I've put an example on this forum (in Dutch). The brouter route (https://tinyurl.com/3nt27szy) shows a profile that is more accurate than the route in Basecamp.
Hi guys,
I came across this video on youtube, where ridewithgps.com improved their elevation profiles by ignoring the underlying altititude data of tunnel and bridges:
I found out that other route planners like brouter, cycle.travel and Osmand also ignores the surface altitude that lies above tunnels and underneath bridges, but mkgmap/Garmin does not. This leads to a much higher elevation gain or loss in the route profile
than the actual ride. Is it possible for mkgmap to correct this issue, or is it hard coded in the Garmin Basecamp software?