outer_multi_polygon

Geometric functions outer_multi_polygon

The outer_multi_polygon function extracts only the outer rings from multi-polygons, removing any holes.

syntax

outer_multi_polygon(multi_polygons: E->MultiPolygon<Point>) -> E->MultiPolygon<Point>

Alias: This is also available as bg_outer_multi_polygon for boost geometry coordinates.

definition

Removes holes from each polygon part in a multi-polygon, keeping only the outer boundary rings. The result has the same number of polygon parts, but each part is a simple polygon without any interior rings.

For multi-polygons without holes, the output is identical to the input.

arguments

argument	description	type
multi_polygons	Multi-polygon geometries	E->MultiPolygon

performance

Time complexity: O(n × p × v) where:

• n = number of multi-polygons
• p = average number of polygon parts per multi-polygon
• v = average number of vertices per polygon part

Memory efficient as it typically reduces geometry size by removing hole vertices.

conditions

• Input should be multi-polygon geometries
• For single polygons, use outer_single_polygon

example

unit<uint32> Parcels: nrofrows = 500;
attribute<DPoint> parcel_geometry (poly, Parcels);  // multi-polygons with possible holes

// Remove all holes (ponds, excluded areas, etc.)
attribute<DPoint> filled_parcels (poly, Parcels) := outer_multi_polygon(parcel_geometry);

// Compare areas
attribute<Float64> original_area (Parcels) := area(parcel_geometry);
attribute<Float64> filled_area (Parcels) := area(filled_parcels);
attribute<Float64> hole_area (Parcels) := filled_area - original_area;

use cases

• Simplifying geometries for visualization
• Computing gross area (including holes)
• Preparing geometries for operations that don't handle holes well
• Data cleaning where holes are artifacts

see also

• outer_single_polygon - for single polygon geometries
• bg_outer_multi_polygon - boost geometry variant
• bg_outer_single_polygon - boost geometry single polygon variant
• polygon operators
• Geometric functions

since version

7.0