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Waypoint accuracy?

Discussion in 'Pro/Adv Discussion' started by Deep6, Jan 3, 2016.

  1. Deep6

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    Was wondering if anyone has had any issues with their phantom not hitting waypoints exactly? I haven't noticed anything, but was considering some flights through an area with trees, and was concerned about drift... Say if I had a point of travel between 2 trees 20 feet apart, should I be concerned?


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  2. ScatSpeak

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    Well, since the P3 is using GPS for navigation and your waypoints can be lat/long specific, it will do a very good job flying to the proper coordinates.... however, the trees and the map upon which you might be basing your GS mission are not guaranteed to properly correlate...

    You can always plan the mission and then while flying it, pay attention through the FPV and stop the mission if the trees aren't where you expect them... Generally, I try to either fly above the tree line on a GS mission or ensure ample separation of any "higher" objects that might not correspond well to actual coordinates...
     
  3. Deep6

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    I plan to fly the "course" plotting the points along the way, so no issues with map vs actual. My street has a lot of trees and I've plotted points and had it slowly fly the course and it seems to hold tight in the middle of the street. While high altitude flights and vids are cool, there's not much to film around here other than the suburbs... Have been practicing in the field behind me, I love how steady the cam is and the way it pans between points, but would like to get a little lower below treetop level, 20-30 feet off the ground and still have her on autopilot.


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  4. Deep6

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    Generally, I try to either fly above the tree line on a GS mission or ensure ample separation of any "higher" objects that might not correspond well to actual coordinates...[/QUOTE]

    Haven't opened GS yet, started to watch a couple vids but figured as a new flyer I needed to concentrate on the base functions and flight operations first. Looking forward to learning more about what GS does soon as I become a bit more confidant.
     
  5. aka1ceman

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    Ive used Litchi to do some very close fly bys to a church steeple, roof tops, etc for videos. I flew it and marked the waypoints when they were close (1-2 ft). I re flew it the following day with no problem, but did launch from same spot due to elevation variations.
     
  6. ScatSpeak

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    It's probably important to keep in mind that GPS accuracy can vary depending on a number of factors... at any given moment it can vary an average of 3.5 meters (~10 feet) or worse...

    Caution for close proximity autonomous flights is warranted.
     
  7. Deep6

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    Any idea if waypoint altitude is gps based or based off of AGL? If A hill is my takeoff point when I make the course, then I take off from a lower elevation nearby on a subsequent flight, will all my waypoints be lower by the difference in my AGL at takeoff.

    I would think that the altitude would be gps based and remain constant, but as I have a course set up over a frozen lake at low altitude I figured I'd ask... Plan to go re-fly as soon as batteries are charged, and don't want to hike up the hill again! Ha ha


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  8. ScatSpeak

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    P3 altitude awareness is based upon take-off point.. that's ground zero, literally and as I recall, the AGL altitude above that is calculated using the barometer.

    So, you're waypoints in a GS mission plan and any altitude assignments have to factor this in... So, if you take-off from the top of a 400' hill and fly up 50' and then horizontally away from it until your P3 is over the base of the hill, as far as it's concerned, you are at 50' even though technically you are actually at 450' AGL.... if there was another hill nearby that had a height of 600', if you wanted to fly to it and hover at 50' over the top, you would have to have the P3 climb to 250' (again assuming you take-off from the top of your 400' hill) as presented on the FPV display.
     
  9. Meta4

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    GPS data is never used for altitude because GPS altitude accuracy is woeful - commonly out by +/-200 feet or more
     
  10. Meta4

    Meta4 Moderator
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    GPS is great for horizontal position but appalling for altitude.
    Because GPS altitude accuracy is not reliable, the Phantom uses a barometer just like a real airplane does for altitude.
    This is from Garmin's FAQ:
    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.
     
  11. PVFlyer

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    We concern altitude a lot more than drifting in waypoint mission as stated in Mission Planner (FPV Camera) Important Notes section below.

    • Aircraft height (altimeter) may easily vary 15-20m (50-66ft) off the actual height after aircraft flying and heating up several minutes. You are recommended to let IMU warn up properly on the ground especially in cold weather, and set waypoint height minimum 25m (82ft). Cold IMU calibration trick is NOT recommended which might incur more discrepancy in altimeter reading.

    For example, if there is a tree 30m height and you set the first waypoint to 40m above that tree to maintain a 10m gap and the last waypoint is also set to 40m above the same tree after 10 minutes mission flight. The aircraft might reach the first waypoint well above the tree, but when it reaches the last waypoint 10 minutes later, it might hit the tree due to discrepancy of barometer though the altimeter is still reporting 40m height.
     
    #11 PVFlyer, Jan 3, 2016
    Last edited: Jan 3, 2016
  12. John Locke

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    As everyone is suggesting, waypoint misssions not super accurate but in general you "may be OK". Don't forget that a weather change in barometric pressure can affect things through the day. What I highly recommend is to never do waypoint missions solely using FPV. I suggest to always (90% of the time) be watching your craft with clear LOS with your finger resting on the mode switch, so if at any time you see something going wrong, flick it quick from F to P. That should stop everything and the craft will stop and hover, fairly quick. That's your safety net in waypoint mission flights.
     
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  13. Deep6

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    Had good luck today setting up a waypoint course by flying it first. Starting at 400 feet, then working down and around, at times the copter was only 10 feet off the ice/water of the lake and flew the mission perfectly! Using the waypoint mission gave me some great continuous shots with very smooth camera transitions. Thanks for all the advice.


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