Join Date: Jul 2008
Location: Austin, TX
Bikes: Giant OCR2, Bridgestone RB-T, Bike-E, Vision R-40, Novara Safari
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GPSs are not particularly accurate when it comes to altitudes, which is why the Edge 305 and some other models has a built-in barometer -- it allows the device to detect small changes in altitude (even a few feet, it can detect it.)
But barometers are affected by the weather. Low pressure zones will make the device think it's higher than it really is, and high pressure zones lower.
The altimeter in an airplane allows you to enter the air pressure from a weather report to make it accurate, but the Edge 305 has no such capability. But it doesn't really matter that much, as you're not usually so worried about altitude as changes in altitude.
If you can turn it on, have it read -140 feet, then go up 20 feet of stairs and have it read -120 feet -- it's likely working properly.
It would be nice if the device would store both the GPS determined altitude and the barometer determined altitude with each data point (and have a way to let you see both), and let you enter in the barometic pressure to calibrate it -- but it doesn't, as it's not really what the device is made for.
When you download the data to your computer and upload to sites like Garmin Connect and Strava, you can usually have the site use known altitudes rather than trust what the GPS says -- so unless you're using it in something that flies, it's not normally a big deal.