The kriging options can be edited with the Kriging Options dialog. This dialog is reached through the 3D Interpolation Options dialog. The options in the Kriging Options dialog are as follows:
The pull-down list in the kriging method section is used to select which kriging technique is used. The options are Simple Kriging or Ordinary Kriging. (See Kriging)
When performing universal kriging, a drift function should be defined. The Drift button brings up the Drift Coefficients dialog. Each of the toggles in the dialog represents a single component of the polynomial equation defining the drift. Initially, all of the toggles off by default. Turning on coefficients enables universal kriging and defines the drift polynomial. For example, to use a planar drift function, only the linear terms should be used.
The Search Options button brings up the Search Options dialog. The Minimum and Maximum values in the Number of points to use for kriging controls how many of the points found in the search radius are actually used in the kriging calculations. If fewer than the minimum value are found, a default value (-999) is assigned to the interpolation point. If greater than the maximum value is found, the closest points are used.
The input data cutoff values are used to screen out data values outside the specified range. Points with values outside this range are ignored.
If the Octant option is selected in the search type section, a maximum of N points in each of the eight octants (for 2D a quadrant is used) surrounding the interpolation point are used in the calculations. This method results in better performance with clustered data. If the Normal method is selected, the octant approach is not used.
The Search Ellipsoid button brings up the Search Ellipsoid dialog. When a value is interpolated to an interpolation point, only a subset of the scatter points in the vicinity of the interpolation point are used in the calculations. The items in the Search Ellipsoid dialog control the shape of a "search space" surrounding the interpolation point. Only points in this search space are considered candidates for use in the kriging calculations.
By default, the search space is a circle (sphere in 3D) centered at the point with a radius defined by the Maximum search radius item. For problems exhibiting anisotropy, the search space can be transformed to an ellipse (ellipsoid in 3D). The anis1 factor and the azimuth angle control the shape and orientation of the ellipse. The azimuth represents the rotation of the major principal axis clockwise from the +y axis. The anis1 factor represents the ratio of the search radius along the minor principal axis relative to the search radius (the maximum radius) in the major principal direction. In most cases, the anis1 factor and the azimuth angle should match the anis factor and azimuth angle defined in the Variogram Editor.
Regardless of which kriging method is selected, a model variogram must be constructed prior to interpolating the values from the scatter points to the target object. In some cases, multiple variograms must be defined. The basic steps involved in constructing a model variogram are to first build an experimental variogram and then construct a model variogram that matches the experimental variogram. Variograms are constructed using the Variogram Editor. The Variogram Editor is activated by selecting the Edit Variogram button in the Kriging Options dialog.
GMS – Groundwater Modeling System
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