[postgis-users] ST_Intersects behaviour with 3D Polygon and Linestring

[Felix Kunde]

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