Consequences of joining a GPX round-trip track somewhere in the middle.

I'm trying to imagine the course you described, but I can't find a figure where everything you describe happens...

Let's say you arrive by car with your bike in the trunk, but there is no parking lot at the beginning of the track; a typical everyday situation in Germany.

So you search for a parking spot by car and eventually find a one, 10 miles away from the actual GPX starting point.

On older Garmin devices, you had to go back 10 miles to the start of the GPX track for the navigation to start. But thanks to firmware advancements from Garmin, it is now possible to join the track anywhere and as soon as the device detects that you are on track, it says "course found" and will navigate you for the remainder of the track from where you joined until the finishing point. But that is not the parking lot, you are lost in the middle of the round trip.

Let's use an example of a round trip track that is 30 miles in length. You join the track at mile #10 and do the math of how long you still have until sundown. Unfortunately, Garmin calculates time and distance only for the remainder of 20 miles, not the full 30, since the track endpoint comes after 20 miles. You "arrive" in time, but realize that you are still 10 miles away from the parked car. These 10 miles are not forecasted and did not contribute to the ETA calculation. Instead, a new navigation must be started for the last 10 miles. You are running late.

Or ride to the start of the ride, navigate to start? yes

The route picked to get to the start also coincides with the end of the route you chose to ride for the day.

You're on track, you arrive to the start. Route finished!

Ok, now I understand what happened.

But - imho - there is no solution but loading courses.

I would also like to have the option of once joining a “know” and “selected” course, if it being a loop, navigation sets you up for the entire loop from “your” starting point/entry.

I think Labbyrun has a valid point/request.

Good idea, or a prompt saying you have arrived at the start, continue with route?

or something along those lines

Let's say the course is a 40 mile loop and you start it at mile 20.

The ETA is calculated to the end of the course (20 miles). Not to returning where you started (40 miles).

Labbyrun said:

Hence my question, is there an option to "close" such courses so that start and end are joined to form a "ring" which can be joined at any position and then distance and ETA are calculated from the location where the course was joined? Any ideas?

No, there's no such option. It's not likely you are going to see this changed. This is (rather) old behavior. 

The way the ETA works is that the distance along the track is calculated for each point before navigation.

The device knows how much distance remains by taking the distance at the current point and subtracting it from the total track distance. (Note that the ETA is not updated if you go off course.)

One reason it's done this way is it reduces the number of calculations needed while using the course (which makes things faster)

Note that this calculation can't be done by subtracting the actual distance traveled since the rider might have deviated off the loaded track.

cycleanalyze said:

Good idea, or a prompt saying you have arrived at the start, continue with route?

What if it's not a loop?

This idea doesn't address the issue the OP has (the messed-up ETA).