One corridor, two passes
The return line may be hidden directly beneath the outward line.
An out-and-back begins at one end, travels to a turnaround and returns towards the start. When both legs use the same path, their map geometry overlaps. The route may look like a single open line even though its point sequence covers the corridor twice.
Order creates the distinction. The outward points run from start to turnaround. Later points run back in the opposite direction. A position halfway along the corridor can therefore represent two valid progress values, one before the turn and one after it.
A GPX that contains only start-to-turnaround points is not a complete out-and-back description. It is a one-way line plus an unstated intention to return. Distance and remaining-progress figures based on that file may cover only half the planned outing.
The state change
The far point separates outbound progress from inbound progress.
The turnaround may be a summit, gate, road junction, waypoint or simply the last point in the file before the sequence reverses. It should be recognisable on both map and ground. A vague reversal on a featureless section makes it harder for the user and software to know that the return has begun.
Route geometry around the turn deserves inspection. A recorded trace may contain a cluster of stationary points. A planned route may reverse between two sparse vertices. A small loop around a building or car park can make the transition appear to continue rather than turn.
- Visible featureA named or unmistakable place reduces uncertainty about where to reverse.
- Complete geometryBoth passes should exist if the route summary is meant to describe the full outing.
- Clean reversalRemove accidental stationary loops only on a copy and only after checking attached data.
After the turn
Direction and route history identify the second pass.
Nearest-route matching alone is ambiguous on the shared corridor. The outward and return geometry may be identical. A navigation system can use recent progress, the fact that the turn was reached and the new direction of travel to continue along the later sequence.
Uncertainty is greatest near the start and turnaround, especially if the user pauses, wanders or begins navigation partway through. Remaining distance can jump if progress attaches to the wrong pass. Treat a sudden large change as a reason to inspect the whole line, not proof that distance has been gained or lost.
The route is not merely the corridor. It is the corridor out, the turnaround, then the corridor back.
A complete handoff
Make the double use of the path explicit before relying on progress.
- Trace
Follow the point order to the far end and back.
Do not infer the return from the visual line. Confirm that later points actually lead towards the finish.
- Count
Compare planned distance with the full route.
If the headline figure resembles the one-way distance, inspect whether the return geometry is missing.
- Mark
Know the turnaround on the ground.
A clear feature prevents an early reversal or an unintended continuation beyond the route.
- Watch
Check progress after reversing direction.
Make sure the next section and remaining distance now describe the inbound pass before relying on later cues.
Sources & scope
What this answer is based on.
The standards define line geometry as an ordered sequence. Out-and-back recognition, progress handling and turnaround tolerance are choices made by the receiving application.