[R-sig-Geo] block and output grid size in block kriging

[subash]

Dear Edzer, 

To understand why block kriging results vary,with different grid sizes, to
keep gridsize identical to grid cells, and smaller than grid cells.

library(gstat)
data(meuse)
coordinates(meuse) = ~x+y
data(meuse.grid)
gridded(meuse.grid) = ~x+y

lzn.vgm<- variogram(log(zinc)~1, meuse)
lzn.fit1 <- fit.variogram(lzn.vgm, model=vgm(psill=1, model="Sph",
range=900, nugget=1))
plot(lzn.vgm, lzn.fit1)

# Ordinary kriging
lzn.ok <- krige(log(zinc)~1, meuse, meuse.grid, model = lzn.fit1)

#Block kriging 
lzn.bok <- krige(log(zinc)~1, meuse, meuse.grid, model = lzn.fit1,
block=c(40,40))

# Choosing a random location with the centre coordiantes(xx,yy) and find the
corresponding block kriged estimate
xx = 181140
yy = 333100
est.bk = lzn.bok$var1.pred[(lzn.bok$x==xx)&(lzn.bok$y==yy)]
[1] 6.499158
est.ok =lzn.ok$var1.pred[(lzn.ok$x==xx)&(lzn.ok$y==yy)]
[1] 6.499624

Why are the kriging estimates at a point different in block and ordinary
kriging, when the grid size is isentical to (40 x 40) or less than(20x20)
the meuse data.

Thanks







diff= m.bk - est.bk



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