TGIS_Topology.ConcaveHull(TGIS_Shape; Double) method

DK11 for Delphi | GisTopology.TGIS_Topology.ConcaveHull | Overloads | Constructors | Methods | Properties | Events

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

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

// Delphi
public
  function ConcaveHull(
    const _shape : TGIS_Shape;
    const _alpha : Double
  ) : TGIS_ShapePolygon; overload;

// C++ Builder
public:
  TGIS_ShapePolygon* ConcaveHull(
    TGIS_Shape* const _shape,
    const double _alpha
  ) /* overload */;

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