Default: 0

How the selection of the algorithm should be managed. One of:

  • 0 — creating new selection

  • 1 — adding to current selection

  • 2 — selecting within current selection

  • 3 — removing from current selection

23.1.17.10.2. Outputs

Label

Name

Type

Description

Input layer

INPUT

[same as input]

The input layer with features selected

23.1.17.10.3. Python code

Algorithm ID: qgis:selectbylocation

import processing
processing.run("algorithm_id", {parameter_dictionary})

The algorithm id is displayed when you hover over the algorithm in the Processing Toolbox. The parameter dictionary provides the parameter NAMEs and values. See Using processing algorithms from the console for details on how to run processing algorithms from the Python console.