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

[Jorge Arévalo]

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)

(...)
BEGIN
  FOR s IN SELECT * FROM T1 LOOP
        select clutter from T2 where ST_Within(ST_Centroid(T1.wkb_geometry),
T2.wkb_geometry))
        (...)
  END LOOP;
  RETURN;
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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