SDO_GEOM.SDO_CONCAVEHULL

Format

SDO_GEOM.SDO_CONCAVEHULL(     
  geom  IN SDO_GEOMETRY,      
  tol   IN NUMBER       
) RETURN SDO_GEOMETRY;

or

SDO_GEOM.SDO_CONCAVEHULL(     
  geom   IN SDO_GEOMETRY,      
  tol    IN NUMBER,      
  radius OUT NUMBER       
) RETURN SDO_GEOMETRY;

or

SDO_GEOM.SDO_CONCAVEHULL(     
  geom  IN SDO_GEOMETRY_ARRAY,      
  tol   IN NUMBER       
) RETURN SDO_GEOMETRY;

or

SDO_GEOM.SDO_CONCAVEHULL(     
  geom   IN SDO_GEOMETRY_ARRAY,      
  tol    IN NUMBER,      
  radius OUT NUMBER       
) RETURN SDO_GEOMETRY;

Description

Returns a polygon-type object that represents the concave hull of a geometry object or of an array of geometry objects.

Parameters

geom: Geometry object (type SDO_GEOMETRY), or array of geometry objects (type SDO_GEOMETRY_ARRAY, which is defined as VARRAY OF SDO_GEOMETRY ).
tol: Tolerance value (see Tolerance).
radius: Output parameter to hold the radius of the circumcircle of the triangles created internally (using Delaunay triangulations) in computing the concave hull.

Usage Notes

The concave hull is a polygon that represents the area of the input geometry, such as a collection of points. With complex input geometries, the concave hull is typically significantly smaller in area than the convex hull.

This function takes all coordinates from the input geometry, uses them to compute Delaunay triangulations, and computes a concave hull. It returns only an exterior ring; any interior rings are discarded.

This function uses the alpha shape in computing the concave hull. By contrast, the SDO_GEOM.SDO_CONCAVEHULL_BOUNDARY function uses exterior boundary points.

The format with the radius parameter returns a radius value that can be useful if you plan to call the SDO_GEOM.SDO_ALPHA_SHAPE function.

An exception is raised if geom has fewer than three points or vertices, or consists of multiple points all in a straight line.

With geodetic data, this function is supported by approximations, as explained in Functions Supported by Approximations with Geodetic Data.

Examples

The following example returns a geometry object that is the concave hull of cola_c. (The example uses the definitions and data from Simple Example: Inserting_ Indexing_ and Querying Spatial Data.

-- Return the concave hull of a polygon.
SELECT c.name, SDO_GEOM.SDO_CONCAVEHULL(c.shape, 0.005)
  FROM cola_markets c WHERE c.name = 'cola_c';
 
NAME                                                                            
--------------------------------                                                
SDO_GEOM.SDO_CONCAVEHULL(C.SHAPE,0.005)(SDO_GTYPE, SDO_SRID, SDO_POINT(X, Y, Z),
--------------------------------------------------------------------------------
cola_c                                                                          
SDO_GEOMETRY(2003, NULL, NULL, SDO_ELEM_INFO_ARRAY(1, 1003, 1), SDO_ORDINATE_ARR
AY(6, 3, 6, 5, 4, 5, 3, 3, 6, 3))

The following example returns a geometry that is the concave hull of an array of three geometry objects.

SELECT sdo_geom.sdo_concavehull(SDO_GEOMETRY_ARRAY(
   SDO_GEOMETRY(2005, NULL, NULL, SDO_ELEM_INFO_ARRAY(1, 1, 4),
      SDO_ORDINATE_ARRAY(6, 3, 6, 5, 4, 5, 3, 3)),
   SDO_GEOMETRY(2005, NULL, NULL, SDO_ELEM_INFO_ARRAY(1, 1, 4),
      SDO_ORDINATE_ARRAY(16, 13, 16, 15, 14, 15, 13, 13)),
   SDO_GEOMETRY(2005, NULL, NULL, SDO_ELEM_INFO_ARRAY(1, 1, 4),
      SDO_ORDINATE_ARRAY(26, 23, 26, 25, 24, 25, 23, 23))), 0.000005)
FROM dual;

SDO_GEOM.SDO_CONCAVEHULL(SDO_GEOMETRY_ARRAY(SDO_GEOMETRY(2005,NULL,NULL,SDO_ELEM
--------------------------------------------------------------------------------
SDO_GEOMETRY(2003, NULL, NULL, SDO_ELEM_INFO_ARRAY(1, 1003, 1), SDO_ORDINATE_ARR
AY(6, 3, 6, 5, 16, 13, 16, 15, 26, 23, 26, 25, 24, 25, 23, 23, 14, 15, 13, 13, 4
, 5, 3, 3, 6, 3))