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Old 06-15-07, 12:18 PM   #1
patrick07
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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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Old 06-15-07, 10:41 PM   #2
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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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Old 06-17-07, 10:49 PM   #3
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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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Old 06-18-07, 06:17 PM   #4
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Quote:
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?
That's what I was thinking but the weather has been very hot and dry lately, especially on that ride. I wondered if anyone had quantified that change, i.e. 1 millibar pressure change = x feet. I'm still very new with these barometric altimiters and I'm not sure how the algorithms work.
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Old 06-19-07, 07:09 AM   #5
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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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Old 06-19-07, 09:17 AM   #6
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1312/1336=0.982

2% sounds OK to me.
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Old 06-19-07, 09:40 AM   #7
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Originally Posted by head_wind
1312/1336=0.982

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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Old 06-19-07, 11:18 AM   #8
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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.
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Old 06-19-07, 01:59 PM   #9
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Maybe you'll find some stuff of interest here: http://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.
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Old 06-19-07, 03:28 PM   #10
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Quote:
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.
This might indicate a switch from GPS to barometric elevation. My GPS elevation is consistently 100 +/- 10 feet too high.
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Old 06-20-07, 04:25 AM   #11
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Thanks, everybody! It appears that I have a lot of research and reading to do!
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Old 06-20-07, 04:31 AM   #12
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Old 06-20-07, 10:31 AM   #13
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Quote:
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?
As I recall, the Edge 305 has an auto calibrate function that uses the GPS altitude to correct the barometric altimeter. However, I believe it only operates if you are stopped for several minutes. I suspect the unit does some long term averaging of the GPS elevation before applying correction factors to the baro altimeter. so, if you are concerned about weather changes affecting your elevation readings, you should be sure to stop along the way long enough for the calibration routine to kick in.
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