Join Date: Jun 2011
Location: Buffalo, NY
Bikes: 2012 Surly LHT, 1995 GT Outpost Trail
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So there are really 2 ways to calculate elevation.
The first is what you'll find on devices like the Garmin Edge computers: Barometric Pressure. These measure the air pressure and calculate the altitude based on that. It's not deadly accurate, but it's usually accurate enough. Changes in the weather will affect it, so typically the longer you ride, the more inaccurate it gets.
Then there's GPS-based elevation. GPS doesn't actually calculate elevation, the signal merely provides a 2D polar coordinate that tells you your latitude and longitude. This value is then interpretted by online mapping software, mixing it in with the National Geographic Survey data to calculate your elevation changes. The problem with this method is that GPS is not as accurate as it looks. Typically it'll be off by up to 20-30 feet, but most mapping software is smart enough to show you on a road when it renders the final graphic. However, most mapping software calculates your elevation before it smooths out the lines. If you ride on a road that follows a ledge, for example, the GPS location can show you as being at the bottom of that ledge in one instant, 30 feet to the right, and then up on the road again a few seconds later. The elevation calculations will say "ok he just descended 20 feet and then climbed 20 feet in the last 20 seconds", thus adding 20 feet to your elevation, and causing your numbers to fluctuate.
The bottom line is; the numbers are "noisy". The equipment required to calculate your precise elevation at all times is far too expensive to put into a cellphone so it merely relies on approximations, and you'll never get a consistent number.