GLEVEL is the vertical level for the grid.
Grids may contain two levels separated by a colon. If the grid to
be selected contains only one level, the colon and second level may
be omitted. In this case, the second level is stored in the grid
file as -1.
Note that the vertical coordinate system for GLEVEL is specified by
The value in GLEVEL may be overridden by specifying @GLEVEL with the
grids to be found. For example, the following two computations are
GFUNC = SUB (TMPF@850,TMPF@500)
GFUNC = LDF (TMPF) and GLEVEL = 850:500.
See the help information on gparm for information on how to specify
a layer in GLEVEL for a LYR_ function.