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if you have a course in txc format you upload it to the garmin training centre program as a course. then when you want to follow it, you start up the garmin and go into "do course", once you are on the course it will guide you with a map. It tells you when you are off course.
It does not include maps, so you cannot navigate (bring your iphone if you think you will get lost!). The 800 has maps, but a friend says the display is too small to navigate properly.