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I'm currently struggling to calculate the area of a slightly non-planar surface in PostGIS. The area is a wall surface from a LOD2 data source.

Here's an example:

SELECT ST_AsText(ST_DelaunayTriangles(ST_GeomFromewkEWKT('SRID=25832;MULTIPOINT(613595.706 5461088.407 428.479,613595.706 5461088.407 421.196,613598.625 5461091.158 421.196,613601.559 5461093.923 421.196,613601.559 5461093.923 428.452,613598.625 5461091.158 433.68,613595.706 5461088.407 428.479)')));

The result of this query is:

GEOMETRYCOLLECTION EMPTY

Which is unexpected for me.

Based on my understanding of https://postgis.net/docs/ST_DelaunayTriangles.html I would expect ST_DelaunayTriangles() to return a set of triangles.

Is my understanding wrong, is there a problem with my usage or is this a bug?

I'm using PostGIS 3.4.0 with SFCGAL 1.3.8.

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  • That geometry seems to be vertical and there are no triangles in X-Y plane. PostGIS is not a real 3D program. I edited of the X or Y coordinates and got a couple of triangles SELECT ST_DelaunayTriangles(ST_GeomFromText('MULTIPOINT Z(613595.706 5461088.407 428.479,613595.706 5461088.407 421.196,613598.625 5461091.158 421.196,613601.559 5461093.923 421.196,613601.559 5461093.923 428.452,613598.625 5461191.158 433.68,613595.706 5461088.407 428.479)'))
    – user30184
    Commented Dec 7, 2023 at 13:10
  • Thanks @user30184 for the explanation. If I got you right then ST_DelaunayTriangles requires the triangles to lay in the XY-plane. That's a bummer, but brings me to an idea (will try it out later). If I rotate the wall to lay rougly lay flat delaunay should work and (hopefully) the area calculation will still stay somewhat accurate.
    – Chris
    Commented Dec 8, 2023 at 9:26
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    So, a not completely generic but for my use-case (vertical surfaces) applicable work-around is to flip the x (or y) and z coordinates, e.g. ST_3darea(ST_DelaunayTriangles(ST_GeomFromText(....))) .
    – Chris
    Commented Dec 8, 2023 at 9:42

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