[GRASS-user] Stream flow direction map

[Micha Silver]

Nikos Alexandris wrote:

> 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
> _______________________________________________
> grass-user mailing list
> grass-user at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/grass-user
>   
It's not clear if your friend needs a flow direction raster (normally, 
AFAIK, flow direction refers to a raster, where each cell has a value 
indicating which is the next downstream cell) or if he want to see the 
flow direction of streams in his stream network.
 
I might be worth mentioning also that, since line vectors already have a 
direction in GRASS, you can display their direction with the "disp=dir" 
option to d.vect. So *if* he digitized his stream network being careful 
to work always downstream, then
d.vect streams disp=shape,dir
will show small arrows indicating the direction of each line segment.
Otherwise, as Jarek said, he should go back to the start and do 
r.stream.carve then rerun r.watershed to extract catchments, streams, as 
well as flowdir along the way, etc.

-- 
Micha Silver
Arava Development Co. +972-52-3665918
http://www.surfaces.co.il