Today I went for a ride with my new Garmin Forerunner 201 GPS. I’m really confused by the data that I’m presented with.
I started the GPS at my home as I began the ride and stopped it when I returned to the same spot. The statistics for the ride are:
Distance: 7.89 Miles
Climb: +589/-624
So, Apparently during my ride my house sunk 35 feet? Is this just an accuracy thing or is there more to these numbers that I’m not considering?
Thanks,
-mg
Are you riding on a 35 foot giraffe?
if the altimeter on your gps works by a barometer a sudden change in the weather could effect the readings like that, or if your gps’s altimeter works off satillites then you need alot of coverage to provide accurate height, the weather can affect this (clouds slow the gps signal down) or if you where in the bush with little overhead view then it would explain it 
It’s an accuracy-on-startup thing. When the GPS first establishes a position the altitude is all over the place, generally up to around 100 feet out. I get round it by turning the GPS on, letting it sit for a minute or two until it gets a decent position then resetting it, or when I upload the track to the computer I just delete the first few trackpoints with the dodgy elevation.
Phil
Re: Altitude anomaly?
giraffe has the right explanation (IMHO). But this thread should be in
Just Conversation (IMHO again).
datz right 
I think your house did sink 35 feet, but at that precise moment in time, a wormhole opened up, distorting time and space so your house, now, and forever will not be 35 feet lower, but as the lateral motion of time didn’t alter your electronic gizmo, it displayed the correct altitude.