SMI Map Format



We will be changing the dimensions of the 9-km SMI maps from 4096x2048 to 4320x2160. The purpose of this change is to match the latitudinal distribution of bins in the Level-3 bin files.

The standard mapped image (SMI) format was originally defined to include a global image with a column x row dimensionality of 4096 x 2048 pixels on a cyclindrical (a.k.a.: equi-rectangular, plate carre) projection. This yields a resolution of approximately 9-km x 9-km for an equatorial map pixel. The actual choice of the "power-of-2" dimensions was based on maximum monitor resoltutions available at the time the format standard was defined.

The SMI maps are generated from Level-3 bin files, which are equal-area bins defined in a sinusoidal distribution of 2160 rows. The number of bins per row varies, with the maximum being 4320 at the equator. The mapping of an integer number of latitudinal bins to an integer number of latitudinally distributed map pixels will result in systematic sampling artifacts, unless the number of map pixels exactly matches the number of bins. The original SMI map format results in a sampling bias on every 18th row. This problem was reported by Kay Kilpatrick, U. Miami, RSMAS, who noted that the same biasing problem also exists in the AVHRR Pathfinder SST maps.