Dear All,
Consider yield as attribute and easting and northing in meters as spatial
coordinates.
I used gls function of nlme package because of spatial dependency of the
residual. I tried to remove large-scale trend by using the easting and
northing as covariates of yield.

model.vc<-gls(yield~easting+northing,method="ML").

Being a spatial data, we are most interested in carrying out variogram
modelling of the spatial structure on the small-scale component i.e. the
residual and equally fit the spatial correlation structure on the residual
as well.

Do I need to obtain the residual from the model.vc above i.e
resid<-residuals(model.vc) and fit the other models as follow

model.sph<-gls(resid~1,corr=CorSpher(form=~easting+northing, nugget=T)) or
model.sph<-update(model.vc,corSpher(form=~easting+northing,nugget=T))

Please which one is correct.

Thank you.
Moshood

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