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Flytrex altitude is way off

Discussion in 'Phantom 2 Vision Discussion' started by Tromac, Dec 13, 2013.

  1. Tromac

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    Hi all,

    I read in the other topic regarding the Flytrex that it uses its own barometric altimeter and this is more accurate than using GPS. I got mine installed today and did a very quick 3 minute flight to make sure it's working. The weather conditions are far from ideal being very windy so I kept the flight short and got it back inside.

    On uploading the mission to Flytrex it reports my starting altitude as 34 meters. Is may be a stupid question, is this reading altitude ASL as opposed to AGL?
     
  2. Driffill

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    It would be ASL otherwise the flytrex would need to have the terrain profile of the world loaded into it. I'm sure there is some way to edit the data to change to take off altitude post flight. The algorithm used to process the data once its uploaded to flytrex could be able to use the GPS co-ordinates to pull up terrain info (if it exists in the public domain I.E. Internet) and edit the data accordingly?
     
  3. Tromac

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    Looking at all the public flights on the Flytrex site the start altitude is obviously ASL. Not a problem I guess.
     
  4. Pull_Up

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    Yep, it's a baromter - it can only measure ASL. However, if you look at your flytex data afterwards you can see "maximum ascent" which will normally show you how high you flew above ground level - it simply takes lowest ASL measurement away from highest. If you take off from high ground and dip below your take-off altitude, though, then that won't be the case.
     
  5. gpauk

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    A barometric altimeter can only measure height change after a calibration point.
    There's no absolute reading as it will vary with time as air pressure changes with weather systems.
    Think about how a barometer on the wall moves -- air pressure varies!
    If you use one to navigate while out walking, you need to reset it against a map from time to time, as it will lose accuracy over time and distance as the air pressure changes.
    It will "zero" itself at power up, and after that measure change in altitude. As the time and distance involved in rc flying is relatively small it will remain accurate.
    So if flytrex is showing any real, rather than relative to power up, altitude it's presumably adding the barometric derived altitude to that gained from the position on the map, or gps perhaps.