[GRASS-dev] r.watershed and swap memory
luca.casagrande at gmail.com
Thu Mar 27 08:40:36 EDT 2008
I made some test using r.water.outlet with Spearfish location and
output coming from GRASS and from TerraSTREAM:
The drainage map created with TerraSTREAM has been reclassified using those
As you can notice, the result are quite different and the one from TS generate
a really small area.
Any idea on that?
Thanks for your help
On Thursday 27 March 2008 01:31:07 Andrew Danner wrote:
> The Terrastream direction coding for single flow direction is as
> 1 = Water flows east
> 2 = SE
> 4 = S
> 8 = SW
> 16 = W
> 32 = NW
> 64 = N
> 128 = NE
> Or pictorially
> 32 64 128
> 16 x 1
> 8 4 2
> In the MFD case, values do NOT represent aspects in the range 0 to 360,
> but instead indicate sums of the directions above. Thus if water flows
> N, S and SW, the value is 64+8+4 = 76.
> In both the SFD/MFD outputs, a value of 0 means this point is local
> minimum and there is no flow direction.
> r.water.outlet reports:
> Input map: drainage direction. Indicates the "aspect" for each
> cell. Multiplying positive values by 45 will give the direction
> in degrees that the surface runoff will travel from that cell.
> The value -1 indicates that the cell is a depression area. Other
> negative values indicate that surface runoff is leaving the
> boundaries of the current geographic region. The absolute value
> of these negative cells indicates the direction of flow. This
> map is generated from r.watershed
> I don't know if the direction it refers to is measured from North or
> East or if it moves clockwise or counterclockwise.
> r.slope.aspect measures ccw from east, TerraStream and TerraFlow are
> measuring clockwise from East, and standard compass direction is
> measured clockwise from North.
> If someone is more familiar with the r.water.outlet drainage directions,
> it should be relatively easy to map the values 1,2,4,8,16,32,64 and 128
> to 1 through 7. log_2(tflow_dir)+1 will map terrastream flow directions
> onto the range 1-7 (handling 0 is special case).
> Is there any consistent way of encoding flow direction? TerraStream and
> TerraFlow I believe used an encoding from Jenson and Domingue, though in
> looking up their reference they started with 1 in the NE and moved
> clockwise. Perhaps ESRI products used 1=E. It seems that aspect should
> be measured clockwise in degrees from north geographically, but it makes
> more sense to measure ccw from east trigonometrically.
> TerraStream could easily rotate or reverse the values 1-255 if there is
> a standard out there to follow.
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