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I've been looking for some function that will consider geometries from a table.

The selection would look something like this:

SELECT geom FROM mytable WHERE geoid IN (1, 10, 15, 30, 2)

How can I tell if the geometries are "contiguous" (sharing at least one point or boundary with no gaps, i.e. geometries are strung together with no breaks).

For more clarity: Assume we use the "^" as a triangle and the SELECT statement above gives this result of three geometries touching and two touching geometries by themselves: ^^^ ^^ The goal is to determine that the above condition is not contiguous, but the following would be: ^^^^^

  • Probably this is hard to do with SQL Server, but if you convert the polygon coverage into a graph and that does not have isolated islands then all boundaries are connected. – user30184 May 11 '16 at 17:55
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If the area of the ST_Union equals the sum of the areas of each polygon. I think you can conclude there are no overlaps.

As I recall if you union two triangles whose corners touch, the result has two rings.

If you buffer the union by some small (epsilon) amount and the resulting geometry/geography has only one exterior ring, I think you could conclude they are contiguous.

  • Thank you! Setting a buffer on all the shapes and use ST_Union, then count the shapes -- if the count is greater than 1, then the shapes (in combination) are not contiguous. – Mike McWilliams May 20 '16 at 14:51

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