[GRASS-user] r.terraflow: several questions for understanding + info about TerraSTREAM

Massimiliano Cannata massimiliano.cannata at supsi.ch
Wed Jun 25 12:42:16 EDT 2008

About terminology....
Maybe I'm wrong but from my understanding...
D4, D8 are two type of single flow direction as in both case the flow 
can go only in one of the 8 or 4 neighbors cells.
Dinfinity, allows to determinate the single direction, as an angle 
between 0 and 360 degree.
In the opposite multiple flow direction, allows flow to route in more 
than one of the neighbors cells.


Moritz Lennert wrote:
> Hello,
> I have a few questions trying to understand r.terraflow details:
> 1) SFD
> r.terraflow offers the possibility to work with single flow direction 
> (SFD) instead of multiple flow direction (MFD). However, the drainage 
> direction output map is strictly identical in both modes, indicating 
> combinations of multiple directions. So, at which stage of r.terraflow 
> does SFD come to play ? Only during accumulation calculation ? Would 
> it be possible to output an SFD direction map from r.terraflow (I know 
> of Hamish's Matlab script, but would be nice to have this directly in 
> the module) ?
> 2) Terminology issue
> The man page says this:
> "Flow directions are computed using either the MFD (Multiple Flow 
> Direction) model or the SFD (Single Flow Direction, or D8) model."
> I'm no expert in flow routing, but it seems weird to equate D8 to SFD. 
> I would have thought that D8 designates the number of possible 
> directions (8 as opposed to 4 in D4 or ininite in D-Infinity) and that 
> SFD/MFD designates the number of directions actually chosen for routing.
> 3) Weighting of directions
> I can't find any information in the man page on whether flow in MFD is 
> weighted by slope, i.e. whether the amount of water attributed to a 
> particular direction depends on the slope, or whether the amount is 
> just equally divided amongst all directions.
> 4) TerraSTREAM
> Finally, are there any plans to integrate the GRASS implementation of 
> TerraSTREAM (http://www.madalgo.au.dk/Trac-TerraSTREAM/wiki) into the 
> Moritz
> _______________________________________________
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Dr. Massimiliano Cannata
Environmental & Geomatic Engineer

Institute of Earth Scences - SUPSI
Trevano, C.P. 72, CH-6952 Canobbio, SWITZERLAND

Tel:    +41 (0)58 / 666 62 14  
Fax:    +41 (0)58 / 666 62 09
E-mail: massimiliano.cannata at supsi.ch


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