[postgis-users] Problem with ST_ExteriorRing and ST_contains

Nicolas Ribot nicolas.ribot at gmail.com
Wed May 19 01:51:03 PDT 2010

> Hi everybody,
> I'm a postgis new user and I would like to submit here a problem I encounter.  Here's what I do :
> from a polygon layer I merge some of them (with ST_UNION) into bigger ones according to an attribute value, then I make an exterior ring (with ST_Collect(ST_ExteriorRing())) around these and then I fill these rings (with ST_Contains) with all the original polygon that are inside ( I do this to have a coherent area without the holes I would have otherwise because of some data particularities).
> All this works very fine for 99.9 % of the polygons but fails on filling the rings for a few of them.  When I look at these I see that they have a loop which is a probably due to a wrong geometry in the original polygons.
> Does any of you know how to deal with this ?
> Thanks for your help

You should first check the geometry validity (st_isvalid,
st_isValidReason) and isolate invalid geometries to correct them
Some "tricks" can correct them automagically (st_buffer(geom, 0.0) for
instance) and some programs can help you identifiy invalid geometries
(Jump for instance, with its validation plugin).
Mailing list contains useful hints on how to correct invalid geometries.



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