[GRASS-dev] r.watershed question

Michael Barton Michael.Barton at asu.edu
Thu Sep 17 16:09:32 EDT 2009

This is VERY helpful. Thanks.

C. Michael Barton
Director, Center for Social Dynamics & Complexity
Professor of Anthropology, School of Human Evolution & Social Change
Arizona State University

Phone: 480-965-6262
Fax: 480-965-7671
www: www.public.asu.edu/~cmbarton, http://csdc.asu.edu

On Sep 17, 2009, at 12:24 PM, Markus Metz wrote:

> Michael Barton wrote:
>> r.watershed can produce a map of "drainage directions".
>> The values of a direction map are in the range of -8 - +8
>> Can someone tell me what these numbers refer to? That is, what
>> direction is 3 or -3? I've tried to run some correlations against an
>> aspect map and am not getting meaningful results yet.
> r.watershed drainage direction is encoded as steps of 45 degree  
> counted
> counterclockwise from East: 1 = 45 degree from East = NE, 2 = 90  
> degree
> from East = N etc. Negative values indicate flow towards a cell with
> undefined (NULL) elevation and thus undefined flow accumulation. Cells
> with undefined elevation are either outside the current region or
> elevation for that cell is not available (MASKed or NULL).
> Correlations against an aspect map may not work because in r.watershed
> water can flow uphill (hydrological DEM correction on-the-fly). MFD  
> does
> additional corrections to flow direction according to maximum flow,
> visible in the difference between SFD and MFD basins (e.g. for
> elev_lid792_1m in the North Carolina sample dataset).
> Hope that helps,
> Markus M

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