Thanks for your answers Eric and Hamish. I had already made sure with d.rast.num and I have double checked now on a "toy" map (a slope on a 100x100 grid): even if there are lower value cells beside a border cell, it won't move from there. Stefano.
2008/2/15, Hamish <[EMAIL PROTECTED]>: > > Stefano Negri wrote: > > I'm a bit confused about r.drain (I'm working with Debian's backport > > version 6.2.1). I have noticed that if you give as input in the > > "coordinate" parameter a point that stands on the region's edge, > > r.drain won't "move" from there: it would output a map with that > > same single point. Is this a bug? > > > feature. > > > > On the other hand I can't understand why in the man page the section > > entitled BUG says: "r.drain currently finds only the lowest point > > (the cell having the smallest category value) in the input file > > that can be reached through directly adjacent cells that are less > > than or equal in value to the cell reached immediately prior to it; > > therefore, it will not necessarily reach the lowest point in the > > input file. It currently finds pits in the data, rather than the > > lowest point present.". Why would a local search be a bug in this > > case? I think it should be like that by design... > > > It is like that by design so really a feature not a bug. > The man page should mention you could fill pits first with a module > like r.fill.dir or r.terraflow if you want that. > (the first is listed in the see also section but not explained AFAICS) > > > > > > I'm not too sure my understanding is correct. What do you think? > > > You are right I think. My understanding is that it works by starting at > the given point and looking for the most downhill cell around it. Then > it moves to that cell and looks for the most downhill cell around that. > And so on until it can go no further. If your starting cell is only > surrounded by cells of higher value (you start in a pit) then it won't > move. By being on the edge of the region you have cut down on the > numbers of cells around your starting point so there is less chances > for finding an outlet point. Zoom right in on the DEM and explore with > d.rast.num to confirm. > > > > Hamish > > > > > > > > > ____________________________________________________________________________________ > Looking for last minute shopping deals? > Find them fast with Yahoo! Search. > http://tools.search.yahoo.com/newsearch/category.php?category=shopping >
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