[postgis-users] Viewshed Isovist

[Paragon Corporation]

I was thinking ST_Difference might work, but unfortunately that would return
the red regions thought it would leave out the green.

Hope that helps,
Regina

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
diplonics
Sent: Thursday, May 14, 2009 2:10 PM
To: postgis-users at postgis.refractions.net
Subject: [postgis-users] Viewshed Isovist


Hi All,
    Just wondering if this can be done in PostGIS and if not does anyone
know of the simplest method to get it done. I have 2 PostGIS polygon
geometry tables and I want to basically do is an isovist/viewshed/line of
sigth operation. I know an intersection operation can isolate where the
polys are spatially equal but what I want is some sort of set complement
operation where it will also remove all polygon sections without direct line
of sight.
Image 1 is a sample of my two data sets. The purple polys are building
outlines while the yellow poly is an unobstructed viewshed from a line.
http://www.nabble.com/file/p23546020/ViewShed01.png
Image 2 is a sample of the desired result. Using a PostGIS intersection
operation I can isolate the green sections, but I want to return a geometry
that is the yellow section with the green and red sections removed.
http://www.nabble.com/file/p23546020/ViewShed02.png 

Any advice on this problem, or links to help,  would be greatly appreciated.
Thanks

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