## GSLIB Help Page: GAM

Description:

- Variogram calculation of gridded data

Parameters:

- datafl: the input data in a simplified Geo-EAS 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 one-line 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 1-D can be handled by setting ny, nz to one and iyd, izd to zero.