[postgis-users] RE: PostGIS cookie cutter

[Kevin Neufeld]

Hi,

First Last wrote:

> i'm a novice PostGIS user, can anybody help me do a cookie-cutter 
> technique in PostGIS? basically, i have a polygon table of political 
> boundary and i want to extract all the polygons that lie within the 
> cookie cutter (which can be a single polygon, or another polygon 
> table). it seems the intersection command works only when the 
> operation is between two polygons but not when a collection of 
> polygons (e.g., table) is involved. by the way, i'm assuming the 
> intersection command is what you use for cookie-cutting?


CREATE TABLE boundary_cutter_relationships AS
  SELECT a.boundary_id,
         b.cutter_id
  FROM   political boundary a,
         cookie_cutter b
  WHERE  a.the_geom && b.the_geom
  AND    intersects(a.the_geom, b.the_geom);

This will create a relationship table between your two polygon tables 
... provided your boundaries and "cookie cutter" polygons have unique ids.
Now, if you wanted to know what boundaries intersect your cutter of 
interest ...
SELECT boundary_id
FROM boundary_cutter_relationships
WHERE cutter_id = ??;

> on the same note, how do i combine two tables of the same geometry type? 

Create a new table with a geometry column.
Insert into the new table values from both tables.

OR

SELECT attribute1, attribute2, the_geom
FROM table1
UNION
SELECT attribute1, attribute2, the_geom
FROM table2;

> also can they be added together even if they don't have the same 
> attribute columns (except for the geometry type). 

No. (http://www.postgresql.org/docs/8.1/static/queries-union.html)

> i'm coming from a ESRI/shapefile background so please bear with me.
>
> any help will be appreciated. thank you.
>
>
>
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Hope this helps.
Cheers,
Kevin

-- 
Kevin Neufeld,
Refractions Research Inc.,
kneufeld at refractions.net
Phone: (250) 383-3022 
Fax:   (250) 383-2140