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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 cn 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 cn has the form
This is illustrated in Figure 34.9 for parameters a0=1, c0=4, and nugget effect cn=1.5.
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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.
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