[R-sig-Geo] scale semivariance befor fit.variogram

[Edzer Pebesma]

The problem is not gstat using floats (it doesn't), it's roughly using
the normal equations to solve a regression problem -- this calls for a
sum of squared values matrix X'X, if the values then are too far apart,
as it seems in your case, even double arithmetic runs into an explosion
of numeric errors.

On 07/19/2010 05:44 PM, Samuel Turgeon wrote:
> Thanks Paul,
> 
> I understand that I can multiplied my data without problem but I'm still
> wondering why I have to this operation. Maybe it's about the precision
> floats in gstat like you said.
> 
> regards,
> Sam
> 
> 2010/7/19 Paul Hiemstra <p.hiemstra at geo.uu.nl>
> 
>> On 07/17/2010 06:19 PM, Samuel Turgeon wrote:
>>
>>> Hi,
>>>
>>> I'm a new user of gstat, so my question is maybe very simple.
>>>
>>> I'm trying to fit a variogram with the function fit.variogram and I get
>>> this
>>> error message:
>>>
>>> Warning: singular model in variogram fit
>>> [1] "a possible solution MIGHT be to scale semivariances and/or distances"
>>>
>>> My data have values between 2.54e-10  et 2.56e-06. If I multiply my data
>>> by
>>> a constant in the semivariogram:
>>>
>>> vario=variogram(NASC*10000000~1,data,cutoff=1000)
>>>
>>> When I do this multiplication I'm able to fit a variogram.
>>>
>>> So my question is, Is it possible to this? Can I back transform (by a
>>> division) my predited values when the ordinary kriging will be done?
>>>
>>>
>>
>> Hi Sam,
>>
>> It is possible, probably the covariance matrix has a lot of zeros because
>> of rounding of your small values. Could this be linked to using single
>> precision floats in gstat (edzer?)? I think there is no problem in
>> 'backtransforming' your values, i.e. you only change the unit of the
>> variable you are using (e.g. from milligram to microgram). Take care though
>> that when predicting whilst using the fitted variogram model, your
>> observations also need to be multiplied by this constant. My suggestion
>> would be to multiply the data at the very beginning of the script, do all
>> you analysis and at the very end do the division. You could also skip the
>> division and present your results in the new unit.
>>
>> regards,
>> Paul
>>
>>  Thanks!
>>>
>>> Sam
>>>
>>>        [[alternative HTML version deleted]]
>>>
>>>
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>>>
>>>
>>
>>
>> --
>> Drs. Paul Hiemstra
>> Department of Physical Geography
>> Faculty of Geosciences
>> University of Utrecht
>> Heidelberglaan 2
>> P.O. Box 80.115
>> 3508 TC Utrecht
>> Phone:  +3130 253 5773
>> http://intamap.geo.uu.nl/~paul <http://intamap.geo.uu.nl/%7Epaul>
>> http://nl.linkedin.com/pub/paul-hiemstra/20/30b/770
>>
>>
> 
> 	[[alternative HTML version deleted]]
> 
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-- 
Edzer Pebesma
Institute for Geoinformatics (ifgi), University of Münster
Weseler Straße 253, 48151 Münster, Germany. Phone: +49 251
8333081, Fax: +49 251 8339763  http://ifgi.uni-muenster.de
http://www.52north.org/geostatistics      e.pebesma at wwu.de