[R-sig-Geo] automap model defaults

[Edzer Pebesma]

Mark, please note that the linear model (with nugget), having only two
parameters where most models have three (nugget, range, sill) or four (+
smoothness) arises as a special case of e.g. the spherical or
exponential model when the range becomes infinite, or at least very
large compared to the spatial extent. I'm not sure if automap allows
fitting of very large range values.
--
Edzer

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
>>>
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>> 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.  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?
> 
> Thanks,
> Mark
> 
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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