[R-sig-Geo] variogram model with gstat

[Moshood Agba Bakare]

Dear All,
I used gstat package of R to fit variogram model to experimental variogram
having normalized the raw data as

VN = (VP -VMean)/VSd

Where VN is the normalized yield value, VP is the original yield value at
sampling point, VMean is average yield  value, and VSd is the standard
deviation of yield value.

The reason for normalization is to be able to assess the stability of
spatial pattern of yield over a number of years. Having normalized the
data, I expected the sill i.e. the variance of the data to be one but to my
surprise the semivariogram plot shows sill of 0.8. What is responsible for
this?

The result of fitting exponential function showed a nugget effect of zero
as showed below

## Exponential variogram model> exp.var<-vgm(psill = 0.8, model="Exp",
range= 15, nugget= 0.1)> exp.mod<-fit.variogram(empvar,model=exp.var)>
print(exp.mod)  model     psill    range
1   Nug 0.0000000  0.00000
2   Exp 0.8066099 11.36279


Does it mean the exponential structure explained 100 percent of total
variation in a normalized yield variable? I am confused! Have a look at the
attached file for the semivariogram plot. Do I need to detrend before
fitting the model? it seems the plot increases monotonically without level
off. Please advise.

Thank you
Moshood
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