[GRASS-dev] bug in v.surf.bspline?

[Michael Barton]

Roberto,

I tried out the update to v.surf.bspline and now I CAN do an interpolation
using a vector attribute column.

However, when I use a z-value for interpolation and then use an attribute
column for exactly the same values, I get very different interpolations.

For example, I ran the following two interpolations, the first using the
z-value (layer 0) and the second using the attribute column that is used for
the z-value (layer 1)

v.surf.bspline input=solutrean_age3D at PERMANENT raster=test_interpolation_z
sie=4 sin=4 type=bicubic lambda_i=1 layer=0

v.surf.bspline input=solutrean_age3D at PERMANENT
raster=test_interpolation_attr sie=4 sin=4 type=bicubic lambda_i=1 layer=1
column=CAL_MID

Then I made a difference map of the two results

r.mapcalc 'test_interpolation_diff=test_interpolation_z -
test_interpolation_attr'

Finally, I ran r.univar to summarize the differences. As you can see there
is a big difference between the 2 maps.

GRASS 6.3.cvs (Europe_ll_wgs84):~ > r.univar test_interpolation_diff
 100%
total null and non-null cells: 21758946
total null cells: 0

Of the non-null cells:
----------------------
n: 21758946
minimum: 280.434
maximum: 24343.5
range: 24063.1
mean: 6819.02
mean of absolute values: 6819.02
standard deviation: 5566.97
variance: 3.09912e+07
variation coefficient: 81.6389 %
sum: 148374632036.5238342285

I'm not sure which is the more accurate, but it LOOKS like the one using
z-values is the correct one. I made a jpg output from each map and attach
them here for visual comparison. It is clear that they are very different,
even though ostensibly using the same data.

Also, in both cases the interpolation does not respect a MASK.

I like this module a lot and hope that this can help to improve it.

Michael


__________________________________________
Michael Barton, Professor of Anthropology
Director of Graduate Studies
School of Human Evolution & Social Change
Center for Social Dynamics & Complexity
Arizona State University

phone: 480-965-6213
fax: 480-965-7671
www: http://www.public.asu.edu/~cmbarton


On 10/19/07 9:35 AM, "Michael Barton" <michael.barton at asu.edu> wrote:

> Excellent! Thanks again.
> 
> Michael
> 
> 
> On 10/19/07 9:34 AM, "Roberto Antolin" <roberto at geomatica.como.polimi.it>
> wrote:
> 
>> Hello Michael
>> 
>> Michael Barton escribió:
>>> Thanks Roberto. Is it in the CVS now?
>> 
>> Yes, it is. Sorry, I forgot to tell you :)
>> Please, let me now if it is ok or there is still something wrong.
>> 
>> Roberto.
>> 
>> 
> 
> __________________________________________
> Michael Barton, Professor of Anthropology
> Director of Graduate Studies
> School of Human Evolution & Social Change
> Center for Social Dynamics and Complexity
> Arizona State University
> 
> phone: 480-965-6213
> fax: 480-965-7671
> www: http://www.public.asu.edu/~cmbarton
> 


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