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

GRASS GRASS user
,
1

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@cirad.fr

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

2

* Ghislain Vieilledent | CIRAD <ghislain.vieilledent@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

3

Ciao Nikos,
The pdf is available at the web link provided in the email. Thanks for giving a look.
Cheers,
Ghislain

Le 6 février 2019 15:19:22 GMT+01:00, Nikos Alexandris nik@nikosalexandris.net a écrit :

* Ghislain Vieilledent | CIRAD <ghislain.vieilledent@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](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

–
Envoyé de mon appareil Android avec Courriel K-9 Mail. Veuillez excuser ma brièveté.

4

Helena Mitasova sent me an answer yestarday on the topic:

I haven’t used (or looked at) r.resamp.rst for a long time, but the tension may be OK becuase it depends on scale
- if you are working in lat/long, the 30sec resolution in decimal degrees is a very small number (compare to e.g. 10m resolution DEM),
so the large tension compensates for that. We normalize the data to mitigate the scale dependency but for latlong
it is still an issue.
Have ou tried some of the other resampling modules, just to get two independent results ? They should be pretty close.

Helena

_______________________________________________________

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@cirad.fr

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

On 06/02/2019 15:24, Ghislain Vieilledent wrote:

Ciao Nikos,
The pdf is available at the web link provided in the email. Thanks for giving a look.
Cheers,
Ghislain

Le 6 février 2019 15:19:22 GMT+01:00, Nikos Alexandris <nik@nikosalexandris.net> a écrit :

    * Ghislain Vieilledent | CIRAD <ghislain.vieilledent@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

--
Envoyé de mon appareil Android avec Courriel K-9 Mail. Veuillez excuser ma brièveté.

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