Hamish’s reply is the closest to what you want I think. However, to actually derive a buffer to define equal ground distance around a point, I think you might need a bit more explanation.
First create a slope map of your dem (r.slope.aspect). Then, as Hamish says, run r.cost. This will calcuate a ‘cost’ based on the slope of the map. He implies that the cost value in the cost map = the ground distance to traverse each pixel (i.e., higher on steep slopes where the ground distance is greater and less on flat terrain where the ground distance is lower). I don’t know if this is how the algorithm works or not (or also includes other factors), but the references he lists will clarify. If not, you can use trig in the mapcalculator to figure ground distance using—as he says—the resolution of the map (i.e., the horizontal distance to traverse each cell) and the slope of the cell.
Assuming that r.cost on a slope map * resolution = ground distance. You can use the resultant map to create a buffer around a point that represents equal ground distance in all directions.
Michael
Michael Barton, Professor of Anthropology
School of Human Evolution & Social Change
Center for Social Dynamics and Complexity
Arizona State University
phone: 480-965-6213
fax: 480-965-7671
www: http://www.public.asu.edu/~cmbarton
From: Bödőcs András bodocs@ludens.elte.hu
Reply-To: bodocs@ludens.elte.hu
Date: Thu, 1 Jun 2006 22:26:32 +0200
To: grassuser@grass.itc.it
Subject: [GRASS-user] real distance analysis
Dear All,
I am trying to solve the following problem, maybe does somebody an idea and can help me…. (thanx in advance)
I would like to examine from an exact point that territory, which is inside an determinated distance from the exact point. BUT I don’t want to use the map projected distance (=Buffer zone), but the datas of a DEM.
I would like to measure the „really” surface, the climbing etc. For example I need the maximum distance „cirlce” (30km or 30 mph doeas not matter) around a point, but measured in the „filed”(=DEM) not on the map (=projected area).
Does somebody have an idea, who can I start with it?
Many thanx
Regards
Andras
Bödőcs András
ELTE Institute of Archaeology
1088 Budapest, Múzeum krt 4/B.
Hungary
T: +36-1-411-65-54,4855200/2916
F: +36-1-411-65-53
