# pgr_alphaShape¶

pgr_alphaShape — Polygon part of an alpha shape.

Availability

• Version 3.0.0
• Breaking change on signature
• Version 2.1.0
• Added alpha argument with default 0 (use optimal value)
• Support to return multiple outer/inner ring
• Renamed in v2.0.0

## 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

## Signatures¶

Summary

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)

## Parameters¶

Parameter Type Default Description
geometry geometry   Geometry with at least $$3$$ points