[GRASS-user] edges in basin map from r.watershed

[MS]

IMO the esri term of a filled DEM being "hydrologically correct" is a  
misnomer.

Natural terrain has real depressions that impact surface water flow.   
Big depressional wetlands can retain water and release via groundwater  
or evapotranspiration.

I like how r.watershed acomodates known depressions and handles the  
flow as interception. Also, then one can use other tools to find  
problematic areas in a raw DEM.  I modeled an internally drained basin  
using known depressions in GRASS, and it worked fantastic.

One problematic example for esri is modeling an internally drained or  
sink-watershed.  These have no surface water outlet.  If one filled it  
to get flow out of a 9 sq. mile watershed, the esri analyses are then  
meaningless.  Which is one reason why filling a dem just to get an  
esri module to work and calling the DEM "hydrologically correct" is a  
misnomer an a limitation.


Mark

On Feb 13, 2009, at 11:09 AM, Markus Metz <markus.metz.giswork at googlemail.com 
 > wrote:

>
>
> Christian Schwartze wrote:
>> Dear GRASS users,
>>
>> with r.watershed I get strange basin boundaries for some areas und  
>> I'm not able
>> to give account of it. Attached you can find that part of the basin  
>> map which
>> looks curiously. I means the sharp-edged regions...
>> Whats the reason?
>>
> This is most probably a flat area (no slope). Flow direction, flow  
> accumulation, stream segments and basins can not reasonably be  
> calculated for flat areas, these are regarded as missing information  
> and some assumption has been made by the algorithm.
> What could help is to use a raster DEM as input that is *not*  
> filled, some would say not hydrologically correct, but r.watershed  
> works better with the raw, not filled DEM.
> What could also help, if this does not work or it really is a flat  
> area, is r.watershed of grass7 with multiple flow direction. Note  
> that the result may look nicer, but it still holds true that  
> drainage direction (and therefore all other output) has to be  
> estimated for flat areas. The A * Search of r.watershed is doing a  
> pretty good job, and multiple flow accumulation can improve it a bit  
> more, within limits.
>> Basis is an Arc Info .adf raster file for DEM data.
>>
> I think Arc Info wants a depressionless, filled, hydrologically  
> correct DEM. r.watershed does explicitely not want such a DEM, it  
> wants a raw DEM with depressions, not filled, and not hydrologically  
> correct.
>
> I hope that helps,
>
> Markus M
>
> _______________________________________________
> grass-user mailing list
> grass-user at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/grass-user