[GRASS-dev] v.surf.bspline

[Paulo van Breugel]

Hi Markus

Thanks for checking. I tried it yesterday on grass 7.8.4.1 (installed using
osgeo4w) and got the same results as you did.

Reinstalled QGIS and GRASS GIS using the standalone installer (because of
some unrelated problems). This installs grass 7.8.3. Tried out the same
again, and the problem reappeared. What seems to be the problem is that the
function always uses the bicubic interpolation method, see below.

g.gisenv set="DEBUG=1"
v.surf.bspline input=samplepoints2 column=value raster_output=bilinear
ew_step=50 ns_step=50

---
D1/1: Interpolation: (1,1): Bicubic interpolation...
 100%
---

g.gisenv set="DEBUG=1"
v.surf.bspline input=samplepoints2 column=value raster_output=bilinear2
ew_step=50 ns_step=50 method=bilinear

---
D1/1: Interpolation: (1,1): Bicubic interpolation...
 100%
---

Cheers,

Paulo


On Sat, Oct 10, 2020 at 11:36 PM Markus Neteler <neteler at osgeo.org> wrote:

> Hi Paulo,
>
> On Sun, Oct 4, 2020 at 3:46 PM Paulo van Breugel <p.vanbreugel at gmail.com>
> wrote:
> >
> > Hi devs,
> >
> > I am running a v.surf.bspline (grass 7.8.3 on Windows) using the
> bilinear and de bicubic algorithm, like e.g. (NC dataset)
> >
> > g.region raster=elev_lid792_1m
> > r.random input=elev_lid792_1m npoints=200 vector=samplepoints seed=20
> > v.surf.bspline input=samplepoints column=value
> raster_output=bspline_bilinear ew_step=50 ns_step=50
>
>  --> raster_output=bspline_bilinear   - ok
>
> > v.surf.bspline input=samplepoints column=value
> raster_output=bspline_bilinear ew_step=50 ns_step=50 method=bicubic
>
>  --> raster_output=bspline_bilinear   - a copy-paste error in this email?
>
> > Results are exactly the same. Any idea if I am missing something here?
>
> I tried myself:
>
> g.region raster=elev_lid792_1m
> r.random input=elev_lid792_1m npoints=200 vector=samplepoints seed=20
>
> # verify the sample point values
> v.univar samplepoints column=value
>  100%
> number of features with non NULL attribute: 200
> number of missing attributes: 0
> number of NULL attributes: 0
> minimum: 104.283
> maximum: 131.352
> range: 27.0692
> sum: 24232.3
> mean: 121.162
> mean of absolute values: 121.162
> population standard deviation: 6.16656
> population variance: 38.0265
> population coefficient of variation: 0.0508953
> sample standard deviation: 6.18204
> sample variance: 38.2176
> kurtosis: -0.548097
> skewness: -0.362208
>
> # now interpolate and check what's being used
> g.gisenv set="DEBUG=1"
> v.surf.bspline input=samplepoints column=value
> raster_output=bspline_bilinear1 ew_step=50 ns_step=50 method=bilinear
> ...
> D1/1: Interpolation: (1,1): Bilinear interpolation...
> ...
>
> v.surf.bspline input=samplepoints column=value
> raster_output=bspline_bicubic1 ew_step=50 ns_step=50 method=bicubic
> ...
> D1/1: Interpolation: (1,1): Bicubic interpolation...
> ...
>
> g.gisenv set="DEBUG=0"
> # calculate differences
> r.mapcalc "diff = bspline_bilinear - bspline_bicubic"
>
> # verify differences
> r.univar diff -eg
> n=525000
> null_cells=0
> cells=525000
> min=-1.35428900518949
> max=1.47700431246362
> range=2.83129331765311
> mean=0.0372330733156362
> mean_of_abs=0.189272111683445
> stddev=0.24882465890669
> variance=0.0619137108800304
> coeff_var=668.289337270996
> sum=19547.363490709
> first_quartile=-0.108568
> median=0.0209677
> third_quartile=0.181387
> percentile_90=0.352807
>
> Looks fine to me.
>
> Markus
>
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