Half-graticule

People in graticules or regions severely restricted by water might consider a modification to the algorithm that will still coincide with the standard coordinates.

Diagram of the approach

1. Take (a,b) as the standard decimal location.
2. Take (c, d) = (a % 0.5, b % 0.5) as the half-size location. (% is the mod operator)
3. New locations:
   1. (c, d)
   2. (c + 0.5, d)
   3. (c, d + 0.5)
   4. (c + 0.5, d + 0.5)

One of those is guaranteed to match (a, b), but one of the other three may be more accessible.