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I am using gstat package for deriving a variogram using exponential model. My R code goes like this:

g <- gstat(id= "HV",formula=HV~X+Y,data=vari)
vari.var <- variogram(g)
vari.var
plot(vari.var,col="black",pch=16,cex=1.3,xlab="Distance",ylab="Semivariance",main="Omnidirectional variogram NDVI")
model.vari <- fit.variogram(vari.var,vgm("Exp",range=range,nugget=0),fit.method=2)
plot(vari.var,model=model.vari,col="black",pch=16,cex=1.3,main="Omnidirectional variogram NDVI").

I am not sure whether it is correct or not.

Also I am not sure how to calculate sill values.

3

Yes, this is correct. When you print the model by typing

model.vari

you'll see sill values, split up in a nugget component (the offset) and the exponential component. The sum of these two is usually indicated by "the sill value" (i.e., around 25).

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