When planning a loop route for use on a Garmin XT XT2 make sure that
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you have your start point a few hundred meters away from where you are this ensures that the unit passes through the start and does not try and take you back through it if you miss it by a bit for example if you start a route at your hotel start it up the road a bit in your intended direction of travel as a slight mistake in the position of the start in a hotel car park might mean you miss the actual start.
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If making a loop route make sure that there is at least one via point at the farthest extent of the loop (On a Garmin these show up as flagged waypoints a hand in MRA) rather than just standard waypoints (a drop point in MRA and a blue dot in Garmin called a shaping point). This means that if you miss a waypoint and the unit recalculates it won't take you directly to the finish(Which is always a Via point.)
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I have an XT and tend to download the route file in the form of a GPX1.1 route and track. I have both visible on the screen when navigating in a contrasting colour so that I can see if the Garmin has recalculated the route and made it different (the track stays as shown on the MRA screen)
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I tend to leave recalculation as prompted on Garmin units and navigate myself back to the route though I do sometimes allow recalculation.