[GRASS-dev] r.watershed question

Markus Metz markus.metz.giswork at googlemail.com
Thu Sep 17 15:24:29 EDT 2009

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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