Markus N and Markus M, Thankyou for your response.
I see. If on the first step, more surrounding cells have the same minimum cost, which is the first cell encountered used (like follows: a is greater than surraounding cells)? I'm sorry the question on the abc. 2 2 2 2 a 2 2 2 2 Thanks Takenori K.
To my knowledge there is no scientific paper available related to the r.drain algorithm. It has been written by Roger S. Miller in 2001 (replacing an older and less functional version). You may see the internals here in the code base: http://trac.osgeo.org/grass/browser/grass/trunk/raster/r.drain Maybe other users know more about this algorithm family? Best Markus N r.drain simply follows the lowest cost in the surrounding cells (must be lower than the current cell). If more surrounding cells have the same minimum cost, the first cell encountered is used. Markus M
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