Hi Patrick, If I understand the question, I haven't done this in GRASS, but one cumbersome approach without using a function like r.drain to find a cost distance *corridor* between two patches is to calculate a separate cost distance surface for every individual patch, from (and only from) that individual patch. Then, for each pairwise combination of two patches that you want to find the least cost path for, you add together (or average them; gives similar results) the two resulting cost distance surfaces for those individual patches, which gives you an accumulative cost distance surface of the cost involved in moving from any one pixel to BOTH patches - basically, a corridor. Basically, there will be a row of pixels between the two cores with the lowest cost values of the accumulative cost distance raster, which is the least cost path. You can either extract out all the pixels with the lowest values or perhaps use a tool like r.drain to find the least cost path (LCP) itself - I haven't done this in GRASS but I believe it can be done.
I think you can use r.drain directly to find the LCP - but importantly, I think it uses the backlink (direction) raster from r.cost as an input to calculate LCPs; if you're connecting, say, patches A and B with an LCP, I think what you do is calculate a cost surface and backlink raster from patch A, then use r.drain to calculate to calculate the LCP by putting in patch B as your starting point, plus the cost surface and backlink raster from the calculation using patch A. If I'm not mistaken you have to do this on a pairwise patch-to-patch basis, and you'll want to make sure you're using the backlink raster. Also, if you're using one of the binary releases for Windows, for some of this you have to use the r.cost and the r.drain module from version 6.5.1 or later (do a search to find the patches on those modules); there's an older version in the WIN 6.4 release that doesn't produce a backlink and thus makes it hard to do this kind of calculation. You might also take a look at some tools built in other GIS platforms for ideas on this kind of connectivity analysis, particularly www.corridordesign.org, Funncon (http://www.nrel.colostate.edu/projects/starmap/FUNCONN%20Users%20Manual_public.pdf), www.circuitscape.org; there are others currently in development by conservation organizations in the US that should come online in the near future. Hope that helps, Tristan -- Landscape Ecology and Conservation Lab University of Washington _______________________________________________ grass-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-user
