The index map is a 2D matrix, of the same (latitudinal and longitudinal) size as the ROMS grid, where the element within each cell is numbered for which box or face in the BGM it falls within or on. This is illustrated in the following pictures.
Box index map:
The picture below illustrates how a box index map would look for the displayed box geometry. All elements which fall within a box, are numbered for that box. So for example, all cells that fall within box 3 (shaded purple) are labeled as '3'.
Face index map:
The face index map is similar to the box index map, but it only has face labels where faces cross cells. For cells not on faces (i.e. inner cells), they are left as NaNs. This is illustrated in the picture below. For example, cells which fall along face 16 (which separates the blue and green shaded boxes) are labeled 16.
