Reverse Geocoding Without Web Access

Question

I am working on an application where one of the requirements is that I be able to perform realtime reverse geocoding operations based on GPS data. In particular, I must be

Answer 1

I suggest using a variant of your first idea: Use a spatial index. A spatial index is a data structure built from rectangles, mapping lat/long to the payload. In this case you will probably map rectangles to state-province pairs. An R-tree may be a good option. Here's an R-tree python package. You could detect roaming by comparing the results of consecutive searches.