[GRASS-user] r.drain documentation

[Hamish]

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





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