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Thread: Garmin 60CSx records elevation change when stationary

  1. #16
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    I imagine that a moving average filter ( https://en.wikipedia.org/wiki/Moving_average ) could take care of some of this type of noise albeit at the "cost" of making the unit appear to be less sensitive to instantaneous changes.

  2. #17
    Senior Member DougPaul's Avatar
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    Quote Originally Posted by iAmKrzys View Post
    I imagine that a moving average filter ( https://en.wikipedia.org/wiki/Moving_average ) could take care of some of this type of noise albeit at the "cost" of making the unit appear to be less sensitive to instantaneous changes.
    A moving average filter is a form of low-pass (smoothing) filter. They are a specific use of windowing. https://en.wikipedia.org/wiki/Window_function. Windows are a well-studied tool in signal processing. (My degree specialty is classical signal processing.)

    I have done some experiments using median filters https://en.wikipedia.org/wiki/Median_filter to see if I could reduce the effects of the noise on the altitude portion of a 1 per second track. They didn't work very well. I think this is a harder problem than it appears at first glance...

    Both median filters and moving averages would work well for a steady climb or descent, but they both clip the peaks and valleys.


    Note also that the GPS has more information available to it than is recorded in the track. The GPS outputs the position (3d), the velocity (3d), and the time (1d) once per second (for consumer GPSes). The track only records the position and time and usually only saves a subset of the track-points. (Garmin GPS tracklogs are limited to 10K points so you can only save ~3hrs at 1/second.)

    Doug

  3. #18
    Senior Member JoeCedar's Avatar
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    Quote Originally Posted by JoeCedar View Post
    I borrowed a Garmin GPSMAP 60 CSx with barometric altimeter from a friend, intending to do some hiking comparisons of iPhone apps (Gaia GPS and Motion X) which run on GPS altitude (no barometric sensor). With the Garmin on my desk under USB power with GPS off, I noticed that the ascent/descent values were increasing with time. Both ascent and descent were increasing. If this increase was due to ambient changes in barometric pressure, one should increase but not both equally. I have relied for years on a separate barometric altimeter, Suunto X6M, for recording ascent/descent while hiking and have found it accurate, reliable, and reproducible. So I did a test over three hours with both units running simultaneously. Results are shown below. Just by luck, there was essentially no change in ambient barometric pressure today during the test.


    Stationary Garmin GPSMAP 60CSx versus Suunto X6M barometric altimeter


    CSx was set in 'variable elevation' and autocalibration on, attached to USB power while stationary.
    GPS was off.
    X6M was running simultaneously on my desk (recording a point every 10 seconds)

    Garmin CSx Feet In Hg
    Time Asc Dsc Barometer Comments
    2:00 0 0 29.84 autocalibration ON, GPS off
    2:30 225 228 29.84
    3:00 463 458 29.84
    3:30 704 699 29.84
    4:00 913 907 29.83
    Reset 4:01 0 0 29.83 autocalibration OFF
    4:30 243 249 29.84
    5:00 502 512 29.84
    Ascent/Descent values increased 450-500 feet per hour with no change in local barometric pressure
    Autocalibration had no effect.
    Barometer readings were from a separate barometer as well as hourly checks from a nearby NWS station

    Suunto X6M
    2:00 0 0
    5:00 20 19
    Essentially no change in Ascent/Descent while recording for three hours.


    I am surprised that the Garmin accumulates significant ascent/descent values when the unit is stationary. I don't know if this is a problem with this particular unit, or it is common to all. I would greatly appreciate if some other users could try this simple test and help me understand what is going on.
    Thanks all, for your comments and discussion. I noticed last night and confirmed today that the cause of the noise and concomitant increase in ascent/descent was having the unit plugged into USB power. It does not occur when under battery power. I also connected the USB cord to the iPhone charger in a wall outlet, located more than 6' from the computer, and the noise occurs there too. So it is not specific to the computer USB power source but just being connected to external power.

  4. #19
    Senior Member CaptCaper's Avatar
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    My Garmins with Glonass/GPS with WAAS ON has the lowest noise out of any of the previous units I've owned since 1996... And the most accurate in tracking,elevation data,etc. Also there is ton's of data on the web telling you exactly why elevation data in these units will never be accurate say as over the ground gps use.. which is more accurate per say in fix then elevation data is. Just the nature of the beast... just a slight change in the air pressure while hiking ( a front coming in) will change the accuracy. Happy reading and searching... why this is.

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