[GRASSLIST:10630] Re: ARC FLOWDIRECTION vs GRASS

[Laura Toma]

It is hard to produce identical outputs for single-flow-direction  
models, because of the cases when a choice must be made. For e.g., if  
there is more than one steepest downslope neighbors, or there is none  
(flat areas).  I believe these choices are not be documented  
(proprietary?) for Arc.

-Laura

On Feb 27, 2006, at 9:48 PM, Dylan Beaudette wrote:

> 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
>
>