Garmin and Strava must be sitting on a gold mine of practical DEM data based on ride and hike records. They have huge databases from mobile devices recording GPS position, speed, and pressure.

My guess is that analyzing the recorded activities is what komoot did to get useful elevation data.

Yet it's unclear to what extent they actually use them - on freeway adjacent bike paths that have different gradients than the adjacent freeway (typically steeper), it's very clear Garmin is using data referencing the freeway.

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.

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.

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.