I must admit, it would be nice if you can download a certain area of maps.
For example, if you are going on holiday, it would be nice to have the maps for Spain already on your SD card so that when you are lost, you can get back to your hotel/restaurant/pub etc. Not necessarily just for TBT, but top down maps too.
I do think (for local journeys) the maps downloads before the navigation starts, so that if you do lose 3g/GPRS signal you still have a visual map, and it also downloads the street view level of the destination (in my experience, it loads very quickly when you reach your destination, so I believe it is pre-downloaded)
Maybe its something that Google would take into consideration if people asked them for it (or a dev for 'hacked' versions of Google maps)
EDIT: should also note that Garmin isn't yet released on Android. They will soon be releasing a Garmin Android phone. Maybe after that they will release the TBT Nav as a standalone app
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