Code table 7 -- Resolution and component flags
from the ECMWF publication "Binary data representation - FM 92 GRIB", May 1992.

Bit      Value     Meaning

1        0         Direction increments not given
         1         Direction increments given
2        0         Earth assumed spherical with radius 6367.47 km
         1         Earth assumed oblate spheroidal with size as 
                   determined by IAU in 1965 (6378.160 km, 6356.775 km,
                   f = 1/297.0)
3-4                Reserved
5        0         Resolved u- and v-components of vector quantities 
                   relative to easterly and northerly directions
         1         Resolved u- and v-components of vector quantities 
                   relative to the defined grid in the direction of increasing
                   x and y (or i and j) co-ordinates respectively