[GRASS-user] Stream flow direction map

[Nikos Alexandris]

On Saturday 01 of May 2010 05:30:04 Nikos Alexandris wrote:
> I am sort of forwarding a friends question related to hydro-processing.
> 
> While there is a proper (vector) contour map and a very clean stream
> network map (digitised from scratch upon topographic maps of 1:50.000)
> available, the flow direction map is missing.
> 
> Is this information easily calculated based on available dem(s), stream
> network(s)? Or should _definitely_ the user add this information while
> digitising the streams?
> 
> I have read many past threads wrt "streams, flow, direction, accumulation"
> but could not trace an answer to the above question.

I read again some stuff and I guess that this is a very basic and simple 
"hydro-gis" question (for which I do not know the answer):

is the direction map bound to a specific stream network, i.e. the one that 
r.watershed or r.stream.extract produces  or  is it an independent one, based 
solely on the input elevation model and its (x-y-z?) resolution?

If the latter is true, then it only takes to use either r.watershed or 
r.stream.extract to get the direction(s).

If not, then the question is if the user can supply his own stream map to 
r.stream.* modules (specifically to r.stream.extract) in order to get the 
desired flow direction map?

Thanks, Nikos