TGIS_Topology.ConcaveHull(TGIS_Shape, double) method

DK11 for .NET | TatukGIS.NDK.TGIS_Topology.ConcaveHull | Overloads | Constructors | Methods | Properties | Events

Creates a concave hull based on the provided shape and edge length.

Available also on: Delphi | Java | ActiveX | Python.

// C#
public TGIS_ShapePolygon ConcaveHull(
  TGIS_Shape _shape,
  double _alpha
);

' VisualBasic
Public Function ConcaveHull(
  ByVal _shape As TGIS_Shape,
  ByVal _alpha As Double
) As TGIS_ShapePolygon

// Oxygene
public
  function ConcaveHull(
    _shape : TGIS_Shape;
    _alpha : Double
  ) : TGIS_ShapePolygon;

The alpha parameter (maximum edge length) tells the algorithm that the length of each edge of the concave hull should not exceed this value. The algorithm starts with finding the convex hull and then attempts to “shrink” it by eliminating edges of length greater than alpha. It is not guaranteed that the concave hull will consist only of edges shorter than alpha because it may not be possible to eliminate some “long” edges in a reasonable manner. However, the output shape is guaranteed to be topologically correct (i.e. no self crossings, contains all input points).

The difference between a convex and a concave hull (sample data unit is kilometer, sample data size is 100×100 km):

Convex Hull	Concave Hull, alpha=1	Concave Hull, alpha=20