Altimeter elevation gain accuracy
 

I've got a Blackburn Neuro 6.0 cyclometer. Very pleased with the unit thus far. I've noticed that the altimeter-based elevation gain doesn't agree all that closely with what comes out of www.mapmyride.com or similar sites. This doesn't come as any great surprise, but I wonder if others out there have compared their cyclometer's calculation with what I would expect is the more accurate figure obtained from actually topography.

mapmyride.com and all the similar web sites are terrible for cumulative elevation gain. It can show half or double what your altimeter says. The problem is that the web sites' data is too sparse for this type of calculation, so they accumulate a lot of errors along your route. I think this problem is mentioned on the mapmyride.com FAQ. Your barometric altimeter should be much more accurate.

my gps with barometric altimeter told me I was 100 ' below sea level when I got to work. as long as the weather does not change much they may be accurate.

100 feet is nothing. The web site cumulative elevation gain can be thousands of feet off for a half-day ride.

Appreciate all the responses. I'm going to see if I can find a more accurate, public source of elevation data. Really accurate data has been available for a while, especially since the Space Shuttle mapping mission several years ago. I have one 40 mile loop which measures 1700 ft. of gain on my cyclometer, but only 1050 or so on these web sites. Makes me wonder what terrain database they are using.

My Blackburn is quite accurate with elevation readings as tested against maps and known elevation points. I do zero the elevation before each ride. The gradient reading is, at best, flaky.

I've also noticed the gradient to be suspect. I'm just getting back into cycling after 20 years and small gradients (and headwinds) are very noticeable. The gradient readout seems stable on more sustained slopes, however. All depends on how they are calculating/filtering the altimeter data. When you think about how much they cram into these tiny instruments it's pretty amazing.