pgr_alphaShape

Name

pgr_alphaShape — Core function for alpha shape computation.

Synopsis

Returns a table with (x, y) rows that describe the vertices of an alpha shape.

table() pgr_alphaShape(text sql [, float8 alpha]);

Description

sql:

text a SQL query, which should return a set of rows with the following columns:

SELECT id, x, y FROM vertex_table
id:int4 identifier of the vertex
x:float8 x-coordinate
y:float8 y-coordinate
alpha:

(optional) float8 alpha value. If specified alpha value equals 0 (default), then optimal alpha value is used. For more information, see CGAL - 2D Alpha Shapes.

Returns a vertex record for each row:

x:x-coordinate
y:y-coordinate

If a result includes multiple outer/inner rings, return those with separator row (x=NULL and y=NULL).

History

  • Renamed in version 2.0.0
  • Added alpha argument with default 0 (use optimal value) in version 2.1.0
  • Supported to return multiple outer/inner ring coordinates with separator row (x=NULL and y=NULL) in version 2.1.0

Examples

PgRouting’s alpha shape implementation has no way to control the order of the output points, so the actual output might different for the same input data. The first query, has the output ordered, he second query shows an example usage:

Example: the (ordered) results

SELECT * FROM pgr_alphaShape(
    'SELECT id::integer, ST_X(the_geom)::float AS x, ST_Y(the_geom)::float AS y
    FROM edge_table_vertices_pgr') ORDER BY x, y;
  x  |  y  
-----+-----
   0 |   2
 0.5 | 3.5
   2 |   0
   2 |   4
 3.5 |   4
   4 |   1
   4 |   2
   4 |   3
(8 rows)

Example: calculating the area

Steps:

  • Calculates the alpha shape
    • the ORDER BY clause is not used.
  • constructs a polygon

  • and computes the area

SELECT round(ST_Area(ST_MakePolygon(ST_AddPoint(foo.openline, ST_StartPoint(foo.openline))))::numeric, 2) AS st_area
FROM (
    SELECT ST_MakeLine(points ORDER BY id) AS openline
    FROM (
        SELECT ST_MakePoint(x, y) AS points, row_number() over() AS id
        FROM pgr_alphaShape(
            'SELECT id::integer, ST_X(the_geom)::float AS x, ST_Y(the_geom)::float AS y
            FROM edge_table_vertices_pgr')
        ) AS a
   ) AS foo;
 st_area 
---------
   11.75
(1 row)

The queries use the Sample Data network.

See Also

Indices and tables