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Go app altitude vs. Metadata

Discussion in 'Pro/Adv Discussion' started by marciano, May 5, 2016.

  1. marciano

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    Looking at some metadata records I found severe differences between Go app log and photos metadata. 300m vs. 220m.
    How is that?
     
  2. alokbhargava

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    Photos will have altitude direct from GPS whereas Go App references are wrt home point.
     
  3. Meta4

    Meta4 Moderator
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    For some reason that only DJI know, in firmware versions after 1.4, DJI replaced the altitude number in the Exif data with altitude data from the GPS.
    GPS altitude is relative to sea level BUT GPS is woefully inaccurate for altitude so why they did this is a mystery and it's screwed up things for photogrammetry.
    But the original barometric altitude data is still in the Exif data in another field and is visible if you use Photoshop to view the full Exif data rather than just a short version.
     
  4. marciano

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    I forgot to say I was no more than 10 meters over the sea level.
    I use exiftool. It shows a long list of shot properties.
    I found Absolute Altitude: +225 and Relative Altitude: +300m
    According to that I should have been diving with my RC. Don't remember that!
    Is it possible to calibrate GPS altitude?
     
    #4 marciano, May 5, 2016
    Last edited: May 5, 2016
  5. Meta4

    Meta4 Moderator
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    You can recalibrate the IMU idf you have problems with the telemetry altitude but there's nothing you can do about GPS altitude.
    As I said, GPS is useless for altitude.
    It is common for it to be +/- 200ft out and swing frequently
    Here's what Garmin say anout GPS altitude accuracy:
    How accurate is the GPS elevation reading?
    GPS heights are based on an ellipsoid (a mathematical representation of the earth's shape), while USGS map elevations are based on a vertical datum tied to the geoid (or what is commonly called mean sea level). Basically, these are two different systems, although they have a relationship that has been modeled.

    The main source of error has to do with the arrangement of the satellite configurations during fix determinations. The earth blocks out satellites needed to get a good quality vertical measurement. Once the vertical datum is taken into account, the accuracy permitted by geometry considerations remains less than that of horizontal positions. It is not uncommon for satellite heights to be off from map elevations by +/- 400 ft. Use these values with caution when navigating.​
     
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  6. marciano

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    Okay, thanks a lot for your explanation.
    In my case Relative Altitude seems to be accurate and enough data.

    I have a Garmin Oregon I have to calibrate Altitude regularly I suppose because of what you have explained.
     
  7. Meta4

    Meta4 Moderator
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    The Garmin Oregon has a built in barometer for altitude.
    Air pressure varies locally and with time of day and you have to calibrate this to ensure your barometer is giving accurate numbers relative to sea level.
    This is what the pilots of real planes do before each flight.
    With the Phantom all you care about is altitude relative to home point.
    The air pressure differences over the duration of a flight aren't enough to worry about.

    Next time you go flying in a jet, pull out the Garmin Oregon and see what it says about altitude.
    It will be completely fooled by the pressurisation of the jet and say you are something like 8000 feet even though you might be flying at 30-40000 feet.
     
  8. marciano

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