The barometric altimeter doesn't work well on rolling hills-- it's much better at long grades. I have a mountain climb that I've done half a dozen times, which gains right around 4,650ft in 20 miles. Every time I get to the top, my Garmin has been within 40 feet (+/-20ft). As far as I'm concerned, that is stunningly accurate for a consumer-grade device. The last time I went up (last Wednesday) my 520 and my buddy's 810 were within 10 feet of one another.