PostGIS - ST_PointOnSurface and ST_Centroid dropping Z

Question

I am using Postgres 9.5.2 and PostGIS 2.2.2.

According to the documentation ST_PointOnSurface should not drop Z. This is what happens. If I run

SELECT ST_AsText(cell) AS pt FROM cells LIMIT 1

I get

POLYGON Z ((157.215247156568 3779.5275740661 76.0620819960913,157.215247156568 3937.00787401575 76.0620819960913,3.4336355447806e-07 3937.00787401575 66.9291338782187,3.43363565136201e-07 3779.5275740661 66.9291338782187,157.215247156568 3779.5275740661 76.0620819960913))

as expected (Z conserved). However, if I run

SELECT ST_AsText(ST_PointOnSurface(cell)) AS pt FROM cells LIMIT 1

or

SELECT ST_AsText(ST_Centroid(cell)) AS pt FROM cells LIMIT 1

I get

POINT(78.6076237499659 3858.26772404092)

and the 3rd coordinate is gone.

Am I doing something wrong?

Answer 1

The ST_PointOnSurface documentation doesn't specifically mention POLYGON Z, so assume it doesn't work for now.

The ST_Centroid documentation doesn't mention Z at all.

However, you can incorporate ST_3DClosestPoint into your statement to get the closest 3D point between the cell and its center (or point on surface).

SELECT ST_AsText( ST_3DClosestPoint(cell,ST_PointOnSurface(cell))) AS pt
FROM cells LIMIT 1

SELECT ST_AsText( ST_3DClosestPoint(cell,ST_Centroid(cell))) AS pt
FROM cells LIMIT 1

POINT Z (78.6076237499658 3858.26772404092 71.495607937155)

Here is a list of all 3D functions