I have just returned from a trip to Scotland, so I can come back to this and add comments that I couldn't from my Ipad.
The route that
@wolfman paosted - Day 1.gpx - I managed to analyse earlier. It doensn't contain any routing information - only the route points, and it identifies each point as being either Via or Shaping. The calculation of the route is left entirely to the Zumo.
This is not an issue, and one which you need to be aware of particularly with the Zumo XT (and probably the XT2), becasue the method of calculating the route is completely different from the way that (say) Basecamp will calculate it. The XT uses 'faster time', and the definition of that seems to have changed with the introduction of the XT. Faster time now seems to mean 'head for the faster roads - even if it means making the route longer in distand and/or time. This is probably not what is wanted - personally I like the way that my 590 calculated the routes - I have got used to it. But the XT behaves differently, and routes have to be planned accordingly.
Basecamp will send the entire route - route points and thousands of additional points to pin the route the the roads that were plotted - so the Zumo will get exactly the same roads. But the XT does like to recalculate the entire route in certain situations (skip, traffic, etc). I expect the XT2 may be the same - but I don't know). This means that we have to take into account what the XT / XT2 will do if it does recalculate.
I am not a believer in adding so many shaping points that the stanav has got to follow thr route precisely. You might as well use a track if you are going to do that. Or a track trip.
I don't have access to USA maps for the Zumo or for Basecamp. But I can use MyRouteApp. This reveals that the identical route plotted on two different maps results in entirely different results.
Here is a section of the Day 1.gpx route that was produced by MRA using HERE maps - the maps used by Garmin. Clck the map for a larger image.
- MRA HERE.jpg (118.86 KiB) Viewed 1701 times
Here is the same route produced by MRA on the OSM maps.
- MRA OSM.jpg (104.62 KiB) Viewed 1701 times
And this shows the two route surperimposed to show the difference
- MRA Merged.gif (153.69 KiB) Viewed 1701 times
THe problem with this route is that there are a lot of major roads around. I have yet to prove a theory, but my current belief is that if there is a major road heading towards the next route point - and that road is closer to your current position thatn the route point - then it will head for the major road. That theory probably needs some working on but at presnt it seems that something like that is happening.
I have never been a believer in pinning the route down by using a load of shaping points. There should be no need, and doing that can lead to all sorts of problems if you have to deviate. But placing route points strategically can solve routing problems.
For example - I never place a route point close to a junction. I place the route point well down the road that I want to be riding - other wise, the Zumo will turn at the junction, visit the route point and then turn back to the road that it was on. Putting another route point at the other end of the road doesn't solve the problem - the Zumos will use the faster road and then visit the junctions at each end.
This isn't new to the XT. In the UK we have many roads that run parallel or close to old major roads. Eg - the northern part of the A6 - a major rout north - runs quite close to the motorway the M6. Similarly once into Scotland the B7079 was once the old dual carriageway A74 into. It is now a single carriageway road that runs parallel to the newer M74 and is brilliant to ride. But plotting a route on either of these requires lots of shaping points - otherwise the routing software will put you onto the motorway and take you off just to visit the shaping point and then return to the same motorway junction.
In cases such as these, it is better to place the shaping points on the road in such a way that it breaks it into thirds. So if the road is 9 miles long, place one SP at 3 miles and another at 6 miles.
In your example, route 75 is the faster route. Points 4 and 5 are far too close to it. If route 58 and 27 is your intention (the red route), than point 5 could be moved closer to (say) Sunbright on route 27. Similarly point 4 could be moved along route 58 - but there are some smaller roads and route 40 to contend with. I managed to get it to follow route 27 just by moving point 4 and 5. I would then need to see how that behaved on the XT. If route 27 insn't your intention, then your route will need many more shaping points - because the Zumo is going to head for that, rather than the side roads. Sorry - the mapping
If you transfer the track of the route that you want it to follow and show the route and the map on the same screen, you can easily see where the Zumo is calculating a different route. Try again. The more you do it, the more able you are to predict what the Zumo is going to do. It then become second nature to produce a route that the Zumo isn't going to screw up. Always get the XT to recalculate the route - set navigation to straight line, then back to Faster time - whatever it does with the route then, that is what it will do if it has to recalculate on the road.