GVCORD

     GVCORD is the vertical coordinate of the grid to be selected.

     The standard values are:
 
	NONE	for surface data
	PRES	data in pressure coordinates (millibars)
	THTA	data in isentropic coordinates (Kelvin)
	HGHT	data in height coordinates (meters)
	SGMA	data in sigma coordinates

     The value in GVCORD may be overridden by specifying %GVCORD with the 
     grids to be found.  For example:
  
	GFUNC = SUB ( TMPC @850 %PRES, TMPC @1500 %HGHT )  
  
     will compute the difference between temperatures on the 850-mb level 
     and the 1500-meter level.