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gps altitudes adjustment

Discussion in 'Inspire 1 Discussion' started by slim.slamma, Jan 27, 2015.

  1. slim.slamma

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    Shouldn't altitude height adjust once you've reached the top of a hill? As it climbs, the ground to craft doesn't adjust altitude. Only from where i originated. Its says im 300 ft high when actually im only 50 above ground on top of a hill.
     
  2. rilot

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    The height is as measured from the take off point, not the actual ground level.
     
  3. slim.slamma

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    That's a problem. Dji should address that in the maps. Not buildings or trees but the actual distance thu maps. Real time flying!
     
  4. Tahoe Ed

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    Are you suggesting that DJI load topo maps in addition to the satellite maps. I am somewhat confused.
     
  5. Ian Fraser

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    Topo Maps would be a good idea.
     
  6. Ultra310x

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    I AGREE craft altitude should be reflective of its actual height to ground not where the controller is.
     
  7. slim.slamma

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    Yeah im flying 700 ft. When in actuality im only 300 ft above ground. That's not real time gps, get it right dji
     
  8. rilot

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    None of the DJI or any other FCs I know of do this. It would require full topological data.
    I'm happy with it as it is. I don't want my craft changing it's altitude uncommanded.
     
  9. slim.slamma

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    Im in love with my craft also, just trying to stay under 400ft. For safety reasons i like to fly 150ft above ground. I just need to do my homework on areas before hand. For 3000 cash topo data should be included.
     
  10. The Editor

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    Actually GPS is extremely inaccurate for altitude measurements. In fact it's pretty useless. Discrepancies of +/- 400ft is not unusual using GPS data.
    The Inspire's Flight controller (like pretty much all other multirotors) uses a small barometric pressure sensor that is sensitive enough to register barometric differences as small as 10cm change in height to regulate it's altitude (I'm not talking about the little ultrasonic transducers on the underside here). If multitorors relied on GPS for altitude information we would all have crashed ours crafts ages ago !! :(

    And for the technically minded amongst everybody.....

    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, they 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.
     
    #10 The Editor, Jan 28, 2015
    Last edited: Jan 28, 2015
    emtea likes this.
  11. slim.slamma

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    Im thinking more along the lines of the fAA. My flight data is being recorded. 700 ft ain't 400. I just want proof that im actually obeying the rules. Accurate not misleading info if bleep hit the fan. Otherwise the i1 is the greatest
     
  12. BaldEagle

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    Actually... the inaccuracy might be more of your friend than you think...:)
     
  13. BaldEagle

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    Are the barometric pressure sensors in airplanes that sensitive, too?
     
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  14. Ian Fraser

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    You can set the current local pressure in aircraft. Radar altimeters give very accurate readings of altitude above terrain
     
  15. The Editor

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    Not sure about that one although I believe commercial aircraft use radar altimeters for take off and landing which are very accurate.
     
  16. Bruce Cuz

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    I've tended to rely on taking a barometric reading - to check height (and thus determine my position) in relation to map contour when ski-touring in a featureless area.

    Although this is a more accurate measurement of altitude - compared to GPS - you need to regularly calibrate such barometers with a known reference height - due to changes in air pressure (often associated with major weather systems).

    Now as a newby (and self-confessed luddite) I was wondering how the Inspire's barometer calibration system works? OK I get it's very sensitive but how does it self-calibrate to compensate for changes in air pressure - so it knows the exact altitude of its launch site on any given day?

    Apologies if this seems a dumb question...there's a certain 'crash paranoia' setting in with this newbie!

    (please use only soft fruit if you feel the urge to pelt me ;-)
     
  17. SanCap

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  18. RaptorMan

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    I think we'll see true AGL elevation within a few years and there's a couple ways to get it. The first would be a radio altimeter and they've been around a long time and are now being made in miniature form suitable for drones. Another way is correlating the lat/lon with topo maps in conjunction with the baro sensor.

    Looking at what high end drones of 2020 might have I could easily see them with:

    1. Radar altimeters
    2. Forward looking radar for collision avoidance
    3. Transponders to report alt, speed, direction, rate of climb and identifier

    A drone so equipped should be able to fly autonomously and although the cost would be prohibitive for the masses in the beginning I suspect the mas production of them will bring it down to an acceptable price for anyone in the Inspire class and above. Remember, an IMU would have cost a small or large fortune not that long ago and just about every drone has them now.

    One last point ... although GPS isn't all that precise on a second-by-second basis they can be very precise if allowed to average for a time. In fact, if the flight is long enough the baro sensor could be corrected for drift and/or weather changes by the averaging of the GPS alt data.


    Brian
     
  19. Bruce Cuz

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    Thanks for the insight into the future, Brian, I'm sure given the speed new technology is being introduced you won't be far off with your predictions!