60cs Altimeter Errors

[mtnc]

While on a recent trip that I have taken often, I noticed the altitude to be way off. My 60CS showed the correct altitude at the top of the mountain, but 30 minutes after getting down into the flat land, it still showed a 1000+ foot error on the high side compared to a similar track on my Vista. I have auto calibration turned on, variable elevation selected and calibrated it earlier in the trip to the satellite altitude. The GPS altitude on the Satellite page also showed the altimeter to be 1000+ feet high. Has anyone else seen this? One more thing to add: After almost 2 hours it was still reading 100+ feet high.

[+wickedsprint]

100 feet is nothing whenya figure the changes in ambient pressure or if a storm is moving in or leaving, that is why the ones in our planes have an adjustment to compensate for this.

[mtnc]

I agree, but 1000+ feet is totally unacceptable. The unit was in error more than 500 feet for at least 1 hour.

[+wickedsprint]

Why is it unnacceptable..?

[GeoBobC]

If the GPS has only a 2D lock it will display horizontal location correctly but could be off thousands of feet in altitude.

[+briansnat]

If the GPS has only a 2D lock it will display horizontal location correctly but could be off thousands of feet in altitude.

The 60CS has a barometric altimeter so it has nothing to do with your lock.

[GeoBobC]

Good point; scratch that one from the list of possibilities.

[peter]

If the GPS has only a 2D lock it will display horizontal location correctly but could be off thousands of feet in altitude.

Besides not applying to the 'cs' models, this generally isn't true anyway. Models without a pressure sensor that are in 2D mode assume that the altitude is unchanged from the last good reading. They then calculate a horizontal position that gives the best agreement with the satellite signals using that assumed altitude value - i.e. they solve for the three unknown variables (lat, long, time) using the satellite time signals and an assumed constant altitude. If the altitude is really off by thousands of feet then the calculated horizontal position is likely to be off by a similar amount.

 

I've observed this behavior at the end of plane flights. After turning my GPS off while up at 35000' I turn it back on again after landing. The initial position fix is '2D' with the altitude still showing 35000' and the horizontal position is generally off by a few miles. Once the unit gets an additional satellite signal and figures out the correct altitude then the horizontal position also jumps to the right spot.