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altitude day
Here's another report re: elevation...
I had a customer at the beach today, so I brought both altimeters along with me. I don't get to the beach all that often so it was a good opportunity.
I walked down to the sand and did the calibration, so that was great to get done at sea level at last. During the first few minutes of the ride home, both the GPS and barometric methods were reading within one point of each other!
Most of the way home, they were about 30 points disparate though. The GPS bounces all over when in motion (sometimes even when still!)... I'm guessing that constantly having to compute information from the multiple satellites leads to all these adjustments before it settles down.
A mile from home, they were still showing about a 25 point difference, with the GPS being in agreement with GoogleMaps' reading. Then, pulling into the driveway, the GPS was 15 points under and the barometric was 15 points over!
A few minutes later, both the units had converged again to within TWO points of each other!... a very satisfying result :> Both seem to believe that the GoogleMaps elevation is about 12 feet too low, though... heh!
I had a customer at the beach today, so I brought both altimeters along with me. I don't get to the beach all that often so it was a good opportunity.
I walked down to the sand and did the calibration, so that was great to get done at sea level at last. During the first few minutes of the ride home, both the GPS and barometric methods were reading within one point of each other!
Most of the way home, they were about 30 points disparate though. The GPS bounces all over when in motion (sometimes even when still!)... I'm guessing that constantly having to compute information from the multiple satellites leads to all these adjustments before it settles down.
A mile from home, they were still showing about a 25 point difference, with the GPS being in agreement with GoogleMaps' reading. Then, pulling into the driveway, the GPS was 15 points under and the barometric was 15 points over!
A few minutes later, both the units had converged again to within TWO points of each other!... a very satisfying result :> Both seem to believe that the GoogleMaps elevation is about 12 feet too low, though... heh!
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Generally speaking I'd have to agree that GPS is not the best source of altitude data for exactly the reasons you mentioned... it's pretty useless without really good satellite coverage, it's generally pretty complex to compute (especially given how accurate the datum is), and the common commercial-grade general-purpose algorithms aren't really optimized for it... and adding motion to this further amplifies those problems.
If it means anything, most general aviation aircraft (at least until fairly recently) rely on good old-fashioned sensitive barometers for determining altitude. Of course that assumes a priori knowledge of accurate barometric pressure at ground level - and what altitude the ground is - where you are. So maybe not an ideal situation either. :P