[R-sig-Geo] automap model defaults

[Paul Hiemstra]

Mark Connolly wrote:
> On 04/15/2010 07:42 AM, Paul Hiemstra wrote:
>> Mark Connolly wrote:
>>> I was wondering what is behind the selection of the default models 
>>> tried by autofitVariogram (model=c("Sph","Exp","Gau","Ste")).  For 
>>> example, why was linear model left out?
>>>
>>> Thanks
>>>
>>> _______________________________________________
>>> R-sig-Geo mailing list
>>> R-sig-Geo at stat.math.ethz.ch
>>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>> Hi Mark,
>>
>> These models are the models that are most often used, so it seemed 
>> logical to use these ones. But why do you want to include the linear 
>> model specifically?
>>
>> cheers,
>> Paul
>>
> Only because it sometimes results in a lower SSError.
But probably the result in terms of kriging prediction is almost the 
same between a spherical with a very large range and a linear variogram.
> I guess that is my real question: why wouldn't I want to try the 
> linear model in the set?  The field I am looking at is a layered ag 
> field in a coastal plain, and the five depth layers (which extend to 
> less than a meter from the surface) I am looking at are often fairly 
> homogeneous for specific properties.  Some of property values I am 
> fitting produce a linear model as the best fit in a specific layer.  
> (Although sometimes when this is the case, the linear model is just 
> barely better and than the next-best non-linear model.)
>
> I assume the selection of the default models is not arbitrary, but I 
> lack the experience to include/exclude linear models in the best fit 
> search.  Best to stick with the default?
I would say yes, because the spherical includes (in practice) the linear 
variogram model as a special case.

cheers,
Paul
>
>
> Thanks,
> Mark


-- 
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 274 3113 Mon-Tue
Phone:  +3130 253 5773 Wed-Fri
http://intamap.geo.uu.nl/~paul
http://nl.linkedin.com/pub/paul-hiemstra/20/30b/770