[postgis-users] is ST_ConvexHull supposed to work like this?

[Frans Knibbe]

Hello,

I am looking for a way to describe a 3D feature that has only partial
geometric coverage with a contiguous geometry. It seemed to me that
ST_ConvexHull could be helpful there. But the results are not as expected.
So I tried a simple test query to get the convex hull of the eight corners
of a cube. The expected result would be a geometry describing the faces of
the cube but instead I get only the bottom face:

*query:*

select st_astext(st_convexhull(st_geomfromtext(
'MULTIPOINTZ(
0 0 0
,1 0 0
,1 1 0
,0 1 0
,0 1 1
,1 1 1
,1 0 1
,0 0 1
)'
)));

*result:*

POLYGON Z ((0 0 0,0 1 0,1 1 0,1 0 0,0 0 0))

This is with PostGIS 2.2.1.

The current description of ST_ConvexHull reads "The convex hull of a
geometry represents the minimum convex geometry that encloses all
geometries within the set". It also says "This function supports 3d". The
test result seems to contradict those statements.

So is ST_ConvexHull operating as expected? If it is, could there be another
way to get the desired result in PostGIS?

Regards,
Frans
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