[GRASS-user] High tension with r.resamp.rst

[Nikos Alexandris]

* Ghislain Vieilledent | CIRAD <ghislain.vieilledent at cirad.fr> [2019-02-06 14:11:04 +0100]:

>Dear GRASS GIS user,
>
>I am using r.resamp.rst to interpolate climatic data at 30sec 
>resolution over Africa.
>
>Grass script, data, and results are available here:
>https://nextcloud.fraisedesbois.net/index.php/s/CKzsZCSCqMTY9rq
>
>#===================
>
># Import Africa shapefile
>v.in.ogr --overwrite input=gisdata/vectors/Africa layer=Africa output=Africa
>
># Rasterize Africa at 30 arc-sec resolution
>g.region -ap vector=Africa res=0:00:30
>v.to.rast input=Africa type=area output=Africa use=val value=1
>
># Import climate
>r.in.gdal --overwrite input=gisdata/rasters/test1.1_044.tif output=test1_1
>
># Resample with RST
>g.region -ap raster=test1_1
># Note: use maskmap=Africa to save computation time
>r.resamp.rst --overwrite input=test1_1 elevation=test1_1_rst \\
>             ew_res=0.00833 ns_res=0.00833 maskmap=Africa
>r.info test1_1_rst
>
># Resample to exact resolution 0:00:30
>g.region -ap res=0:00:30
>r.resample --overwrite input=test1_1_rst output=test1_1_30s
>
># Export
>r.out.gdal --overwrite input=test1_1_30s \\
>             output=output/test1_1_30s.tif type=Float32 \\
>             createopt='compress=lzw,predictor=2'
>
>#===================
>
>When I use r.info on the interpolated raster, it indicates a tension 
>of 7278 in the comments which seems very high to me as the default 
>value is 40.
>
>|   Comments:
>|    tension=7278.552524
>|    dnorm=5.495598, zmult=1.000000
>|    KMAX=50, KMIN=35, errtotal=0.000535
>|    zmin_data=0.056488, zmax_data=3.756866
>|    zmin_int=-0.066779, zmax_int=3.214336
>
>On the other side, results do not look so bad (see test.pdf figure).

I am following this thread out of curiosity/interest on resampling.  The
.pdf file is not attached.  How big is it?

Nikos


>Would you have any comment on this high value for the tension and do 
>you think interpolation has been performed correctly ?
>
>Best regards,
>
>Ghislain