24.1.18. Vector selection

Extract by attribute
Extract by expression
Extract by location
Random extract
Random extract within subsets
Random selection
Random selection within subsets
Select by attribute
Select by expression
Select by location

24.1.18.1. 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.

Viz také

Select by attribute

Parameters

Label	Název	Type	Popis
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…

Outputs

Label	Název	Type	Popis
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

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.

24.1.18.2. 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 Výrazy.

Viz také

Select by expression

Parameters

Label	Název	Type	Popis
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…

Outputs

Label	Název	Type	Popis
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

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.

24.1.18.3. 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 — Obr. 24.100 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).

Viz také

Select by location

Parameters

Label	Název	Type	Popis
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…

Outputs

Label	Název	Type	Popis
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.

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.

24.1.18.4. 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.

Viz také

Random selection

Parameters

Label	Název	Type	Popis
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

Outputs

Label	Název	Type	Popis
Extracted (random)	`OUTPUT`	[same as input]	Vector layer containing randomly selected features from the input layer

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.

24.1.18.5. 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.

Viz také

Random selection within subsets

Parameters

Label	Název	Type	Popis
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.

Outputs

Label	Název	Type	Popis
Extracted (random stratified)	`OUTPUT`	[same as input]	Vector layer containing randomly selected features from the input layer

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.

24.1.18.6. 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

Viz také

Random extract

Parameters

Label

Název

Type

Popis

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

Outputs

Label	Název	Type	Popis
Input layer	`INPUT`	[same as input]	The input layer with features selected

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.

24.1.18.7. 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

Viz také

Random extract within subsets

Parameters

Label	Název	Type	Popis
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

Outputs

Label	Název	Type	Popis
Input layer	`INPUT`	[same as input]	The input layer with features selected

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.

24.1.18.8. 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.

Viz také

Extract by attribute

Parameters

Label	Název	Type	Popis
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 — removing from current selection 3 — selecting within current selection

Outputs

Label	Název	Type	Popis
Input layer	`INPUT`	[same as input]	The input layer with features selected

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.

24.1.18.9. 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 Výrazy.

Viz také

Extract by expression

Parameters

Label

Název

Type

Popis

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 — removing from current selection
3 — selecting within current selection

Outputs

Label	Název	Type	Popis
Input layer	`INPUT`	[same as input]	The input layer with features selected

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.

24.1.18.10. 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.

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

Viz také

Extract by location

Parameters

Label	Název	Type	Popis
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

Outputs

Label	Název	Type	Popis
Input layer	`INPUT`	[same as input]	The input layer with features selected

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.