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

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