Does 'supports 3d' not mean in this context in the documentation: the

function does not forget about each z coordinate of the input in the end

result, but does not take it into account inside the calculation/analysis?

It seems to me that the convex hull resulting is around the x-y

coordinates, but for the extreme points the z-coordinate will be present

in the result (so convex hull while looking from above ;-)

select st_astext(st_convexhull(st_geomfromtext(

'MULTIPOINTZ(

0 0 0

,1 0 0

,1 1 0

,0 1 0

,0 1 1

,2 2 1

,1 0 1

,0 0 1

)'

)));

st_astext

---------------------------------------------

POLYGON Z ((0 0 0,0 1 0,2 2 1,1 0 0,0 0 0))

The z=1 for point 2,2,1 is preserved however...

Martijn

On 15-03-17 12:09, Frans Knibbe wrote:

> 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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