Oh, your elevation profile should be fine without a barometic altimeter. What goes astray is the cumulative climb totals as in "I climbed 3,212 ft in 100 miles." A GPS should give you a decent recording that the base of the hill was at X feet and the top was at x feet of elevation. But if the road has any undulations on the way up (which just about all do), the total climbing will be off.

Make sense?