[GRASS-user] r.mapcalc: compare two maps [RESOLVED]

[Rich Shepard]

On Thu, 30 Mar 2017, Nikos Alexandris wrote:

>> For the record: there is a "differences" color table which is useful for
>> this.

> Don't forget to try r.colors with -e or -a -g if the first attempt does not
> show much differences.

   I _had_ not noticed the 'differences' color table. Applying that produces
interesting results (see attached .png).

   The four blue cells are ~0.6m higher in the blocked map than in the open
culvert map. I don't know the reason for the lighter blue cell to the
southwest of the group of four. I suspect, but don't know, that the pinkish
cells represent slightly higher water accumulation in the blocked map than
in the open map.

  Germane to what Markus M. wrote, "The point locations used to edit the
original DEM are probably wrong (not close to the center), ..." I now think
that's where I made one mistake. When I isolated those four cells and
queried them I truncated the easting and northing coordinates to the integer
value assuming that all elevations in a 1m x 1m cell were the same. If this
assumption is incorrect then I'll re-create the change and start the model
running again with a slightly different set of elevations for those cells.

   If anyone can suggest other changes to the model input to better show
where water accummulates behind a blockage please let me know.

Rich
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