[postgis-users] Viewshed Isovist

[Martin Davis]

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