Pitney Bowes MapInfo Vertical Mapper - Manual (Page 162)

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Vertical Mapper 3.7
The sill is a variance value that the model curve ideally approaches but does not cross.
The range is the distance value at which the variogram model determines where the
sill begins.
Anisotropic Modeling
It is quite natural for the behaviour of a data set to vary differently in one direction as compared to
another. For example, a steeply sloping hill will typically vary in two directions. The first is up and
down the hill where it varies sharply from the top to bottom, and the second is across the hill where it
varies more gradually. When this occurs in a data set, it is called anisotropy. When performing
anistropic modeling, you are essentially using sample data points that will most accurately reflect the
behaviour of the surface. This is achieved by creating additional models for each direction analyzed.
When interpolating points are oriented in a north-south direction, kriging weights can be influenced
to use the parameters of one model while the points oriented in an east-west direction will be
weighted using a different model.
A two-directional, two-model semivariogram.