you shouldn't need a GPS fix, because you should know your position to a much better accuracy using the visual odometer. for large scale 3d navigation, this ap

you shouldn't need a GPS fix, because you should know your position to a much better accuracy using the visual odometer. for large scale 3d navigation, this approach is what we do, use a rough GPS location to cut down the search area. This was because we were using a stateless system. Its much more efficient to have a "rough localizer" to find that first fix, the optimise map loading based on current precise position and likely heading. On this system smaller system, once you have found your initial position (which will at worse require you to go through the entire DB once) you can keep most things on disk and selectively load your active area (the maximum possible travel time + load time + fudge factor.)