I have a Polar 725. Depending on the weather, the altituge of my house ranges several hundred feet. But the relative changes in altitude remain very constant. Some complain that a barometric altimeter can shift during the ride. I've only seen this once. On a 4 hour metric, I had the beginning and ending points (i.e., the same point) end up different by 10 vertical feet. That's really not much shift over 4 hours.