Digby The variance/sill relationship is theoretical and does not depend on the layout of the samples, regular or clustered. Since the sill only uses pairs where samples are uncorrelated from one another, the clustering is irrelevant.
It does depend on the distribution of the samples values being 'stationary', that is having constant mean and variance over the study area. It also depends on that distribution having a valid variance. For example, the variance of samples from a lognormal distribution depends on the average of those samples - hence the proportional effect. All of this is explained in any basic geostatistics book, including Matheron's original Theory of Regionalised Variables and my Practical Geostatistics (Chapter 3) which cn be freely downloaded from http://geoecosse.bizland.com/practica.htm Isobel http://uk.geocities.com/drisobelclark/seasonsgreetings.htm
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