viewshed analysis

Malcolm Williamson malcolm at
Mon Jan 18 14:41:42 EST 1993

We are currently utilizing viewshed analysis as a cultural resource management
tool. We are using r.los, but we have ideas for a couple of functions that this
program doesn't perform. Has anyone out there developed or modified GRASS 
programs for either of the following functions? (or simply considered the
necessary algorithms!):

1) Rather than return the viewing angle of viewable cells as r.los does, return 
the additional elevation necessary to make non-viewable cells viewable. This 
would tell you how tall a structure could be built behind, say, a ridge before
it would be visible from a particular vantage point.

2) Some objects, such as winter vegetation, provide only a partial hindrance to
viewability. By referencing a second map layer that contained visual impedance
data of such objects (0=no visual impedance, 10=complete visual impedance?) the
viewshed-generating program would restrict the MAXIMUM viewable distance of 
cells that would be viewed through these objects. As an example, viewsheds
generated for summer might have visibility entirely blocked by hardwood foliage,
while the same viewsheds generated for winter might show limited visibility 
through the same trees.

Any programs, algorithms, or thoughts on these functions would be much 
appreciated, before we start burning any programming hours. Thanks!


Malcolm D. Williamson - Technical Assistant      E-mail: malcolm at
Center for Advanced Spatial Technologies      Telephone: (501) 575-5736
Ozark Rm. 12                                        Fax: (501) 575-3846 
University of Arkansas              
Fayetteville, AR 72701


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