[postgis-users] GeometryCollection after ST_Centroid(ST_Dump)

[Obe, Regina]

Birgit,

So you are saying ST_Centroid sometimes returns geometry collections.
Hmm I don't see how that is possible unless there is bug somewhere.

Can you provide an example of that.  Regarding the below query - it
would be a bit easier to follow if you did a subselect first.

Something like

 SELECT
  abdump.gid,
  abdump.objart_07,
 abdump.cat_07,
ST_X(ST_Centroid(abdump.newgeom)) As x_centroid,
ST_Y(ST_Centroid(abdump.newgeom)) As y_centroid
ST_Area(ST_Centroid(abdump.newgeom)) As area,
ST_Perimeter(ST_Centroid(abdump.newgeom)) as perimeter
FROM (SELECT (ST_Dump(ST_Intersection(a.the_geom, 
b.the_geom))).geom As newgeom, a.gid, a.objart_07, a.cat_07, a.the_geom
as a_the_geom, b.the_geom as b_the_geom
FROM 
 bfn.test_mv_dlm07 a, bfn.test_mv_dlm07 b
WHERE ST_Overlaps(a.the_geom,b.the_geom)=true AND 
Intersects(a.the_geom,b.the_geom)=true
) As abdump
GROUP BY ST_X(ST_Centroid(abdump.newgeom)), 
ST_Y(ST_Centroid(abdump.newgeom)), ST_Area(ST_Centroid(abdump.newgeom)),

ST_Perimeter(ST_Centroid(abdump.newgeom)), abdump.gid, abdump.objart_07,
abdump.cat_07, a_the_geom, b_the_geom
ORDER BY x_centroid asc, y_centroid asc, area asc, perimeter asc, a.gid
asc;

Hope that helps,
Regina

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
Birgit Laggner
Sent: Tuesday, October 14, 2008 6:56 AM
To: PostGIS Users Discussion
Subject: [postgis-users] GeometryCollection after ST_Centroid(ST_Dump)

Dear list,

I have a SELECT statement in which I try to group by the polygons 
resulting from a self intersection. After I realized that I can't group 
by the_geom because this would group polygons which are only similar but

not equal, I decided to group by the area, the perimeter and the x- and 
y-coordinates of the centroid of every polygon. The SELECT statement I 
wrote, worked so far for some test datasets and showed seemingly 
reasonable results. But for at least one of my datasets, the statement 
does not work, because the ST_Centroid produces some GeometryCollections

instead of POINTS. What could cause that problem and how can I solve
it???

This is my statement:

SELECT
  a.gid,
  a.objart_07,
  a.cat_07,
  ST_X(ST_Centroid((ST_Dump(ST_Intersection(a.the_geom, 
b.the_geom))).geom)) as x_centroid,
  ST_Y(ST_Centroid((ST_Dump(ST_Intersection(a.the_geom, 
b.the_geom))).geom)) as y_centroid,
  ST_Area((ST_Dump(ST_Intersection(a.the_geom, b.the_geom))).geom) as
area,
  ST_Perimeter((ST_Dump(ST_Intersection(a.the_geom, b.the_geom))).geom) 
as perimeter,
  (ST_Dump(ST_Intersection(a.the_geom, b.the_geom))).geom as the_geom
FROM bfn.test_mv_dlm07 a, bfn.test_mv_dlm07 b
WHERE ST_Overlaps(a.the_geom,b.the_geom)=true AND 
Intersects(a.the_geom,b.the_geom)=true
GROUP BY ST_X(ST_Centroid((ST_Dump(ST_Intersection(a.the_geom, 
b.the_geom))).geom)), 
ST_Y(ST_Centroid((ST_Dump(ST_Intersection(a.the_geom, 
b.the_geom))).geom)), ST_Area((ST_Dump(ST_Intersection(a.the_geom, 
b.the_geom))).geom), ST_Perimeter((ST_Dump(ST_Intersection(a.the_geom, 
b.the_geom))).geom), a.gid, a.objart_07, a.cat_07, a.the_geom,
b.the_geom
ORDER BY x_centroid asc, y_centroid asc, area asc, perimeter asc, a.gid
asc;

The ST_Dump results in POLYGONs, LINESTRINGs and POINTs.

Thank you for any help!

Regards,

Birgit.

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