[GRASS-dev] Re: [GRASS GIS] #807: r.watershed doesnt consider longer distance to diagonal neighbouring pixels

[GRASS GIS]

#807: r.watershed doesnt consider longer distance to diagonal neighbouring pixels
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  Reporter:  aread   |       Owner:  grass-dev at lists.osgeo.org
      Type:  defect  |      Status:  new                      
  Priority:  major   |   Milestone:  6.4.0                    
 Component:  Raster  |     Version:  6.4.0 RCs                
Resolution:          |    Keywords:  r.watershed              
  Platform:  All     |         Cpu:  All                      
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Comment (by mmetz):

 Replying to [comment:11 helena]:
 > this is where the negative values are:
 > http://skagit.meas.ncsu.edu/~helena/grasswork/accum_mfd_neg.png
 > [[BR]]
 > why does it switch from positive to negative in the middle
 > of the slope even for the flow accumulation that apparently originates
 > on the top of the hill well inside the area?

 The main fow originates from the top of the hill, but cells on the slope
 also receive flow from other neighbouring cells with higher elevation.
 Tracing this back, particularly for MFD flow dispersion, can lead to the
 edge of the region

 > I can imagine the high accumulation values starting from outside,
 > but the low negative accumulation values (in tens of cells) close to the
 center
 > of the region have no way of starting outside.
 Yes, they have, although the absolute amount of surface flow making it
 from the edge of the region to there is minimal, <<< 1. If the edge of the
 region has higher elevation than the center of the region, and if there is
 an ever so slight downhill slope leading from the edge to the center, some
 edge flow will reach the center.

 Long explanation:

 As the manual says, "Negative numbers indicate that those cells possibly
 have surface runoff from outside of the current geographic region." or
 from cells with unknown elevation (MASKed or NULL), and this is what the
 results should show, if not, it's a bug.

 The general slope direction in elev_lid792_1m is from about Northwest to
 Southeast and the terrain is quite flat. Many hillslopes seem to be
 neither strongly convex nor concave in tangential direction, and in MFD
 mode, flow is distributed to many neighbours. Therefore it can be expected
 that the sign of flow accumulation will also be spread considerably.
 Still, is it true that so many cells receive offmap flow or is it a bug?

 In elev_lid.acc.mfd.png

 http://skagit.meas.ncsu.edu/~helena/grasswork/accum_mfd_neg.png

 only negative accumulation values are displayed, overlayed by
 elev_lid792_cont1m (1m contour lines). The negative sign is not propagated
 uphill (except for egressing depressions), but, starting from the borders
 of the region, slowly descends downhill (edge between coloured and white
 cells).

 Looking at negative sfd accumulation values, displayed in
 elev_lid.acc.sfd.highlight.png

 http://sites.google.com/site/markusmetzgiswork/elev_lid.acc.sfd.highlight.png

 one of the crucial differences is where the red ellipse is located in the
 Northwest. Here, the hillslope seems to be rather flat in the tangential
 direction, if not convex, and surface flow including the negative sign is
 spread quite far.

 Comparing elev_lid792.acc.sfd.zoom.png

 http://sites.google.com/site/markusmetzgiswork/elev_lid.acc.sfd.zoom.png

 and elev_lid792.acc.mfd.zoom.png

 http://sites.google.com/site/markusmetzgiswork/elev_lid.acc.mfd.zoom.png

 both show only negative flow, it is apparent that SFD struggles to find a
 reasonable flow direction on this hill and shows a straight line where the
 negative sign is propagated from the region's edge downhill into the main
 basin. MFD on the contrary spreads out the surface flow, typical for
 convex hills and similar to flat areas. The negative sign is spread
 together with the flow, and because this is at the top of the main basin
 in the current region, the negative sign is propagated wide and far
 downhill.

 Negative flow also appears within the region midway down the large hill in
 the Northwestern quarter, keeping in mind that this is not really a hill.
 Elevation difference from the top of the hill to the first occurence of
 negative flow is about 60cm and from that point the elevation difference
 slowly increases. AFAICT, the negative sign is only propagated downhill.

 If you are confident that there are indeed negative flow accumulation
 values where there should be positive flow accumulation values, please
 post the exact coordinates, so I can look in more detail what is going
 there.

 Markus M

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