## GSLIB Help Page: GAMV

Description:

- Variogram calculation with scattered data

Parameters:

- datafl: the input data in a simplified Geo-EAS formatted file.
- icolx, icoly and icolz: the columns for the x, y, and z coordinates (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)
- nvar and ivar(1) \ldots ivar(nvar): the number of variables and their column order 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 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).
- nlag: the number of lags to compute (same for all directions).
- xlag: the unit lag separation distance.
- xltol: the lag tolerance. This could be one-half of xlag or smaller to allow for data on a pseudo-regular grid. If xltol is entered as negative or zero it will be reset to xlag/2. A pair will report to multiple lags if xltol is greater than one-half of xlag
- ndir: the number of directions to consider. All these directions are considered for all of the nvarg variograms specified below.
- azm, atol, bandwh, dip, dtol, and bandwd: the azimuth angle, the half window azimuth tolerance, the azimuth bandwidth, the dip angle, the half window dip tolerance, and the dip bandwidth. The azimuth is measured in degrees clockwise from north, e.g., azm=0 is north, azm=90 is east, and azm=135 is south-east. The dip angle is measured in negative degrees down from horizontal i.e., dip=0 is horizontal, dip=-90 is vertical downward, and dip=-45 is dipping down at 45 degrees. bandwh is the horizontal ``bandwidth'' or maximum acceptable horizontal deviation from the direction vector. bandwd is the vertical ``bandwidth'' or maximum acceptable deviation perpendicular to the dip direction in the vertical plane.
- 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.
- 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).