[postgis-users] ST_UNION drops M coordinate

[Paul Ramsey]

Yes, we just don't support the higher dimension in the operation. GEOS
was extended to 3 dimensional handling some time ago, but never got
pushed farther when we added M at postgis 1.0. Probably fixable, but
with a reasonable amount of careful work to catch every case.

P

On Wed, Aug 5, 2009 at 10:22 AM, Satish Murthy<satish_m at hotmail.com> wrote:
>
> When I do this (notice the second ST_COLLECT):
> select ST_AsEWKT (ST_COLLECT (geom)) from (
> 		select (ST_DUMP(geom)).geom from (select p_geom as geom from (
> 			select ST_COLLECT (geom_col, st_GeomFromEWKT('Point(1 -1 0 10)')) as
> p_geom from tab) a
> 		)as b ) as c
> I get:
>     "MULTIPOINT(71.1 -42.31 10 10,1 -1 0 10,1 -1 0 10)"
> This is fine, except that "1 -1 0 10" gets repeated.
>
> BUT---
> When I do (I use ST_UNION here):
> select ST_AsEWKT (ST_COLLECT (geom)) from (
> 		select (ST_DUMP(geom)).geom from (select p_geom as geom from (
> 			select ST_UNION (geom_col, st_GeomFromEWKT('Point(1 -1 0 10)')) as p_geom
> from tab) a
> 		)as b ) as c
> I get this:
>    "MULTIPOINT(1 -1 0,71.1 -42.31 10)"
> No repetition, but the M coordinate is missing.
>
> Any pointers on what's going wrong here?
>
>
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