[GRASSLIST:10618] Re: ARC FLOWDIRECTION vs GRASS

[Dylan Beaudette]

On Monday 27 February 2006 05:31 pm, Jonathan Greenberg wrote:
> I'm trying to replicate preparation of data for the DHSVM hydro model in
> GRASS following an AML script, and I'm a bit confused why every GRASS
> module with flow direction computation produces a different output than the
> ARC one -- they even differ between themselves (e.g. r.watershed and
> r.terraflow). Does anyone know of a flow direction computation that
> produces the EXACT same outputs as ARC?  I'm not "judging" which one is
> better, I'm just trying to follow as closely as possible the first time
> around the "idealized" algorithm.
>
> --j
>


Good questions -- tough answers. I would be sceptical of suggesting that the 
Arc-derived output is any better... but to the real subject:

Can you provide more information on the "type" of flow model used by the Arc 
AML script? i.e. is it using D4,D8,Rho8,FRho8,DEMON,D-INF flow calculation?

note that D4,D8, and Rho8 are SFD (single input flow direction - single 
output)
FD8,FRho8 are MFD (multiple input flow direction - single output)
DEMON and D-INF are similar in many ways, but not identical (multiple input 
multiple output?) [1]

Each one of these different treatments of adjacent cells will influence the 
output flow map/ 

Some examples in GRASS see man pages for each command

1. simplest case of the D8 flow direction, stored in new raster dir_d8 :
r.fill.dir in=elev_meters elev=e_filled direction=dir_d8 

note that direction can be in the form of 0-360 or 1-8

2. example of the D-INF algorithm (vector output stored in new vector flow)
r.flow elevin=e_filled aspin=aspect flout=flow

3. example of MFD aka FRho8 :
r.terraflow....

note that the direction output will be from 0-255

[1] Wilson and Gallant. Terrain Analysis. 2000


-- 
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341