[R-sig-Geo] gstat: Parameter estimate Matern vs exponential variogram model

[Edzer Pebesma]

This is well known, see e.g. Ribeiro & Diggle, "Model-based
geostatistics", 2007, page 51. The point of the Matern model is that
different values of kappa represent random fields with different degrees
of smoothness, the exponential, fixing kappa at 0.5, does not provide
that. Nobody can tell you upfront how smooth the variable is from which
you obtained data.

On 04/10/2014 12:17 AM, Moshood Agba Bakare wrote:
> Hi all,
> I used gstat to fit Matern variogram model to my crop yield by setting the
> kappa = 0.5. To my surprise, its parameters estimate are exactly the same
> with that of exponential. Does it mean there is no point in fitting matern
> model? When I set the kappa = 0.7, the parameters estimate are now
> different from that of exponential. Could anyone tell me the appropriate
> value for kappa? Having known that matern is a generalized form of
> theoretical variogram, does it worth fitting matern model when I have
> already fitted the common theoretical variogram (spherical Gaussian, and
> exponential)
> 
> 
> exponential model
> 
> model      psill   range
> 1   Nug 0.04281109 0.00000
> 2   Exp 0.76810071 5.45954
> 
> 
> 
> 
> 
> Matern Model
> 
>   model      psill   range kappa
> 1   Nug 0.04281109 0.00000   0.0
> 2   Mat 0.76810071 5.45954   0.5
> 
> 
> 
> 
> 
> R Script
> 
> ## Exponential variogram model
> exp.var<-vgm(psill = 0.7, model = "Exp", range = 10, nugget= 0.1)
> exp.mod<-fit.variogram(empvar,model=exp.var)
> print(exp.mod)
> plot(empvar,model=exp.mod, xlab = "Lag distance (m)", ylab =
> "Semivariance",main="Exponential variogram model")
> 
> 
> 
> 
> 
> ## Matern variogram model
> mat.var<-vgm(psill = 0.7, model = "Mat", range = 10, nugget = 0.1,kappa = 0.7)
> mat.mod<-fit.variogram(empvar,model = mat.var)
> print(mat.mod)
> plot(empvar,model = mat.mod, xlab = "Lag distance (m)", ylab =
> "Semivariance", main="Matern variogram model")
> 
> 
> 
> Thanks
> 

-- 
Edzer Pebesma
Institute for Geoinformatics (ifgi), University of Münster
Heisenbergstraße 2, 48149 Münster, Germany. Phone: +49 251
83 33081 http://ifgi.uni-muenster.de GPG key ID 0xAC227795

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