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STFilterRings: Removing rings from Polygon based on area.

A function that have found use in Oracle and PostGIS is one that allows a user to filter out the rings of a polygon or multipolygon based on area.

Here is presented a function to do this in SQL Server Spatial. This function requires Denali (2012) as it uses unionAggregate. In addition, the function requires my Extract function.

 USE [GISDB]  -- Change to your database
 GO
 -- Drop dbo.STFilterRings
 --
 DROP   FUNCTION dbo.STFilterRings;
 -- Create dbo.STFilterRings
 --
 CREATE FUNCTION dbo.STFilterRings(
   @p_geometry geometry,
   @p_area     FLOAT
 )
 RETURNS geometry
 AS
 BEGIN
   DECLARE
      @v_filtered_geom geometry;
   BEGIN
     IF ( @p_geometry IS NULL )
       RETURN NULL;
     IF ( @p_geometry.STGeometryType() NOT IN ('CurvePolygon','Polygon','MultiPolygon') )
     RETURN @p_geometry;
     SELECT @v_filtered_geom = a.outer_rings.STDifference(f.inner_rings)
       FROM (SELECT geometry::UnionAggregate(e.geom) AS outer_rings
               FROM dbo.EXTRACT(@p_geometry,1) AS e
        WHERE e.sid = 1
          AND e.geom.STArea() > @p_area
           ) AS a,
            (SELECT geometry::UnionAggregate(d.geom) AS inner_rings
               FROM (SELECT e.geom
                       FROM dbo.EXTRACT(@p_geometry,1) AS e
                  WHERE e.sid <> 1
                        AND e.geom.STArea() > @p_area
                    ) AS d
            ) f;
     RETURN @v_filtered_geom;
   END;
 END
 GO

Some tests.

Polygon

 -- Original
 --
 SELECT geometry::STGeomFromText('POLYGON((0 0,20 0,20 20,0 20,0 0),(10 10,10 11,11 11,11 10,10 10),(5 5,5 7,7 7,7 5,5 5))',0) AS geom;

Visually this looks like this.:

 -- Rings <= 2sq M removed
 --
 SELECT dbo.STFilterRings(geometry::STGeomFromText('POLYGON((0 0,20 0,20 20,0 20,0 0),(10 10,10 11,11 11,11 10,10 10),(5 5,5 7,7 7,7 5,5 5))',0),2) AS geom

Visually, this looks like this:

MultiPolygon

 -- Original
 --
 SELECT a.geom.STArea() AS area, a.geom
   FROM (SELECT geometry::STGeomFromText('MULTIPOLYGON (((0 0,20 0,20 20,0 20,0 0),(10 10,10 11,11 11,11 10,10 10),(5 5,5 7,7 7,7 5,5 5)), ((30 30, 50 30, 50 50, 30 50, 30 30)), ((0 30, 20 30, 20 50, 0 50, 0 30)), ((30 0,31 0,31 1,30 1,30 0)))',0) AS geom) AS a;

This looks like this.

area geom
1196 0×0000000001041E0000…..

Visually….

 -- Rings <= 2sq M removed
 --
 SELECT e.geom.STArea() AS area, e.geom
   FROM dbo.EXTRACT(dbo.STFilterRings(geometry::STGeomFromText('MULTIPOLYGON (((0 0,20 0,20 20,0 20,0 0),(10 10,10 11,11 11,11 10,10 10),(5 5,5 7,7 7,7 5,5 5)), ((30 30, 50 30, 50 50, 30 50, 30 30)), ((0 30, 20 30, 20 50, 0 50, 0 30)), ((30 0,31 0,31 1,30 1,30 0)))',0),2.5),1) AS e

This looks like this:

area geom
400 0×0000000001040500…
400 0×0000000001040500…
400 0×0000000001040500…
4 0×0000000001040500…

Visually….

I hope this is useful to someone.