A unit's movement options are vectors. As it's designed, you've got vectors in the due directions and the medians between those, each scaled to be the size necessary to make the grid work as mapped from a sphere for the particular location. So the east vector at 45 N is root(2)/2 the size it is at the equator.

So if you've got an east vector for every square that is a function of the location, that means there's an east vector field. Same goes for the other seven options. Might as well make a zero at the poles, since one zero is weird, zeroes that don't line up are weird, and no one wants to settle at the poles anyway.