[R-sig-Geo] Discontinuities in KED prediction with `nmax` set

[Piero Campalani]

Dear list,

I have a set of ~130 ground stations measurements and several grid
covariates with which I am spatialising the ground point values.
I am using external drift kriging: I noticed discontinuities in the
predicted map when using a _localized_ prediction, hence restricting the
neighbors count with the *nmax* parameter in the gstat function.

I am a bit surprised since I guessed that localising the prediction
wouldn't change that much the final map: the more distant the ground point,
the smaller the kriging weight associated with it.
I attach a screenshot to visually understand the situation: on the left
side is the KED prediction, on the right side the localized (nearest 50
points) KED prediction.
I don't know if these evident discontinuities are a normal consequence of
localized models, or if I am doing something wrong.

This is an example of what I did, using the meuse dataset:
___________________________
library(gstat)
library(sp)
data(meuse)
data(meuse.grid)
coordinates(meuse) <- ~x+y
coordinates(meuse.grid) <- ~x+y

cvgm <- variogram(zinc~dist, data=meuse, width=100, cutoff=1000)
fitted <- fit.variogram(cvgm, vgm(psill=1, model="Exp", range=100,
nugget=1))
KED_fit <- gstat(id="KED_fit", formula=zinc~dist, data=meuse, model=fitted)
KED10_fit <- gstat(id="KED10_fit", formula=zinc~dist, data=meuse,
model=fitted, nmax=10)

 KED_pred <- predict.gstat(KED_fit, meuse.grid)
KED10_pred <- predict.gstat(KED10_fit, meuse.grid)

KED_pred$KED10_fit.pred <- KED10_pred$KED10_fit.pred
gridded(KED_pred) <- TRUE
spplot(KED_pred, zcol=c("KED_fit.pred", "KED10_fit.pred"),
col.regions=colorRampPalette(brewer.pal(7, "BuPu")[-(1:1)])(21))
___________________________

Thanks for any clarification in advance.
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