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

Details

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.

Ramifications

Would this lead to fragmenting of the geohash community? Perhaps not, since anyone using this technique would likely not be able to make it to the standard coordinates. This approach merely serves as a way of deterministically generating alternate meetup locations when the standard one is unreachable.