Process operations (Clean/Simplify category)

Generalises geometry using a variation of the Douglas-Peucker algorithm that is limited by the number of output points.

Property

Number of points*

The number of output points, as a simple value, or as an expression.

Example: 50/100/500

The following examples show the effect of increasing the Number of points value.

Initial Polygon Item:

Number of points = 50:

Number of points = 100:

Number of points = 500:

Generalises geometry using the Douglas-Peucker algorithm.

Property

Tolerance*

The Douglas-Peuker algorithm tolerance, as a simple vale, or as an expression.

All vertices in the simplified geometry will be within this distance of the original geometry.

Example: 50/100/300

The following examples show the effect of increasing the Tolerance value.

Initial Polygon Item:

Tolerance = 50:

Tolerance = 100:

Tolerance = 300:

Generalises geometry using the Perpendicular Distance algorithm.

Properties

Repeat*

The number of times to repeat the algorithm, as a simple value, or as an expression.

Example: 4

Tolerance*

The Perpendicular Distance algorithm tolerance, as a simple value, or as an expression.

Example: 50/500/1000

The following examples show the effect of increasing the Tolerance value.

The original map:

Tolerance = 50:

Tolerance = 500:

Tolerance = 1000:

Generalises geometry using the Radial Distance algorithm.

Property

Tolerance*

The Radial Distance algorithm tolerance, as a simple value, or as an expression.

Example: 50/500/1000

The following examples show the effect of increasing the Tolerance value.

The original map:

Tolerance = 50:

Tolerance = 500:

Tolerance = 1000:

Generalises geometry using the Reumann-Witkam algorithm.

Property

Tolerance*

The Reumann-Witkam algorithm tolerance, as a simple value, or as an expression.

Example: 50/500/1000

The following examples show the effect of increasing the Tolerance value.

The original map:

Tolerance = 50:

Tolerance = 500:

Tolerance = 1000:

Removes duplicate items based on an expression.

Property

Expression*

The test expression, e.g. 'FC&' or 'building_type$="industrial"'

As an example consider the following:

The overlay Units contains Industrial and Office buildings:

Select the Units overlay:

Click Next.

The Processing Operations dialog will be displayed.

Select General > Filter in the left-hand pane and click the right arrow button to add Filter to the right-hand pane.

Click the ... button.

The Expression Builder dialog will be displayed:

Set the Process command General > Filter to show only the Industrial buildings and click OK.

This will be building_type$="Industrial"

Click OK.

The Filter will now appear in the Processing Operations dialog:

Click Next.

The Processing Output dialog will be displayed. Select the required output and click Finish.

The Processing Input dialog will again be displayed:

Check the Units overlay is still selected and click Next.

The process will run and the final display will be shown:

The new overlay will be displayed with all duplicates removed.

Note: The item remaining will be the last item drawn or edited.

Removes duplicate items based on their geometry.

Removes self-intersections, e.g. a bow-tie.

Removes segments shorter than a tolerance.

Property

Tolerance*

The tolerance, as a simple value, or as an expression, e.g. '0.1234' or '0.001234*_length#'

Removes vertices that cause a spike.

Property

Tolerance*

The tolerance, in degrees, as a simple value, or as an expression, e.g. '0.1' or 'Acos(0.999998)*180/3.14'

Removes vertices between straight, or near straight segments.

Property

Tolerance*

The tolerance, in degrees, as a simple value, or as an expression, e.g. '0.1' or 'Acos(0.999998)*180/3.14'.

Simplifies the geometry of an item, so that it matches the OGC 'simple' form (e.g. by removing all self-intersections).