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FAQ: How can one determine the lag size to use in Geostatistical Analyst?

Question

How can one determine the lag size to use in Geostatistical Analyst?

Answer

There are no set rules for determining what lag size should be used. It is the craft of the researcher, their knowledge of the phenomenon they are analyzing, and the reason(s) for modeling a variogram that help to determine the appropriate lag size.
Consider what happens when we take lag size to its extremes. A lag size = 0 will produce a variogram cloud that perfectly displays all possible pairings, but makes interpretation of the variogram structure difficult. At a lag size = infinity (or a distance at least as large as the maximum distance between any two samples), we get one value represented by a single point that represents the average distance and average variogram value for all sample pairings. Selecting an appropriate lag size between these extremes allows for the creation of a manageable semivariogram to aid in interpretation.

In most cases, the goal is to have as many pairs of points as possible represented in any one variogram point. The more pairs a variogram point contains, the more it is of the sample values overall. More pairs per variogram point, however, means a wider bin, and a wider lag distance typically results in less structure for the first points in a variogram.

There are two rules of thumb for selecting a lag size:
(1)Have at least 30-50 pairs minimum for any one variogram point. Smaller bins or lag size means less pairs and probably better structure, but too small a bin or lag size typically introduces more noise into the variogram.
(2)Multiply the lag size by the number of lags, which should be about half the largest distance among all points.