First a bit introduction: For good or bad I am from the "lucky" young people that grow up with technology. I was used to get directions from my smartphone. Recently I got into motorcycles, and one thing led to another, now I'm about to do my first group ride around Europe with fellow bikers! Without doubt you need a proper satnav device on your bike - you can't use your precious phone. Given that we are doing a group ride you can expect that every member of the group needs to be involved in the process of creating the perfect route for our trip
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Without further ado let me tell you how I managed to get the Google Maps multiple destination route to work flawless on our Zumo XTs.
The first step is to open Google Maps on your PC with your account logged in. On the left side you will see "Saved". When you click on that a menu appears with your saved places, visited ecc. What you are looking for is the last tab - "Maps". Click on that tab and you'll see - "Create and manage maps in My Maps". Click on "Open My Maps". A new tab should open. Click on "+ Create a new map". When a new map has opened you can name it and then under the search bar you'll find a directions button (hover with the mouse on the buttons under the search bar and you'll find the one labeled "add directions"). On the left side appears a new layer with two destinations you can enter. Don't worry - you can add as many destinations as you want. You can modify the route by simply dragging it on the map - same as on Google Maps. After you are done modifying your route click on the three dots on the right side of the map name -> export to KML/KMZ. On the pop-up window you can chose to export the entire map or only layers from it. (I prefer exporting the layer with our final route as in our bike group every member created different layer.) Leave the boxes UNchecked. Download the file. When you download the file go to: https://www.gpsvisualizer.com/ . Upload the KMZ file you just downloaded from G Maps and select to convert it in GPX. Download the GPX file. Here comes the tricky part.
When you download the GPX file you have to open it in empty Notepad file. When you open it things are pretty straight forward. You need to modify the following: Delete every line that starts with "<wpt", until you reach line "<trk>" - DON'T delete that. Check the name of your route in line "<name> </name>". When ready, save with "ctrl+S". Now you are all set and can send the GPX to your Zumo XT. It will load the route as Track. Open it and save it as Trip. It may suggest to recalculate the route but if you want the route to be 1:1 as in G Maps - decline. Now you are ready for the trip! Simply go to Apps -> Trip Planner -> Saved Trips and select the route you imported!
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I know it is a bit long but since I couldn't find anything on the internet about this topic that just works, I decided to share my experience. I am aware that there exist ways of doing this with Basecamp, Garmin Explorer but I just don't like them. I wanted something that everyone knows to use, like Google Maps, and that was easy to modify the route.
Safe trips to everyone and don't be mad if I don't answer frequently - a trip is awaiting me

P.S. The lines in the GPX file "<wpt" are the points/destinations in your route. I prefer to delete them because the Zumo XT added them to my saved places. And I simply didn't like the idea. Feel free to leave those lines in the GPX file if you want to add the destinations to your favorites.