[GRASS-dev] [GRASS GIS] #2820: r.surf.idw gives broken results - Don't match v.surf.idw

[GRASS GIS]

#2820: r.surf.idw gives broken results - Don't match v.surf.idw
---------------------+-------------------------
  Reporter:  lrntct  |      Owner:  grass-dev@…
      Type:  defect  |     Status:  new
  Priority:  major   |  Milestone:  7.0.3
 Component:  Raster  |    Version:  7.0.1
Resolution:          |   Keywords:
       CPU:  x86-64  |   Platform:  Linux
---------------------+-------------------------

Comment (by mlennert):

 I can reproduce the issue in the NC data set:

 {{{
 g.region vect=nc_state res=500 -a
 v.surf.idw precip_30ynormals col=annual out=vsurfidw --o
 v.to.rast precip_30ynormals use=attr attr_col=annual out=precip --o
 r.mapcalc "precip_int = int(precip * 100)" --o
 r.surf.idw in=precip_int out=rsurfidw_tmp --o
 r.mapcalc "rsurfidw = float(rsurfidw_tmp) / 100" --o
 r.mapcalc "diff = rsurfidw - vsurfidw"
 }}}


 Analysing the differences, you get:

 {{{
 r.univar map=diff
 total null and non-null cells: 24840200
 total null cells: 0
 Of the non-null cells:
 ----------------------
 n: 24840200
 minimum: -370.28
 maximum: 1020.98
 range: 1391.26
 mean: 48.8595
 mean of absolute values: 127.803
 standard deviation: 180.027
 variance: 32409.7
 variation coefficient: 368.458 %
 sum: 1213680178.7663
 }}}

 and


 {{{
 r.covar -r map=rsurfidw,vsurfidw
 r.covar: complete ...
 N = 24840200
 1.000000 0.284021
 0.284021 1.000000
 }}}

 Looking at the individual result maps:


 {{{
 r.univar map=rsurfidw
 total null and non-null cells: 24840200
 total null cells: 0
 Of the non-null cells:
 ----------------------
 n: 24840200
 minimum: 947.42
 maximum: 2329.17
 range: 1381.75
 mean: 1346.31
 mean of absolute values: 1346.31
 standard deviation: 183.013
 variance: 33493.9
 variation coefficient: 13.5937 %
 sum: 33442685677.211
 }}}

 and


 {{{
 r.univar map=vsurfidw
 total null and non-null cells: 24840200
 total null cells: 0
 Of the non-null cells:
 ----------------------
 n: 24840200
 minimum: 1120.14
 maximum: 1546.86
 range: 426.72
 mean: 1297.45
 mean of absolute values: 1297.45
 standard deviation: 92.1991
 variance: 8500.67
 variation coefficient: 7.10616 %
 sum: 32229005498.4201
 }}}

 I will also attach some maps, including a map of the original data points.
 The interpolated maps use the following color table:


 {{{
 947.42 255:255:255
 2329.17 0:0:255
 }}}

 v.surf.idw smoothes the map much more...

--
Ticket URL: <https://trac.osgeo.org/grass/ticket/2820#comment:2>
GRASS GIS <https://grass.osgeo.org>