Jorge Arévalo wrote:
... If is useful, I tested the method "ST_isvalid" with the
multipolygon and returns "false". Maybe the multipolygon is not
closed? I loaded the data from a shapefile. Is it possible to create a
"non-valid" multipolygon? Does PostGIS accept this?
Ah, yes. Most spatial predicates in PostGIS assume the input geometry
is valid. Suppose you had defined a POLYGON with a hole or inner ring
outside of the exterior ring. What is the area? Does the question even
make sense? PostGIS allows invalid geometries in the database so users
can make full use of the PostGIS toolset to do whatever they need (ie.
breakdown a polygon to it's constituent linework and rebuild it back up
again to a valid polygon)
Ok, being even more specific. I'm working with data about Spain. I
have a HUGE multipolygon that represents "water" (this is, the coasts
around Spain and its islands). Then, the "holes" inside this
multypolygon have the shape of Spain, Balearic Islands and Canary
Islands. Of course, I have more multipolygons that represent
"forests", "airports", "cities", etc, that fit into these holes.
Really, my problem is with some points that belong to an airport in an
island. Using "ST_Within" and "ST_Contains", the result is that these
points belong to the multipolygon "airport" and multipolygon "water"
at same time. Obviously, the island (and its airport) is surrounded by
water, but the airport's points shouldn't be part of the multopolygon
"water". And, as I said, when I apply "ST_isvalid" to the multipolygon
"water", returns false. Maybe is not closed?
Yeah, as mentioned before, a point that is not on the surface of a
(multi)polygon (whether in a hole or completely outside) is not
considered within the (multi)polygon. It does sound like your ocean
polygon has validity issues.
Oh, btw, what's the difference between "ST_Within" and "Within". Does
"ST_Within" use index instead of geometry? Am I right?
No, not instead of. Both use the actual geometry for testing within.
ST_Within will also use the index to narrow down the candidate list first.
As you can see, the definition of ST_Within is just a simple SQL wrapper
that first invokes the index.
postgis=# select prosrc from pg_proc where proname = 'st_within';
prosrc
---------------------------------------
SELECT $1 && $2 AND _ST_Within($1,$2)
(1 row)
http://postgis.refractions.net/documentation/manual-svn/ST_Within.html
Cheers,
Kevin
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