Altimeter Strangeness

[valkyrie]

Did a DIY by GPS 600 audax at the weekend and for the main part my etrex Vista HCx worked perfectly. For the first 400km the profile corresponds pretty closely to that given by ridewithgps.com, but somewhere around Carlisle it all goes a bit strange. First the Vista throws in a non-existent hill then it thinks I'm going under the sea. The altitude dropped to -150m at one point and when I noticed it I did a quick re-calibration. For about 50km after that the altitude reading slowly oscillated up and down regardless of what I was actually doing then started working normally again.

We did catch some rain before this problem started but the symptoms look different to my previous experiences of the "water in hole" problem.

Anyone any idea what the problem might be?

[Kim]

Could a sudden change in atmospheric pressure cause that?  There was some Weather on Sunday...

[Feanor]

My Edge 800 altitude did something similar a couple of weeks ago.
It jumped suddenly from +143 to -74m 89k into this local loop:

http://connect.garmin.com/activity/201487783

A'v nae idea fit's gaun on.

[Arn0]

I noticed a similar strangeness that Sunday around noon (30th July) with my HCx.
I think it was because of the weather conditions: sudden heavy showers and even a bit of hail at some point...

[Toady]

Silly question, but it didn't correspond with a battery change did it?  I sometimes see the altitude plot on my 60csx go haywire when I change the batteries, presumably as it needs to re-auto-calibrate the altimiter.