I hate not knowing how something works. My childhood is littered with electronic devices pulled apart and analysed … radios, motors, cameras, computers.
I couldn’t figure out how the eTrex navigates along a route so I had to run some field tests which for a GPS unit means going for a walk. The first few tests were inconclusive so on the way to work this morning I walked along a 3-point route and checked how the Garmin calculated the next waypoint at four points along the route: before the route, between the first two waypoints, between the second two and after the route. I checked it both ways, so following the route to the northmost waypoint and then to the southmost, so eight datapoints all up.
The results are depicted in the diagram below; the results for following the route south are the same as the reverse of following the route north so I’m just showing the four scenarios here:
The eTrex assumes that you are standing at the first waypoint so it doesn’t bother to navigate you to the first waypoint. Whilst you can create a two-waypoint route it doesn’t really make sense as it will navigate directly to the second and final waypoint so you might as well just GOTO that waypoint – except that if you have the two waypoints as a route you can see a line between them on screen which might help you orient.
The Garmin eTrex chooses which waypoint to send you to depending on where you are in relation to the waypoints. If you’re closer to a waypoint further along the route it’ll just skip earlier waypoints and won’t make you U-turn.
In this following additional test I walked past the second waypoint instead of going to it. Once the eTrex determined I was giving the second waypoint a miss bypassing it by 70 meters it switched over to the third and final waypoint: