Elevation discrepancy - Edge 305
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Elevation discrepancy - Edge 305
I recently posted an out-and-back ride on MotionBased and it listed the elevation gain as +1312/-1336 ft. Will barometric pressure change enough during a two-hour ride to affect elevation readings on the 305 or is this a problem with the GPS accuracy?
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When I got my GPS in December there was a well-known bug in Garmins that prevented them from accurately calculating accumulated elevation change. It was worse than just a few feet, though. Not sure if that might be your problem.
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Large discrepancies in elevation recorded by barometric instruments, like your 305, is often the result of rapid pressure rise/drop do to weather. Was there a storm from moving in? Clouds or high-wind to indicate hi/low pressure storm front?
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Originally Posted by Logan B
Large discrepancies in elevation recorded by barometric instruments, like your 305, is often the result of rapid pressure rise/drop do to weather. Was there a storm from moving in? Clouds or high-wind to indicate hi/low pressure storm front?
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Turn it on a few minutes before you ride. Locks in the sattlelites and the altimeter will stablilize. I do it every time I use mine. The altimeter seems to jump about 100 feet once it stabilizes.
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Originally Posted by head_wind
1312/1336=0.982
2% sounds OK to me.
2% sounds OK to me.
It was OK by me, too. I'm just so happy to have one of these computers that a discrepancy that small doesn't bother me in the least but I am intrigued by how pressure plays a role in how the machine records altitude.
I think I'll check the NOAA site after my next long ride and compare the pressure and altitude at the beginning of the ride versus at the end of my ride. If anyone else has done this, were the results what you expected them to be?
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If I recall correctly, one millibar pressure drop (at around sea level) is about what you get with a 27-foot increase in altitude. So your variation is about one millibar or so.
Don't forget that there are tides in the atmosphere as well as in the oceans. Twice a day there is atmospheric high tide, and twice a day there is atmospheric low tide. The ambient pressure goes up at high tide and down at low tide if you're standing still and the weather is completely constant; there's just more or less air above you pressing down.
If you're riding for more than an hour or two I'd expect to see a visible effect from atmospheric tides, unless you were right around a high or low point.
Don't forget that there are tides in the atmosphere as well as in the oceans. Twice a day there is atmospheric high tide, and twice a day there is atmospheric low tide. The ambient pressure goes up at high tide and down at low tide if you're standing still and the weather is completely constant; there's just more or less air above you pressing down.
If you're riding for more than an hour or two I'd expect to see a visible effect from atmospheric tides, unless you were right around a high or low point.
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Maybe you'll find some stuff of interest here: https://gpsinformation.net/
Some of the stuff there is old but only references to SA (selective
availability) are obsolete. SA was the dithering of the signal contents
so that private users had less precision than the military.
I don't know anything about enhancement of the data using aneroid data.
Some of the stuff there is old but only references to SA (selective
availability) are obsolete. SA was the dithering of the signal contents
so that private users had less precision than the military.
I don't know anything about enhancement of the data using aneroid data.
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Originally Posted by Torgrot
Turn it on a few minutes before you ride. Locks in the sattlelites and the altimeter will stablilize. I do it every time I use mine. The altimeter seems to jump about 100 feet once it stabilizes.
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Thanks, everybody! It appears that I have a lot of research and reading to do!
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Originally Posted by patrick07
I recently posted an out-and-back ride on MotionBased and it listed the elevation gain as +1312/-1336 ft. Will barometric pressure change enough during a two-hour ride to affect elevation readings on the 305 or is this a problem with the GPS accuracy?