Polygon composition

A function for composing polygons is not-so-surprisingly named ST_MakePolygon. For a solid polygon without holes, it accepts one argument-a closed LineString.

1. For example, a triangle-shaped polygon can be created using the following query:

        SELECT
        ST_MakePolygon(ST_MakeLine(ARRAY[ST_MakePoint(20,50),
        ST_MakePoint(19.95,49.98), 
        ST_MakePoint(19.90,49.90),ST_MakePoint(20,50)]));  

1. An attempt to create a polygon from fewer than four points will result in an error:

        SELECT
        ST_MakePolygon(ST_MakeLine(ARRAY[ST_MakePoint(20,50),
        ST_MakePoint(19.95,49.98), ST_MakePoint(19.90,49.96)]));
        ERROR:  lwpoly_from_lwlines: shell must have at least 4 points

And if a ring is not closed:

        SELECT
        ST_MakePolygon(ST_MakeLine(ARRAY[ST_MakePoint(20,50),
        ST_MakePoint(19.95,49.98),     
        ST_MakePoint(19.90,49.96),ST_MakePoint(20,50),
        ST_MakePoint(20.01,50.01)]));
        ERROR:  lwpoly_from_lwlines: shell must be closed

When creating simple shapes using coordinates typed by hand, it's rather obvious if a ring is closed or not. But what if there are hundreds or thousands of vertices? Luckily, PostGIS has a function for finding out whether a LineString is closed or not--it's called ST_IsClosed:

       SELECT
       ST_IsClosed(ST_MakeLine(ARRAY[ST_MakePoint(20,50),
       ST_MakePoint(19.95,49.98),   
       ST_MakePoint(19.90,49.96),ST_MakePoint(20,50),
       ST_MakePoint(20.01,50.01)]));
         st_isclosed
         -------------
         f

        SELECT
        ST_IsClosed(ST_MakeLine(ARRAY[ST_MakePoint(20,50),
        ST_MakePoint(19.95,49.98),   
        ST_MakePoint(19.90,49.96),ST_MakePoint(20,50),
        ST_MakePoint(20,50)]));
        st_isclosed
        -------------
        t

PostGIS can also snap the ends of a ring to create a polygon from an unclosed LineString. Just replace ST_MakePolygon with ST_Polygonize:

        SELECT
        ST_Polygonize(ST_MakeLine(ARRAY[ST_MakePoint(20,50),
        ST_MakePoint(19.95,49.98),   
        ST_MakePoint(19.90,49.96),ST_MakePoint(20,50),
        ST_MakePoint(20.01,50.01)]));
         st_polygonize    
         --------------------
         010700000000000000
        (1 row)

Beware though--this is an automated process, and for some shapes it can lead to undesirable effects, like weirdly-shaped MultiPolygons.

1. For constructing a polygon with holes, the interior ring(s) should be supplied as an ARRAY:

       SELECT ST_MakePolygon(
       ST_MakeLine(ARRAY[ST_MakePoint(20,50),ST_MakePoint(19.95,49.98),             
       ST_MakePoint(19.90,49.90),ST_MakePoint(20,50)]),    
       ARRAY[ST_MakeLine(ARRAY[ST_MakePoint(19.95,49.97),ST_MakePoint
       (19.943,49.963),     
       ST_MakePoint(19.955,49.965),ST_MakePoint(19.95,49.97)])]
       );