GSLIB Help Page: GAM

Description:

Variogram calculation of gridded data

Parameters:

datafl: the input data in a simplified GeoEAS formatted file.
The data are ordered rowwise (x cycles fastest, then y, then z).

nvar and ivar(1) ... ivar(nvar): the number of variables
and their columns in the data file.

tmin and tmax: all values, regardless of which variable,
strictly less than tmin and greater than or equal to tmax
are ignored.

outfl: the output variograms are written to a single output
file named outfl The output file contains the variograms
ordered by direction and then variogram type specified in the parameter
file (the directions cycle fastest then the variogram number). For
each variogram there is a oneline description and then nlag
lines each with the following:

lag number (increasing from 1 to nlag

average separation distance for the lag.

the {\em semivariogram value (whatever type was specified).

number of pairs for the lag.

mean of the data contributing to the tail.

mean of the data contributing to the head.

the tail and head variances (for the correlogram).
The {\tt vargplt program documented in section VI.1.8 may be used to
create PostScript displays of multiple variograms.

igrid: the grid or realization number.
Recall that realizations or grids are written one after another;
therefore, if igrid=2 the input file must contain at least
2 x nx x ny x nz values and the second
set of nx x ny x nz values will be taken
as the second grid.

nx, xmn, xsiz: definition of the grid system (x axis).

ny, ymn, ysiz: definition of the grid system (y axis).

nz, zmn, zsiz: definition of the grid system (z axis).
One or two dimensional data may be considered by setting the number of
nodes in some directions to 1. Often, gam is used to check the
variogram reproduction of realizations from a simulation program.

ndir and nlag: the number of directions and lags to
consider. The same number of lags are considered for all directions
and all directions are considered for all of the nvarg variograms
specified below.

ixd, iyd and izd: these three arrays specify the unit
offsets that define each of the ndir directions (see
section III.2).

standardize: if set to 1, the semivariogram values will be
divided by the variance

nvarg: the number of variograms to compute.

The "variogram type" is specified by an integer
code

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, e.g., if
ivtail(i) is set to 1, ivhead(i) is set to 2, and
ivtype(i) is set to 2, then
distance measure i will be a cross semivariogram between
variable 1 and variable 2. Note that ivtype(i) should be set to
something that makes sense (e.g., types 1,2, or 3); a cross relative
variogram would be difficult to interpret. Further, note that for the
cross semivariogram (ivtype=2) the two variables
ivtail and ivhead are used at both the tail and
head locations. The ivtype variable corresponds to the
integer code in the list given in section III.1.

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).

Application notes:

Regularly spaced data in 1D can be handled by setting ny, nz
to one and iyd, izd to zero.