Calculating the outer boundary of several Geometry objects in SQL Server 2008

Question

I have lots of Polygons of datatype Geometry in SQL Server 2008. The image below shows how a select of all these Geometrys looks visualized.

What I need to do is create a Polygon which represents the outer boundary of all these polygons. So I used the response given to a previous spatial question I asked to create the following code:

DECLARE @test TABLE(geom GEOMETRY);

INSERT INTO @test SELECT geom FROM ForceBoundary

DECLARE @geom GEOMETRY
SELECT @geom = (SELECT TOP 1 geom FROM @test)
SELECT @geom = @geom.STUnion(geom) FROM @test

SELECT @geom

This produced the following result, which has cracks in it due to holes between the polygons:

So I updated my query with the following change:

INSERT INTO @test SELECT geom.Reduce(0.001).STBuffer(100) FROM ForceBoundary

Which improved the result, however it's not solving the issue completely and it also damages the outer boundary accuracy.

What is the correct way to achieve this? From looking through a list of the STxxxx functions I couldn't see one that seemed to provide the results I need?

Answer 1

Answer provided by geographika at GIS StackExchange:

It sounds like you want to remove slivers. There is a function for this in the SQL Server Spatial Tools project - FilterArtifactsGeometry.

A blog post on using the function can be found here.

This has an option for filtering out small poylgons using the ringTolerance parameter:

Remove all polygon rings thinner than provided tolerance (e.g. ring.STArea < ringTolerance x ring.STLength). A value of 0 will not remove any rings.

In practice, this allows very thin polygon rings (slivers) to be detected and removed while leaving more typically shaped polygon rings alone. The presumption is, of course, that slivers are undesirable but non-sliver rings are desirable.