Supported versions: latest (3.8) 3.7 3.6 3.5 3.4 3.3 3.2 3.1 3.0 main dev
Unsupported versions:2.6 2.5 2.4 2.3 2.2 2.1 2.0

pgr_alphaShape¶

pgr_alphaShape — Polygon part of an alpha shape.

Availability

Version 3.0.0
- Breaking change on signature
- Old signature no longer supported
- Boost 1.54 & Boost 1.55 are supported
- Boost 1.56+ is preferable
  - Boost Geometry is stable on Boost 1.56
Version 2.1.0
- Added alpha argument with default 0 (use optimal value)
- Support to return multiple outer/inner ring
Version 2.0.0
- Official function
- Renamed from version 1.x

Support

Description¶

Returns the polygon part of an alpha shape.

Characteristics

Input is a geometry and returns a geometry
Uses PostGis ST_DelaunyTriangles
Instead of using CGAL’s definition of alpha it use the spoon_radius
- $spoon\_radius = \sqrt alpha$
A Triangle area is considered part of the alpha shape when $circumcenter\ radius < spoon\_radius$
When the total number of points is less than 3, returns an EMPTY geometry

Summary

pgr_alphaShape(geometry,   [spoon_radius])
RETURNS geometry

Example: passing a geometry collection with spoon radius $1.5$ using the return variable geom

SELECT ST_Area(pgr_alphaShape((SELECT ST_Collect(the_geom) FROM edge_table_vertices_pgr), 1.5));
 st_area
---------
    9.75
(1 row)

Parameter	Type	Default	Description
geometry	`geometry`		Geometry with at least $3$ points
spoon_radius	`FLOAT`		The radius of the spoon

Kind of geometry	Description
GEOMETRY COLLECTION	A Geometry collection of Polygons