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Shares of GPS device makers Garmin and TomTom plummeted today through a combination of their quarterly results and the launch of Google Maps Navigation. Following both low guidance for Garmin's next quarter as well as poor results from TomTom, shares for the two fell 16.4 percent and 20.8 percent respectively and remained low through the entire trading day after news of Google's free, turn-by-turn mapping service became public....