[postgis-users] Problems with ST_Within in an island of a multipolygon

[Larry Reeder]

I have a layer called "foo" of about 5000 polygons.   One of the
polygons is a multipolygon with island shells.  ST_Within isn't
working as expected for points within that island shell.


Here's my table:

foo_spatial=# \d foo
                              Table "public.foo"
  Column  |   Type   |                        Modifiers
----------+----------+---------------------------------------------------------
 gid      | integer  | not null default nextval('foo_gid_seq'::regclass)
 the_geom | geometry |
Indexes:
    "foo_pkey" PRIMARY KEY, btree (gid)
    "foo_the_geom_key" gist (the_geom)
Check constraints:
    "enforce_dims_the_geom" CHECK (st_ndims(the_geom) = 2)
    "enforce_geotype_the_geom" CHECK (geometrytype(the_geom) =
'MULTIPOLYGON'::text OR the_geom IS NULL)
    "enforce_srid_the_geom" CHECK (st_srid(the_geom) = (-1))


ST_Within returns true when asked if a point inside the island belongs
to the polygon that contains the island:

foo_spatial=# select ST_Within(ST_GeomFromText('POINT (-115.04252
36.05137)', -1), the_geom) from foo where gid = 4933;
 st_within
-----------
 t
(1 row)

However, when I ask if any polygons contain this point, I don't get any back:

foo_spatial=#  SELECT gid from foo where
ST_Within(ST_GeomFromText('POINT (-115.04252 36.05137)', -1),
the_geom);
 gid
-----
(0 rows)


Does this make sense?    I know PostGIS can handle the general problem
of ST_Within for island sub-polygons because when I extract this
multipolygon into a separate table by itself, the queries work as
expected.  There seems to be something happening with other polygons
in the layer that is breaking this.   Initially I did have some bad
polygons in the layer, but I used the cool script here:
http://linfiniti.com/2010/07/cleaning-geometries-inside-postgis/.
That fixed my bad polygons, and now all polygons are valid:

foo_spatial=# select count(*) from foo where ST_IsValid(the_geom) = 'f';
 count
-------
     0
(1 row)


But still ST_Within isn't working as expected.   Here's my version info:

foo_spatial=# select version();
                                                       version
---------------------------------------------------------------------------------------------------------------------
 PostgreSQL 8.4.7 on x86_64-pc-linux-gnu, compiled by GCC gcc-4.4.real
(Ubuntu/Linaro 4.4.4-14ubuntu5) 4.4.5, 64-bit
(1 row)

foo_spatial=# SELECT postgis_full_version();
                                         postgis_full_version
-------------------------------------------------------------------------------------------------------
 POSTGIS="1.5.1" GEOS="3.2.0-CAPI-1.6.0" PROJ="Rel. 4.7.1, 23
September 2009" LIBXML="2.7.6" USE_STATS
(1 row)

dash_spatial=# select count(*) from foo where ST_IsValid(the_geom) = 'f';
 count
-------
     0
(1 row)


Anyone have any ideas?



Thanks............                            Larry