I now realize I needed to think more carefully about using strava segments for this purpose. I dismissed the idea before posting my question because it seems to get the altitude information from the GPS data used to create the segment, so when the segment at the north end of the B-line was created, it used totally bonkers altitude data, and we seem to be stuck with that.
The grade on Rogers road is way less than it used to be, so the existing strava segment is now wrong. But maybe I can create a new (private) segment. I think there are 3 possibilities for where new altitude data can come from: watch altimeter, map data (not updated yet, but also not accurately sensitive to the original difference between sidewalk and road), and triangulation with GPS satellites. I think I’d have to use a GPS with an altimeter. My current one doesn’t have it, but my old one does. So I’ll head back out there to try again 🙂