PRO GRIDINDEX_TO_LATLON, INDEX, RES, LAT, LON

; For an equal angle earth grid spanning the range -180 to +180 in longitude
; and -90 to +90 in latitude, compute the latitude and longitude of the 
; lower left corner of the 1D grid cell given the zero-based 1D grid cell index
; and grid resolution (in degrees).
;
; The origin of the grid is at -180W, -90S.
;
; For a grid with 10 degree resolution, there are 648 grid cells covering the
; Earth (18 rows and 36 columns). Grid cell index 0 is at lower left of the
; global grid, and grid cell index 647 is at the upper right.

;- Get number of columns in the grid
numcol = floor(360.0 / res)

;- Compute row and column in the grid (zero-based)
row = index / numcol
col = index mod numcol

;- Compute lat and lon
lat = float(row * res) - 90.0
lon = float(col * res) - 180.0

END