[R-sig-Geo] automap model defaults

[Mark Connolly]

On 04/15/2010 09:56 AM, Paul Hiemstra wrote:
> 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.
> 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
>
>
Thanks.  This and the related responses from you and Edzer are very 
helpful and make perfect sense.  I think I was losing the forest for the 
trees, or in the case the the maize stand for the maize stalks.

Mark