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

[Ghislain Vieilledent | CIRAD]

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).

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

Best regards,

Ghislain

-- 
_______________________________________________________

Ghislain VIEILLEDENT

CIRAD - UMR AMAP
botAnique et Modélisation de l'Architecture des Plantes et des végétations
TA A51/PS2
F-34398 Montpellier cedex 05
FRANCE

Phone: +33.6.24.62.65.07
Skype: ghislain.vieilledent
E-mail : ghislain.vieilledent at cirad.fr

Research unit web site: http://amap.cirad.fr
Personal web site: https://ecology.ghislainv.fr
BioSceneMada project: https://bioscenemada.cirad.fr
_______________________________________________________