Meng-ying,
Sorry I don't have time to process the data yet, but a summary so far,
is that geostatisitcs was originally developed in the mining field, and
for the application of mineral resource assesment with which I am involved
where sampling patterns are generally clustered and the use of the sill of the
variogram can be used as an estimate of the variance at your discression.
As Isobel says this is where the half originates in the equation for the
experimental variogram, as the sill will often approximate the variance.
However with many applications of geostatisitics much more detailed and
regular sampling patterns are used (even in mining e.g. soil geochemistry),
in which case the sill is not an exact estimator (I'm not sure of the correct
terminology here) of the variance and should be treated as such, though Isobel
seems to be more familiar with the mathematics of this, and it might be
interesting a further study of this.
Digby
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