I have noticed more than once that my eTrex Legend shows inconsistent elevation on a local lake. At the south end of the lake it showed 512 feet and 3 miles north it showed 529 feet which is approximately correct. At the time it had an accuracy of 8-10 feet. I'm pretty sure there is not a 17 foot difference in elevation or I would notice the current. Any explanation?
I bought an Legend a month ago and I was little impressed by its capacity. Outside under a sunshield (parasol) I had no satelites!!!! Brought the unit back and had a better reception although far from what I wanted (compare to Nuvi 650).
Brought that unit back also and bought a Vista HCx.... $ or 5 satelites even in my basement!
The accuracy of a GPS seems to depend on the number of satelites it connects to. Is it possible that the lake at the other end has more foliage and or clouds? Accurary levels can range from 10ft to 100ft horizontaly so also easily 17 ft in the elevation, it seems to me.
After Googling GPS and altitude accuracy it seems that no consumer GPS is very good at altitude calculations. They obviously focus on North-South, East-West determination. Inaccuracies for altitude are much greater.
It also seems that they use a "rubber banding" feature where if the GPS is reading 10 feet off the road it will still display the location as being on the road. You can demonstrate this effect when driving into parking lots. The GPS will still show you on the road for a brief period before showing you as being off the road. This may account for why you see your altitude rising and falling even when stopped, while the GPS display shows you parked in one place.
Altitude readings are cool but less important for an automotive GPS. The moral of the story: Don't take your GPS hang gliding for its altitude readings!
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