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

[GRASS GIS]

#3860: GRASS GIS producing different slope than GDAL
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  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 mazingaro):

 Replying to [comment:3 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
 > }}}

 This is a normal difference, I don't see problems here. The univar stats
 of the defference map were made for GRASS and GDAL produces slopes?

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Ticket URL: <https://trac.osgeo.org/grass/ticket/3860#comment:4>
GRASS GIS <https://grass.osgeo.org>