[jeffbee]

They must be doing something that integrates user-contributed data because one quirk of their course planner[1] is an out-and-back plan along the same course will have non-coincident elevation profiles, even along trails where it can't be explained by switching sides of the road.

1: And their planner, in general, is garbage. I usually take the output file and run it through an AI system to fix it so it doesn't try to cue me to pointlessly switch to the other side of a major road and ride on the sidewalk.

[GorbachevyChase]

If they’re sourcing some of their information from GPS data, the vertical precision for elevation is pretty poor, even if you’re using professional equipment. I’ve been being off a foot or tube is perfectly normal. You really need to use survey monumentation as controls. Also many states known invest in a statewide light R elevation program. It’s really too bad because they are so useful for planning and design.

[qwhelan]

Yeah, it's understandable that the real world is messy and user-sourced data can be very suspect.

The case I'm thinking of is the eastern end of SR-520 over Lake Washington in the Seattle area. It has hundreds/low thousands of bicycle crossings/day (so surely dozens of Garmin users/day), the background is a dammed freshwater lake with a well known elevation, and clear view of the sky. Yet the elevation data is garbage somehow.

It doesn't really matter for planning purposes as any alternative has such a huge delta but it does signal that user data isn't being utilized to refine their data set even on high volume segments.