Dear GRASS experts: I report a strange behavior of GRASS 7.0.3 on Ubuntu 16.04. It happens in r.slope.aspect. When I tried to create a slope map using a resolution bigger than the original DEM's resolution, the result becomes strange.
Here is a simple example (please look at the code at the end of this e-mail). First, it creates an imaginary DEM on which "elevation" is a column number in 100 pix x 100 pix region of 0N~1N, 0E~1E . It is called testdem. Using that DEM, r.slope.aspect gives right answer (test_slope =0.51 deg. It is equal to atan (100 m/((40000000 m)/360)). Please take a look at the attached picture. However, after setting the resolution twice as big (0.02 deg) as the original one (0.01 deg), r.slope.aspect happens to give a wrong answer (test_slope1 =0.0257, which is ohe-half of the right answer). However, when I resample the original DEM by r.mapcalc command, and try r.slope.aspect command using the resampled DEM (called test_dem2), this time it gives right answer (test_slope =0.51 deg). Obviously these three results should be the same because the original DEM is a homogeneous slope. I suspect something wrong in the treatment of resolution in r.slope.aspect commands. (code) g.region w=0 e=1 s=0 n=1 rows=100 cols=100 r.mapcalc expression="testdem=col()" --o r.slope.aspect elevation=testdem slope=test_slope0 --o g.region res=0.02 r.slope.aspect elevation=testdem slope=test_slope1 --o r.mapcalc expression="testdem2=testdem" --o r.slope.aspect elevation=testdem2 slope=test_slope2 --o d.mon wx0 d.rast test_slope0 d.rast test_slope1 d.rast test_slope2 --------------- Kenlo Nishida Nasahara Faculty of Life and Environmental Sciences University of Tsukuba, Japan 305-8572 [email protected]
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