Chapter Contents |
Previous |
Next |
The KRIGE2D Procedure |
For all the variogram models considered previously, the following property holds:
However, a plot of the experimental semivariogram may indicate a discontinuity at h=0; that is, as , while . The quantity c_{n} is called the "nugget effect"; this term is from mining geostatistics where nuggets literally exist, and it represents variations at a much smaller scale than any of the measured pairwise distances, that is, at distances , where
There are conceptual and theoretical difficulties associated with a nonzero nugget effect; refer to Cressie (1993, section 2.3.1) and Christakos (1992, section 7.4.3) for details. There is no practical difficulty however; you simply visually extrapolate the experimental semivariogram as . The importance of availability of data at small lag distances is again illustrated.
As an example, an exponential semivariogram with a nugget effect c_{n} has the form
This is illustrated in Figure 34.9 for parameters a_{0}=1, c_{0}=4, and nugget effect c_{n}=1.5.
You can specify the nugget effect in PROC KRIGE2D with the NUGGET= option in the MODEL statement. It is a separate, additive term independent of direction; that is, it is isotropic. There is a way to approximate an anisotropic nugget effect; this is described in the following section.
Chapter Contents |
Previous |
Next |
Top |
Copyright © 1999 by SAS Institute Inc., Cary, NC, USA. All rights reserved.