Sorry, I found that it is avoided by "-a" option. It is described in the manual. "If the user really wants the raster elevation map resampled to the current region resolution, the -a flag should be specified. " https://grass.osgeo.org/grass73/manuals/r.slope.aspect.html
Kenlo Nasahara On Mon, 15 Jan 2018 12:28:41 +0900 NASAHARA Kenlo <[email protected]> wrote: > 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] -- 奈佐原 顕郎 （旧姓西田） 筑波大学生命環境系 〒305-8572 つくば市天王台1-1-1 電話・FAX： 029-853-4897 Kenlo Nishida Nasahara Faculty of Life and Environmental Sciences University of Tsukuba, Japan 305-8572 [email protected] _______________________________________________ grass-user mailing list [email protected] https://lists.osgeo.org/mailman/listinfo/grass-user
