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To use Routes successfully on Garmin Satnavs you must use the same map to generate the Route before sending it to the unit, this is because the Route file does not contain any map data, it only has a series of waypoints. In this example let us say you use a map containing the green road and you pick points A,B and C from it, the Route file only contains the 3 points.
A Satnav will take the 3 Route points and connect them by auto-routing between them, that means finding a road on it's map to follow using any rules like, shortest, quickest, no motorways etc., BUT if the map on the Satnav only has the purple road it will use that because it doesn't know about the green road.
Some Satnavs may accept
Tracks as well as
Routes, Tracks are not auto-routed, the same 3 points in a Track are joined by straight lines to form the red line, however in my experience most Satvans only allow Routes and not Tracks.
If you use
Garmin Basecamp to generate the Route you don't get these problems because Basecamp will copy maps from your Satnav into it's own area, you then have exactly the same maps on both Basecamp and Satnav, you also get the same shortest, quickest etc. rules applied so you know that the Route on Basecamp and Satnav will be the same and the maps are updated when the maps on your Satnav are updated
Basecamp will perform auto-route exactly the same as your Satnav will to suggest a Route, you can alter it if you want to change it before downloading to your Satnav