Class OverlapUnion

Unions MultiPolygons efficiently by using full topological union only for polygons which may overlap, and combining with the remaining polygons. Polygons which may overlap are those which intersect the common extent of the inputs. Polygons wholly outside this extent must be disjoint to the computed union. They can thus be simply combined with the union result, which is much more performant. (There is one caveat to this, which is discussed below).

This situation is likely to occur during cascaded polygon union, since the partitioning of polygons is done heuristically and thus may group disjoint polygons which can lie far apart. It may also occur in real world data which contains many disjoint polygons (e.g. polygons representing parcels on different street blocks).

Inheritance

Object

OverlapUnion

Inherited Members

Object.Equals(Object)

Object.Equals(Object, Object)

Object.GetHashCode()

Object.GetType()

Object.MemberwiseClone()

Object.ReferenceEquals(Object, Object)

Object.ToString()

Namespace: NetTopologySuite.Operation.Union

Assembly: NetTopologySuite.dll

Syntax

[Obsolete("Due to impairing performance")]
public class OverlapUnion

Remarks

Algorithm

The overlap region is determined as the common envelope of intersection. The input polygons are partitioned into two sets:

OverlappingPolygons which intersect the overlap region, and thus potentially overlap each other
DisjointPolygons which are disjoint from (lie wholly outside) the overlap region

The Overlapping set is fully unioned, and then combined with the Disjoint set. Performing a simple combine works because the disjoint polygons do not interact with each other(since the inputs are valid MultiPolygons). They also do not interact with the Overlapping polygons, since they are outside their envelope.

Discussion

In general the Overlapping set of polygons will extend beyond the overlap envelope. This means that the union result will extend beyond the overlap region. There is a small chance that the topological union of the overlap region will shift the result linework enough that the result geometry intersects one of the Disjoint geometries. This situation is detected and if it occurs is remedied by falling back to performing a full union of the original inputs. Detection is done by a fairly efficient comparison of edge segments which extend beyond the overlap region. If any segments have changed then there is a risk of introduced intersections, and full union is performed.

This situation has not been observed in JTS using floating precision, but it could happen due to snapping. It has been observed in other APIs(e.g.GEOS) due to more aggressive snapping. It is more likely to happen if a Snap - Rounding overlay is used.

NOTE: Test has shown that using this heuristic impairs performance.

Constructors

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OverlapUnion(Geometry, Geometry)

Creates a new instance for unioning the given geometries.

Declaration

public OverlapUnion(Geometry g0, Geometry g1)

Parameters

Type	Name	Description
Geometry	g0	A geometry to union
Geometry	g1	A geometry to union

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OverlapUnion(Geometry, Geometry, UnionStrategy)

Creates a new instance for unioning the given geometries.

Declaration

public OverlapUnion(Geometry g0, Geometry g1, UnionStrategy unionFun)

Parameters

Type	Name	Description
Geometry	g0	A geometry to union
Geometry	g1	A geometry to union
UnionStrategy	unionFun	Function to union two geometries

Methods

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Union()

Union a pair of geometries, using the more performant overlap union algorithm if possible.

Declaration

public Geometry Union()

Returns

Type	Description
Geometry	The union of the inputs

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Union(Geometry, Geometry)

Union a pair of geometries, using the more performant overlap union algorithm if possible.

Declaration

public static Geometry Union(Geometry g0, Geometry g1)

Parameters

Type	Name	Description
Geometry	g0	A geometry to union
Geometry	g1	A geometry to union

Returns

Type	Description
Geometry	The union of the inputs

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Union(Geometry, Geometry, UnionStrategy)

Union a pair of geometries, using the more performant overlap union algorithm if possible.

Declaration

public static Geometry Union(Geometry g0, Geometry g1, UnionStrategy unionFun)

Parameters

Type	Name	Description
Geometry	g0	A geometry to union
Geometry	g1	A geometry to union
UnionStrategy	unionFun	Function to union two geometries

Returns

Type	Description
Geometry	The union of the inputs