postgis (INTERSECTION)

[Matthew Perry]

Thip,

On 8/30/05, Thip <thip at ait.ac.th> wrote: I need to create one layer on fly 
using intersection. (Layer name are twy_sec and twy_su, new layer name is 
twy_intersection) SO I write down in the map file which is not working. Does 
anyone have suggestion?


At the minimum you'll need to include a CLASS item in your LAYER.

CLASS
COLOR 255 0 0 
OUTLINECOLOR 0 0 0
END

In addition you might want to check the postgis connection string. Three 
things that jump out at me are that 

1) you must use "using UNIQUE gid" 
2) you should probably rewrite it as nested selects like "SELECT geom from 
(SELECT intersect(...) as geom from ...) as table using unique gid using 
srid = -1"
3) be forewarned that intersections in postgis can be painfully slow. 
Depending on your users need and the size of the table, it may be worth it 
to pre-process this data if speed is a factor.

Also remember to put indexes on the gid column, and gist indexes on geom. 
Check out the postgis docs for more details - 
http://postgis.refractions.net/docs/

Hope this helps,
-- 
Matt Perry
perrygeo at gmail.com
http://www.perrygeo.net
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