[GRASS-user] Longest flow path

[Jarek Jasiewicz]

Rich Shepard pisze:
> On Tue, 3 Aug 2010, Jarek Jasiewicz wrote:
>
>> r.watershed elevation=elev drainage=dirs streams=stream 
>> treshold=<some value>
>> r.stream.order streams=streams dirs=dirs hack=longest
>>
>> the stream with order 1 will be the longest streams of the catchment
>
> Jarek,
>
>   Please correct me if I'm wrong, but I don't believe this answers 
> David's
> question. 
yes this is correct answer. I suggest hack ordering not horton or 
strahler. Hack ordering ascribe first order to the longest stream in the 
catchement, second order to its tributaries etc. see more in documentation.

If you need more options also you can use:

r.stream.order streams=streams dirs=dirs hack=longest table=ordering

it will create table which has several attributes including longest 
length from head (cumlength) for every segment and distance to outlet 
(distance) where all segments has its more downstream cell distance to 
the outlet. Also see documentation.