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Altitude when changing terrain heights.

Discussion in 'Pro/Adv Discussion' started by MHaerialphoto, Aug 4, 2015.

  1. MHaerialphoto

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    Hey guys, just wondering if A, this is a feature or bug, and B, i can, and should change it?

    I flew from a mountain road in Lake Tahoe this weekend out over the lake. My 'altitude' clearly went from 0' above the ground near me, to hundreds of feet overt the lake. The display only showed me at 80 feet the whole time. That's clearly the height from MY ground, or takeoff point. Is there a way to make it readjust to the terrain it's over? If so, is there any reason I should change it?

    Thanks in advance,
     
  2. bbfpv

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    Nope. The bird only knows AGL from it's launch point.
     
  3. rockydog

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    Height shown is always the height above your take-off point.
     
  4. RyanK

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    I wonder if what he's asking would ever be possible. I think it would be great to not only know the AGL of where you took off, but also of where you currently ARE. Would that even be possible with the barometer?
     
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  5. Monte55

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    Are you sure about that? I have read where people have negative numbers if flying below takeoff point
     
  6. MHaerialphoto

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    Thanks guys. I kind of thought so, but it would be a great option to toggle, or at least see a different view. GPS supplies this info because I use apps when offroading in areas with no cell signal that give that data so it is possible.
     
  7. bbfpv

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    ... or below take off point.
     
  8. MHaerialphoto

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    I think he means it's 'relative' to your takeoff point. Not necessarily 'above'.
     
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  9. MHaerialphoto

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    Do we know if the map view gives that info?
     
  10. bbfpv

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    It doesn't. Everything is AGL. I don't believe the GPS chip in the bird even knows MSL altitude.
     
  11. Monte55

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    Isnt that what I said?
     
  12. bbfpv

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    Yes. Just thought below was implied when referring to altitude.
     
  13. Monte55

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    Ok. You guys are losing me. We are saying the same thing but the posts keep coming.
     
  14. Bret Lucas

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    To clear this up. The P3 does have a barometer to measure altitude but it uses this sensor to help measure height AGL (above ground level) along with GPS. The datum is your take off point always.
     
  15. TAZ

    TAZ

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    It is possible with additional coding/implementation, but not with a great degree of accuracy. Google Maps API have a service that gives you an estimate of ground height based on GPS coordinates. So, provided an implementing application had wifi or cell service connectivity, it could query the google API of current GPS coordinates and do a simple calculation to display an estimate of Above Ground Level.

    The problem here is not every point on earth has an associated elevation data point. In this case, it averages values based on nearby known elevations. I think the Google API claims accuracy of +-30M (although often in highly mapped areas, it is very accurate). There are other data sets (like the USGS that covers only the U.S.) that claim accuracy within 3M yet their coverage isn't complete ... So, depending on where you are, the estimate could be several meters off. Kinda of not very useful for navigation purposes. Plus it would need to keep querying the API as your bird moved...potentially very taxing on already taxed processor and requiring wifi/cellular, all just for an estimate of above round level elevation that could be many meters off.
     
  16. N017RW

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    +1 TAZ
    There's been a lot of discussion in the past re GPS and altitude.
    It's just not practical in this toy quad to use it and is further bolstered by the fact that many, if not most, portable GPS devices also include a barometer for such data.
     
  17. SteveMann

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    Even manned aircraft with GPS receivers costing thousands of dollars are still required to use the barometric altimeter on most GPS approaches. GPS altitude is notoriously unreliable.
     
  18. JohnK

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    Haven't heard someone use the word "datum" in many years. I read "battery's" as the plural of "battery" more times than I can count. Sorry for the off topic, but a breath of fresh air is always appreciated. ;-)
     
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  19. Meta4

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    The Phantom gets no altitude information from GPS. It's all from it's barometric altimeter.
    While GPS gives good horizontal accuracy, it is woefully inaccurate in the vertical.
    It's commonly out by 100 feet of more and can swing quickly..
     
  20. Bret Lucas

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    GPS information is three dimensional so the P3 will get information with VNAV (vertical navigation). Of course the barometer is much more sensitive so will be the primary altitude sensor.