Meng,
You wan't to have an evenly spaced sample pattern for you estimation
of the variance, if you use samples within range of each others then these
are clusters of samples which will overweight that area, hence by removing
samples below the range, you remove "clusters" of samples. A common method of performing statistics on spatial data is first to perform data declustering, than calculate your statistics, however as Isobel points
out a fast way to do this is remove samples below the range.
Digby
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