Before beginning, the data should be examined to ensure that geostatistical analysis is appropriate. At least 30-50 data points are recommended, and some authors have suggested that the minimum number of data needed is as much as 100 (e.g., Webster and Oliver 1993), especially for data that exhibit a large amount of short range variability.
Use the Data Analysis module under the Tools menu in VSP to check for extreme outliers and see whether the data follow a highly skewed distribution. Kriging also requires the assumption that the data are spatially continuous, which will be determined during the variogram analysis.
The Exclude column on the Data Entry tab is a binary indicator that indicates whether or not a data point should be included in the variogram calculations. The user might set the Exclusion indicator to 1 (i.e., exclude the point) if a data point is highly redundant. An example might be the large number of wells present along a remediation barrier. It might be useful to exclude such a large number of highly redundant points because they can dominate the variogram. The user can calculate the variogram both with and without points marked for exclusion.
The Reserved column on the Data Entry tab is a binary indicator for whether or not a well should be included in the well redundancy analysis. In some cases there may be wells that cannot be dropped from the sampling schedule, for regulatory or other reasons. In that case, they can be included as data points in kriging, but would not be included in the list of wells that could be removed from the sampling schedule, even if they appear to be redundant.
Cameron, K, and P Hunter. 2002. Using Spatial Models and Kriging Techniques to Optimize Long-Term Ground-Water Monitoring Networks: A Case Study. Environmetrics 13:629-59.
Deutsch, C.V. and A.G. Journel. 1998. GSLIB Geostatistical Software Library and User's Guide, 2nd Edition, Applied Geostatistics Series, Oxford University Press, Inc. New York, NY.
Gilbert, RO. 1987. Statistical Methods for Environmental Pollution Monitoring. Van Nostrand Reinhold, New York.
Isaaks, EH, and RM Srivastava. 1989. An Introduction to Applied Geostatistics. Oxford University Press, New York.
Webster, R, and MA Oliver. 1993. How Large a Sample Is Needed to Estimate the Regional Variogram Adequately? . Geostatistics Troia '92 , ed. A Soares, Vol 1, pp. 155-66. Kluwer Academic Publishers, Dordrecht.