This seems like a fairly complex spatial computation, so I don't think
you'll find anything in PostGIS to help out with it. There are probably
some standard algorithms for computing this kind of polygon - a Web
search might turn up something.
Regina - at first I thought Difference was a possibility here, but then
I noticed that the precise shape of the viewshed in the gaps between
buildings depends on the relative location of the viewing line segment,
in a subtle way.
diplonics wrote:
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
--
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022
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