[postgis-users] Problem with ST_Within, Polygons and Multipolygons

Jorge Arévalo jorge.arevalo at gmail.com
Thu May 7 03:09:25 PDT 2009


I have additional info. When running the method "isvalid", I get this
warning message:

NOTICE:  Self-intersection at or near point -2749.99 4.7955e+06

What is exactly a "self-intersection"? May this "self-intersection" be the
cause of my problem?


2009/5/6 Jorge Arévalo <jorge.arevalo at gmail.com>

> Hi,
> Thanks for your response Kevin. I can provide some data:
> - Screenshot (over geoserver) of the "water" multipolygon. You can see the
> shape of Spain and the Balearic Islands, and a hole on the left (Portugal).
> The multipolygon is open, isn't it? -->
> http://www.nebulared.com/tmp_geo/water_multipolygon.jpg
> - Zoom over Menorca (one of the Balearic Islands). As you can see, the
> point tested is inside the "airports" multipolygon, but ST_Within and
> ST_Contains returns that the point is inside both, "water" and "airports"
> multipolygons -->
> http://www.nebulared.com/tmp_geo/zoom_in_water_multipolygon.jpg
> - The "ST_AsText" data for the "airport" multipolygon -->
> http://www.nebulared.com/tmp_geo/airports_multipolygon.txt
> - The "ST_AsText" data for the "water" multipolygon (rar file, 2.5 MB of
> plain text...) -->
> http://www.nebulared.com/tmp_geo/water_multipolygon.rar
> So, how can I check the validity of my multipolygons? Do someone could help
> me with this?
> Now, the query. My context is the next:
> - I have a table with polygons that I want to check (to say if they are in
> an airport, in a forest, in water, etc). We can call this table T1. I use
> the centroid of the polygons to check, not the entire polygon.
> - In other different table, that we'll call T2, I have all the
> multipolygons ("water", "airports", etc). The name of the field that says if
> the multipolygon is "water" or "airport" or anything is "clutter"
> With this code, I pretend to loop over T1, checking if the centroid of its
> polygons are inside of one (and ONLY one) of the multipolygons of T2.  (Is
> part of a procedure)
> (...)
>         select clutter from T2 where
> ST_Within(ST_Centroid(T1.wkb_geometry), T2.wkb_geometry))
>         (...)
> END;
> Most of the times, the select inside the loop, returns more than one result
> ("water" and other one, normally). That's my problem
> I have another version of the query, with Java. Again, I loop over the
> table T1, and check the centroid of its polygons with all the multipolygons
> (water, airports...) that are in T2. Then, my query, inside the loop, is:
> SELECT clutter FROM T2 WHERE ST_Within(?, T2.wkb_geometry)
> The '?' is replaced with the centroid of the polygon in T1, of course.
> Basically, is the same idea of the previous SQL code, but with Java.
> Apart from this, other problem is that ST_Within takes a long time. 1 sec
> per query. And I have like 2 millions of polygons (queries) to check...
> My multipolygons' table (T2) has only 29 rows (small table with large
> geometries), and I found this
> http://postgis.refractions.net/documentation/manual-1.3/ch05.html. Seems
> to be good for me. But using "SET enable_seqscan TO off", the queries take
> the same time (~ 1 sec). Maybe the other approach (create an additional
> column that "caches" the bbox, and matching against this) could help me. Any
> ideas on how to create this bbox with my huge multipolygons?
> Thanks in advance
> Regards
> Jorge
> On Wed, May 6, 2009 at 4:28 AM, Kevin Neufeld <kneufeld at refractions.net>wrote:
>> 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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