I had looked into local geocoding databases but did not want to add a database file in the git repository. reverse_geocoder looks really interesting though and I may have a look at adding that.

For the time being, elodie does cache responses from the MapQuest API. And to reduce the number of API calls it does some approximation by seeing if an entry exists in the cache within 3 miles of the current photo --- if so, it uses that instead of looking up the location. [1]

[1] https://github.com/jmathai/elodie/blob/75e65901a94e14e6fd1ff...