PRO LATLON_TO_GRIDINDEX, LAT, LON, RES, INDEX

; For an equal angle earth grid spanning the range -180 to +180 in longitude
; and -90 to +90 in latitude, compute the zero-based 1D grid cell index corresponding
; to a given latitude, longitude, and grid resolution (all 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 = floor((lat +  90.0) / res)
col = floor((lon + 180.0) / res)

;- Compute grid index (zero-based)
index = numcol * row + col

END