[postgis-devel] Issue 51 in postgis: Enhancement to ST_Summary to include info about invalid geometries

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Comment #8 on issue 51 by ke... at refractions.net: Enhancement to ST_Summary  
to include info about invalid geometries
http://code.google.com/p/postgis/issues/detail?id=51

I'm just getting around to testing this.  Good work Paul.  It does seem  
like a good
start in a collection of geometry cleanup functions.

For my use cases, however, it would need to be a set returning function  
that returns
all the locations where a geometry falls down ... as geometries.

ie.
select st_isvalidreason(
   'POLYGON (( 0 0, 0 4, 4 4, 4 0, 0 0 ),
             ( 3 5, 2 5, 2 6, 3 6, 3 5 ),
             ( 2 2, 2 3, 3 3, 5 2, 2 2 ))'::geometry);
      st_isvalidreason
---------------------------
  Self-intersection [4 2.5]
(1 row)

should also report a 'Hole lies outside shell [3 5]' ... but this is  
expected since
this function is not a set returning function.

We already have ST_Dump, ST_DumpRings, (and ST_DumpPoints on the  
wishlist).  It would
be really nice to add ST_DumpInValidReason that returns a 'setof  
geometry_dump'.  IE.
return all the self-intersection points/lines, all holes that are outside,  
etc.

-- Queries like this would then be possible.
SELECT feat_id, (ST_DumpInValidReason(the_geom)).geom
FROM myfeatures
WHERE NOT ST_IsValid(the_geom);

Martin says that this information is available in JTS, just not exposed  
yet, so it
would be some work to get it into PostGIS.  But, does this sound like a  
do-able thing
to add to the wishlist?  I know I've definitely needed something like this  
in the
past.  Is this overloading the is_valid concept too much?  I'm just  
thinking that
having an exhaustive set of cleaning functions would be a very useful thing  
to add to
PostGIS.

In the mean time, is it possible the prototype of this function change to  
something
that returns a geometry instead of a textual description?  Perhaps both ..  
a single
instance of
CREATE TYPE invalid_reason AS
    (description text,
     geom geometry);

-- Kevin

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