Hi everybody,
I have a general question about the behaviour with 2D lines intersecting 3D
polygons. In my example I want to filter walls of a building which are crossed
by a shortest line between address point and street geometry. When regarded in
2D most 3D polygons are invalid, of course. Those polygons which have "too few
points" in 2D (e.g. a rectangle of 5 points) do not return any intersection
with a crossing linestring. My question is: How is this polygon then handled
internally because functions like ST_Relate still return a result and not an
exception.
SELECT ST_Relate('POLYGON Z ((0 1 1,2 1 1,2 1 2,0 1 2,0 1
1))'::geometry,'LINESTRING (1 0,1 3)'::geometry);
-- returns FFFFFF102 = disjoint, even though the geometries overlap in 2D space
When there are more points, a self-intersection is reported as expected. But,
PostGIS' topology functions can still operate on these invalid polygons and
return TRUE on intersection.
SELECT ST_Relate('POLYGON Z ((0 1 1,2 1 1,2.1 1 1,2.1 1 2,2 1 2,0 1 2,0 1
1))'::geometry,'LINESTRING (1 0,1 3)'::geometry);
-- returns 1F20F0102
One trick is to extract the exteroir ring of the polygons to get around the
"too few points" issue. They I get the result I want. But I thought it might be
an interesting question, if the described behaviour is really consistent.
Kind regards
Felix
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