I made some test using r.water.outlet with Spearfish location and output coming from GRASS and from TerraSTREAM:
http://www.movedigital.com/go/doktoreas/114926/grass_terrastream.jpg The drainage map created with TerraSTREAM has been reclassified using those rules: 0=-1 128=1 1=2 2=3 4=4 8=5 16=6 32=7 64=0 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 Luca On Thursday 27 March 2008 01:31:07 Andrew Danner wrote: > Luca, > > The Terrastream direction coding for single flow direction is as > follows > > 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: > drainage=name > 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. > > > -Andy _______________________________________________ grass-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-dev
