24.1.23. Vector coverage

24.1.23.1. Clean coverage

Added in 4.4

Operates on a coverage (represented as a list of polygon features) to fix cases where the geometry does not in fact exactly match, repairing small overlaps and gaps to restore a valid topological coverage.

The algorithm uses a combination of snapping and gap/overlap merging. Snapping to nearby vertices and line segments improves noding robustness and eliminates small errors. If the snapping distance is not specified, an automatic distance based on the data extent is used. A distance of 0 disables snapping.

Gaps and holes that are narrower than the a speified width are merged with an adjacent polygon. Gaps which are not fully enclosed (“inlets”) are not removed. When repairing overlaps and gaps, a strategy can be used to determine which adjacent polygon the problematic area is merged into (e.g., merging with the polygon that shares the longest border, or the one with the maximum/minimum area). Polygons that have collapsed during cleaning will be returned as empty polygons.

Fig. 24.89 Cleaning gaps and overlays within a coverage

Allows features in-place modification of polygon features

Parameters

Label	Name	Type	Description
Input layer	INPUT	[vector: polygon]	Input polygon vector layer
Maximum gap width	MAXIMUM_GAP_WIDTH	[numeric: double] Default: 0.0	Gaps which are narrower than this distance are merged with an adjacent polygon. Polygon width is determined as twice the radius of the maximum inscribed circle of the gap polygon. Empty holes in input polygons are treated as gaps, and may be filled in. Gaps which are not fully enclosed (‘inlets’) are not removed.
Snapping distance Optional	SNAPPING_DISTANCE	[numeric: double]	Snapping to nearby vertices and line segment snapping is used to improve noding robustness and eliminate small errors in an efficient way. By default the snapping distance is not set, which means that the clean operation uses a very small snapping distance based on the extent of the input data. A distance of zero prevents snapping from being used.
Overlap merge strategy	OVERLAP_MERGE_STRATEGY	[enumeration] Default: 0	Determines the strategy used to merge gaps with adjacent polygons. One of: 0 — Longest border: the feature sharing the longest border with the gap 1 — Maximum area: the biggest adjacent feature 2 — Minimum area: the smallest adjacent feature 3 — Minimum index: the first encountered adjacent feature (lowest ID)
Cleaned	OUTPUT	[vector: polygon] Default: [Create temporary layer]	Specify the output vector layer with cleaned coverage. One of: Create Temporary Layer (TEMPORARY_OUTPUT) Save to File… Save to Geopackage… Save to Database Table… The file encoding can also be changed here.

Outputs

Label	Name	Type	Description
Cleaned	OUTPUT	[vector: polygon]	The output polygon vector layer with cleaned gaps and overlays.

Python code

Algorithm ID: native:coverageclean

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.

24.1.23.2. Dissolve coverage

Added in 3.36

Operates on a coverage (represented as a set of polygon features with exactly matching edge geometry) to dissolve (union) the geometries. It provides a heavily optimized approach for unioning these features compared with the standard Dissolve tools.

See also

Dissolve

Parameters

Label	Name	Type	Description
Input layer	INPUT	[vector: polygon]	Input polygon vector layer
Dissolved	OUTPUT	[vector: polygon] Default: [Create temporary layer]	Specify the output vector layer. One of: Create Temporary Layer (TEMPORARY_OUTPUT) Save to File… Save to Geopackage… Save to Database Table… The file encoding can also be changed here.

Outputs

Label	Name	Type	Description
Dissolved	OUTPUT	[vector: polygon]	The output polygon vector layer with dissolved geometries.

Python code

Algorithm ID: native:coverageunion

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.

24.1.23.3. Simplify coverage

Added in 3.36

Operates on a coverage (represented as a set of polygon features with exactly matching edge geometry) to apply a Visvalingam–Whyatt simplification to the edges, reducing complexity in proportion with the provided tolerance, while retaining a valid coverage (i.e. no edges will cross or touch after the simplification). Geometries will never be removed, but they may be simplified down to just a triangle. Also, some geometries (such as polygons which have too few non-repeated points) will be returned unchanged.

If the input dataset is not a valid coverage due to overlaps, it will still be simplified, but invalid topology such as crossing edges will still be invalid.

Allows features in-place modification of polygon features

Attention

Running this algorithm requires QGIS installed with GEOS >= 3.12 (see Help ► About menu).

See also

Simplify, Smooth

Parameters

Label	Name	Type	Description
Input layer	INPUT	[vector: polygon]	Input polygon vector layer
Tolerance	TOLERANCE	[numeric: double] Default: 0.0	The maximum distance (in unit of choice) between two consecutive vertices to get merged.
Preserve boundary	PRESERVE_BOUNDARY	[boolean] Default: False	When enabled, the outside edges of the coverage will be preserved without simplification
Simplified	OUTPUT	[vector: polygon] Default: [Create temporary layer]	Specify the output vector layer. One of: Create Temporary Layer (TEMPORARY_OUTPUT) Save to File… Save to Geopackage… Save to Database Table… The file encoding can also be changed here.

Outputs

Label	Name	Type	Description
Simplified	OUTPUT	[vector: polygon]	The output polygon vector layer, with a lower number of vertices.

Python code

Algorithm ID: native:coveragesimplify

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.

24.1.23.4. Validate coverage

Added in 3.36

Analyzes a coverage (represented as a set of polygon features with exactly matching edge geometry) to find places where the assumption of exactly matching edges is not met. Invalidity includes polygons that overlap or that have gaps smaller than the specified gap width.

Attention

Running this algorithm requires QGIS installed with GEOS >= 3.12 (see Help ► About menu).

See also

Check validity, Small gaps, Overlaps

Parameters

Label	Name	Type	Description
Input layer	INPUT	[vector: polygon]	Input polygon vector layer
Gap width	GAP_WIDTH	[numeric: double] Default: 0.0	The maximum width of gap to detect
Invalid edges	INVALID_EDGES	[vector: line] Default: [Create temporary layer]	Specify the output vector layer. One of: Skip Output Create Temporary Layer (TEMPORARY_OUTPUT) Save to File… Save to Geopackage… Save to Database Table… The file encoding can also be changed here.

Outputs

Label	Name	Type	Description
Invalid edges	OUTPUT	[vector: line]	The output line vector layer showing error edges for each invalid polygon.
Validity result	IS_VALID	[boolean]	Returns whether the coverage is valid or not.

Python code

Algorithm ID: native:coveragevalidate

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.