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  1. #1
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    New question here. Edge Altimeter Functions ?

    I am curious, since my Edge 705 features a "Barometric" Altimeter, if it is GPS corrected for accuracy? My RINO features a Barometric Altimeter too, but gives the option to Manually Calibrate it (IF there is a known elevation) OR have it Automatically corrected by the GPS data it receives. (makes sense since barometric altimeters are not accurate based on my limited experiences with them due to constantly changing weather conditions).
    So, Since there is no option or calibration mode on the EDGE, i am curious how it gets it Elevation accuracy, or if it IS accurate?
    Whenever i download my tracks and view them in my NG Topo Software or even Garmin Connect, the Elevation Gains NEVER match the Units' readings, they usually differ by a VERY noticeable amount, 20-40% usually, i always assumed this was due to the software interpreting the data differently, although i never really know WHICH one is more accurate. I assumed the UNIT's record the more subtle Up and Down gains and losses as i ride, but the Software usually shows HIGHER Elevation Gains than the units i use, so i am not sure which to believe for my elevation gains on my rides...
    Anyone KNOW how these EDGE units really work with regards to Elevation and Altimeter Functions?
    Last edited by UN-COG-KNEE-TOE; 09-05-2009 at 10:04 AM.

  2. #2
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    I recall reading that the barometric altimeter was corrected with the GPS on the 705, I have never figure out if that is true or how it works. It is/was documented on Garmin someplace though. I also have noticed some of the variance in altitude, especially doing the exact same workout on the same route can have +/- 500ft difference in ascent.

  3. #3
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    I used to deal with stuff like this all the time when climbing. Keep in mind that barometric pressure is constantly changing. It was nothing for us to go to sleep and wake up with our altimeters reading 500' higher. Which really stunk because it meant another low front was moving in.

  4. #4
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    Yes, changing weather will cause a barometric altimeter to 'drift'. But keep in mind that calculating altitude via satellites requires a MINIMUM of 4 satellites, and a horizontal position fix requires a minimum of 3. So, it's possible for you to have a position fix, with NO altitude readout unless you have a receiver with a barometric altimeter. Since the barometric altimeter is an independent sensor, it can operate independently of satellite reception issues. The GPS corrections of the barometric altimeter are simply a sort of auto calibration. If you can manually calibrate the altimeter from a USGS benchmark or a known peak/pass altitude, it will be more accurate.

  5. #5
    trail rat
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    The 705 has some known issues with how well and how fast the barometric altimeter calibrates the GPS for elevation. There are discussions on the Garmin Connect forums, MotionBased, and GroundSpeak, and other places. There have been a number of firmware releases with no fix to the elevation calibration, and a few users are quite vocal about it.
    http://regex.info/blog/2006-04-16/179
    https://forums.garmin.com/showthread.php?t=77
    http://forums.motionbased.com/smf/in...?topic=8083.20

    Positive thread about barometric altimeter calibration working, look at post #45 for 305 data, he does not test the 705, but implies that it uses the same system as the 305 and that it works.
    http://forums.groundspeak.com/GC/ind...owtopic=194177

    I have found no way to manually calibrate the altitude from a know point, like my 60CSx, and other models. What I have been doing is to turn the GPS on and set it on the table on the deck for 30 minutes or more before I ride. My altitude readings are much more accurate, less than 100 feet from known points, and usually less than 50 vertical feet. It seems to auto calibrate, but very slowly. When I turn the GPS on and ride, the elevation accuracy is off by a greater factor.
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  6. #6
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    Quote Originally Posted by NateHawk
    The GPS corrections of the barometric altimeter are simply a sort of auto calibration. If you can manually calibrate the altimeter from a USGS benchmark or a known peak/pass altitude, it will be more accurate..
    Thanks for your reply... if you know some way of actually Calibrating a 705, would love to learn about it.

  7. #7
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    Good job! Lots of 705 info

    Quote Originally Posted by slocaus
    The 705 has some known issues with how well and how fast the barometric altimeter calibrates the GPS for elevation. There are discussions on the Garmin Connect forums, MotionBased, and GroundSpeak, and other places. There have been a number of firmware releases with no fix to the elevation calibration, and a few users are quite vocal about it.
    http://regex.info/blog/2006-04-16/179
    https://forums.garmin.com/showthread.php?t=77
    http://forums.motionbased.com/smf/in...?topic=8083.20

    Positive thread about barometric altimeter calibration working, look at post #45 for 305 data, he does not test the 705, but implies that it uses the same system as the 305 and that it works.
    http://forums.groundspeak.com/GC/ind...owtopic=194177

    I have found no way to manually calibrate the altitude from a know point, like my 60CSx, and other models. What I have been doing is to turn the GPS on and set it on the table on the deck for 30 minutes or more before I ride. My altitude readings are much more accurate, less than 100 feet from known points, and usually less than 50 vertical feet. It seems to auto calibrate, but very slowly. When I turn the GPS on and ride, the elevation accuracy is off by a greater factor.

    Thanks for Your reply - these are a lot of good info Links, i need to spend the time to read through it. Appreciate your apparent Shared curiosity and research into the 705, which i think is a great unit Overall.

  8. #8
    trail rat
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    Quote Originally Posted by UN-COG-KNEE-TOE
    Thanks for Your reply - these are a lot of good info Links, i need to spend the time to read through it. Appreciate your apparent Shared curiosity and research into the 705, which i think is a great unit Overall.
    My friends call me a "techno-weenie", their eyes roll back in their heads when I get into my explanations.
    Oh, I can get far worse than this. I just have to know the underlying "how it works", drives my friends nuts.
    Last edited by slocaus; 09-07-2009 at 07:41 AM.
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