GLEVEL

     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 
     GVCORD.

     The value in GLEVEL may be overridden by specifying @GLEVEL with the 
     grids to be found.  For example, the following two computations are 
     identical:
  
		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.