[postgis-users] how to select points outside polygons?

[Stephen Woodbridge]

On 9/29/2011 5:55 PM, Vishal Mehta wrote:
> *Hi Steve, *
>
> **
>
> I tried that
>
> select powerplants.gid, plantname
>
>       from powerplants, pca_huc
>
>     where state ='CA'
>
>                 AND NOT st_contains(pca_huc.geom,powerplants.geom)
>
>     order by powerplants.gid;
>
> but got some 44,000 rows in the result. There are 1477 powerplants
> (points) and 299 pca_huc polygons. The powerplants don’t all fall inside
> the polygons, and I want to get the list of those that fall outside.
>
> In a desktop gis I can quickly select and see that 102 powerplants fall
> outside: so I should not be getting 44,0000 rows in the result…

Ahhh! that is a different problem! you want the points the do not fall 
in ANY polygon, so you need to first union or collect all you polygons 
into a single object and then ask which point are out that object.

So maybe this will work:

select powerplants.gid, plantname
   from powerplants,
        (select st_collect(geom) as geom from pca_huc) as pca
  where state ='CA'
        AND NOT st_contains(pca.geom,powerplants.geom)
  order by powerplants.gid;

If that does not work then try:

select powerplants.gid, plantname
   from powerplants,
        (select st_union(geom) as geom from pca_huc) as pca
  where state ='CA'
        AND NOT st_contains(pca.geom,powerplants.geom)
  order by powerplants.gid;

-Steve

> V
>
> **
>
> **
>
> Did you try:
>
>
>
> select powerplants.gid, plantname
>
>       from powerplants, pca_huc
>
>     where state ='CA'
>
>                 AND NOT st_contains(pca_huc.geom,powerplants.geom)
>
>     order by powerplants.gid;
>
>
>
> This should give you all power plants in CA and not in your contains
>
> clause. What is the coverage of pca_huc table? If this covers all of CA
>
> then there is not possible result. The would be like say where a=1 and
>
> not a=1.
>
>
>
> -Steve W
>
>
>
> On 9/29/2011 1:21 PM, Vishal Mehta wrote:
>
>>/  I have a point layer and a polygon layer. How do I select points that/
>
>>/  are NOT contained within polygons?/
>
>>/      /
>
>>/  For example, this query correctly selects points contained by polygons/
>
>>/      /
>
>>/  --/
>
>>/      /
>
>>/  select powerplants.gid, plantname/
>
>>/      /
>
>>/  from powerplants, pca_huc/
>
>>/      /
>
>>/  where state ='CA'   /
>
>>/      /
>
>>/  AND/
>
>>/      /
>
>>/  st_contains(pca_huc.geom,powerplants.geom)/
>
>>/      /
>
>>/  order by powerplants.gid;/
>
>>/      /
>
>>/  --/
>
>>/      /
>
>>/  I tried to get the inverse selection using st_disjoint instead of/
>
>>/  st_contains , but that did not work. I also tried using NOT and EXCEPT/
>
>>/  without success (although I may not have constructed the latter queries/
>
>>/  correctly..)/
>
>>/      /
>
>>/  Thanks!/
>
>>/      /
>
>>/  Vishal/
>
> /Vishal K. Mehta <http://sei-us.org/about/staff_person/19>, Ph.D. /
>
> /Staff Scientist, /
>
> /Stockholm Environment Institute-US /
>
> /400 F St, Davis, CA 95616 /
>
> /http://sei-us.org/ /
>
> //
>
>
>
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