GSLIB Help Page: VARMAP
Description:
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Variograms are traditionally presented as 1-D curves as a function of
the distance h along a particular direction. It is often useful
to have a global view of the variogram values in all directions.
The variogram map is a 2-D plot of the sample semivariogram for all
experimentally available separation vectors
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Parameters:
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datafl: the input data in a simplified Geo-EAS formatted file.
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nvar and ivar(1) \ldots ivar(nvar): the number of
variables and their column order in the data file.
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tmin and tmax: all values, regardless of which variable,
strictly less than tmin and greater than or equal to tmax
are ignored.
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igrid: set to 1 if the data are on a regular grid and set to 0 if
the data are irregularly spaced.
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nx, ny and nz: the number of nodes in the
x, y, and z coordinate directions (used if
igrid is set to 1). Any of the numbers may be set to
one if that coordinate is not present in the data, e.g., 2-D data may
be handled by setting nz to 1.
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xsiz, ysiz and zsiz: the separation distance between the
nodes in the x, y, and z coordinate directions
(used if igrid is set to 1).
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icolx, icoly and icolz: the columns for the
x, y, and z coordinates (used if igrid is
set to 0). Any of the column numbers may be set to zero if that
coordinate is not present in the data, e.g., 2-D data may be handled
by setting icolz to 0.
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outfl: the output file for the variogram maps/volumes are
written to a single output file named outfl This file contains
each variogram map/volume written sequentially as a grid with the
x direction cycling fastest, then y, then z.
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nxlag, nylag and nzlag: the number of lags to compute
in the X, Y and Z directions
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dxlag, dylag and dzlag: the lag tolerances or ``cell
sizes'' in the X, Y and Z directions
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minpairs: the minimum number of pairs needed to define a
variogram value (set to missing if fewer than minpairs is
found).
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standardize: if set to 1, the semivariogram values will be
divided by the variance
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nvarg: the number of variograms to compute.
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The "variogram type" is specified by an integer
code
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ivtail, ivhead and ivtype: for each of the nvarg
variograms one must specify which variables should be used for the
tail and head and which type of variogram is to be computed.
For direct variograms the ivtail array is identical to
the ivhead array. Cross variograms are computed by having the
tail variable different from the head variable.
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cut: whenever the ivtype is set to 9 or 10, i.e., asking
for an indicator variogram, then a cutoff must be specified immediately
after the ivtype parameter on the same line in the input file.
Note that if an indicator variogram is
being computed then the cutoff/category applies to variable
ivtail(i) in the input file (although the ivhead(i)
variable is not used it must be present in the file to maintain
consistency with the other variogram types).
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Application notes:
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Directions of anisotropy are usually evident from a variogram map.
Visualization of a variogram volume is best done using a graphical
routine that allows continuous sectioning of the volume along any of