[GRASS-dev] [GRASS GIS] #3860: GRASS GIS producing different slope than GDAL

[GRASS GIS]

#3860: GRASS GIS producing different slope than GDAL
------------------------+-------------------------
  Reporter:  mazingaro  |      Owner:  grass-dev@…
      Type:  defect     |     Status:  new
  Priority:  major      |  Milestone:  7.6.2
 Component:  Raster     |    Version:  unspecified
Resolution:             |   Keywords:  slope
       CPU:  x86-64     |   Platform:  Linux
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Comment (by mlennert):

 Replying to [comment:2 mazingaro]:
 > Replying to [comment:1 mlennert]:
 > > What does GRASS GIS give you when you work at the resolution of the
 data, i.e. 1m, instead of 0.5m ?
 > The result is the same (with the -a flag)

 Weird. Are you sure you are working on the same region extent ?

 I just tried with the NC demo data elevation map, setting the region with
 g.region rast=elevation in GRASS. Here are the univar stats of the
 difference map between the two:


 {{{
 > r.univar diff
  100%
 total null and non-null cells: 8100000
 total null cells: 22784

 Of the non-null cells:
 ----------------------
 n: 8077216
 minimum: -9.67979e-05
 maximum: 9.72748e-05
 range: 0.000194073
 mean: -1.50386e-09
 mean of absolute values: 1.30826e-05
 standard deviation: 1.69973e-05
 variance: 2.88908e-10
 variation coefficient: -1.13025e+06 %
 sum: -0.0121469677105779
 }}}

 IOW: very small (IMHO negligeable) differences.

 However, when I calculate the slope in GRASS with the same region extent,
 but a resolution set to 5m, I get much more significant differences (diff
 and univar stats calculated at 5m resolution):


 {{{
 > r.univar diff_restest
  100%
 total null and non-null cells: 2025000
 total null cells: 5696

 Of the non-null cells:
 ----------------------
 n: 2019304
 minimum: 0
 maximum: 19.4712
 range: 19.4712
 mean: 3.74369
 mean of absolute values: 3.74369
 standard deviation: 2.68803
 variance: 7.22549
 variation coefficient: 71.8015 %
 sum: 7559653.93641658
 }}}

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Ticket URL: <https://trac.osgeo.org/grass/ticket/3860#comment:3>
GRASS GIS <https://grass.osgeo.org>