automap model defaults
Edzer Pebesma wrote:
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.
See the following example: data(meuse) coordinates(meuse) = ~x+y vgm1 <- variogram(log(zinc)~1, meuse) vgm1$gamma = 1:15 + 0.00001*runif(15) v1 = fit.variogram(vgm1, vgm(15,"Sph",1000,1)) v2 = fit.variogram(vgm1, vgm(15,"Lin",1000,1)) plot(vgm1, v1) #Sph plot(vgm1, v2) #Lin Probably the difference in the estimated covariance matrix is quite small between the linear and the spherical variogram. In this example the range is quite a bit larger than the spatial extent, so I think automap can handle this. cheers, Paul
-- 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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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