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How precise are waypoints? Had two crashes...

Discussion in 'Pro/Adv Discussion' started by Octoruss, Mar 21, 2016.

  1. Octoruss

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    Hi All,

    I plotted a rather sophisticated course between two trees and through a fence, but ended up crashing both times. It appeared the course line was ok, but the altitude was quite a bit off ...maybe by about 5-10 feet...which resulted in the Phantom crashing into some small branches.

    The GPS signal remained strong the whole time, so I'm confused why it wasn't (or isn't) more accurate. Do you know if this is a limitation of the Waypoints feature?
     
  2. msinger

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    The altitude is determined by the barometer within your Phantom. It's not 100% accurate, so I wouldn't suggest relying on it for flying in close quarters.
     
  3. CaptainDrone798

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    Could be because it climbs or decends from one plotted point to the next. The only time it's at the altitude you set is when it's actually at the waypoint, it then begins to climb/decend to the next waypoint.
     
  4. souldronedlyer

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    Get a P4 with the new sensor and this will never happen again. Maybe


    Sent from my iPhone using PhantomPilots mobile app
     
  5. exit 4

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    I hate to say it but you are experiencing the exact issue I feared would happen with waypoints. My barometer is also considerably off. I think you may have expected it to do what I expected, to go on a diagonal line from point to point to reach the altitude you programed. Maybe you need the altitude to be set one step ahead of each point? Good luck.
     
  6. tcope

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    This may be a little confusing. Altitude of always based on the take off point being 0. If you climb a 50' hill and hover at 100' that is 100' above the take off point but is now only 50' above the ground under the Phantom. If you set the waypoint using the Go app, that way point is still where the Phantom is when set.

    So the altitude above ground or above the take off point should not matter anyway. Which is perhaps what you are stating.
     
  7. tcope

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    Very useful. The solution is spending $1,400 which may or may not address one small issue. :(
     
  8. FantomFlier

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    As indicated by @msinger, your quad uses a barometer which measures air pressure. Air pressure changes all the time and should not be considered accurate beyond 20 feet or so. In fact, an approaching weather front can cause barometric differences of several hundred feet.

    It is seldom advisable to fly below anything with your Phantom, unless you have a huge margin for error.
     
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  9. TheLightSpeedz

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    The P4 has the same challenge with Altitude readings.

    When I had Litchi I found that app to be very accurate, but flying between trees is complicated and dangerous. It doesn't matter what phantom you have...


    Sent from my iPhone using PhantomPilots mobile app
     
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  10. Air Ontario

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    I never plot closer than about 5 foot horiz. when using sat maps and Litchi. I also know the area(tree line) and can see it 200m away.

    I have flown about 15-20 ft high and parallel with it will videoing.

    I have a tablet with a barometer and a barometer app. I set the barometer app to the baro pressure at launch point to get precise altitude ASL and validate that jives with reference data for that launch point.

    I then launch, compare attitude reported by the drone to actual height and again at first waypoint. If they don't match I adjust or abort.
     
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  11. grottoli

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    Will be trying some of that (weather permitting). I will be setting a waypoint prior to entering the 'tight' space and a waypoint clear of that same 'tight' space before moving on. This should bring the craft to the desired heights during the mission.

    In closing, every tight waypoint will have 3; starting point and height, the actual mission waypoint (assume some recording being done here), and finally the exit waypoint prior to changing altitude.

    Should be fun (or not; we'll see)

    Cheers

    P.S. Had this open on my pad for a while before sending. Appears to be already answered... Gonna try anyhow. Wish me luck


    Sent from my iPad using PhantomPilots mobile app
     
  12. CaptainDrone798

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    What I meant was that if you set a waypoint at 100 feet and set the next waypoint at 200 feet, the Phantom will leave the first waypoint at 100 feet and will keep climbing until it reaches 200 feet at exactly the same time it reaches waypoint 2... not before.
     
  13. grottoli

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    Correct... with the exception that your rate of ascent governs how quickly your bird rises. Otherwise when the waypoint is reached but at a lower altitude, the AC will finish the move by going up to the designated altitude. I'm trying to find specifics (which are eluding me for the moment) and will amend when I do. Feel free to chime in folks.

    Cheers


    Sent from my iPad using PhantomPilots mobile app
     
  14. Traveler

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    With regards to altitude I have personally tested the with and without VPS turned on and I have concluded that VPS provides a much more accurate account of altitude. This has limitations of course but for close to the ground flying I'm a VPS fan. It can even follow you up and down a slope. Better going down than up though.
     
  15. Trackman1

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    Just look at your flight log in Msingers viewer.
    In regular flight I start up and take off from a piece of plywood. I land back on the plywood. The flight log says I am 5.2 ft away.
    That is with 18 to 20 sats. So that is as accurate as the waypoint could possibly be for me at that time.
     
  16. Imabiggles

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    unless he is flying sideways or backwards. The avoidance only works going forward.
     
  17. JerEl

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    From all the replies it seems folks have many different safety guidelines. I live in a pine tree infested rural area and after flying around for over a year at 100' I felt pretty good at clearing everything within 2 miles of my takeoff point. Until...My P3P found a pine tree at 120' just about 300' distance from takeoff and of course on a Litchi orbit mission decided to run right into it. Learned my lesson about "assuming". One hundred twenty fee is, for this area and type of pine, a very tall pine tree and it being so close and me not noticing it before (despite it being on the road I go in and out of all time) convinced me to add 25' to 50' of the height that Google Earth shows PLUS the height of any obstacle. In a piloted plane one sets the barometer at the takeoff point to get a more accurate altitude indication and then adjusts it along the way or at least to the landing point by listening in to the radio frequencies of the area. While DJI's method of just setting the takeoff position as "0" works out okay most of the time you can't really trust Google maps elevation or the barometer and it's probably best to manually fly around an area you plan to do your missions to check the height of obstacles.

    It would be interesting if we could find out the accuracy of the on-board barometer like +-15' or something like that. Also, I wonder if anyone has tested how long it takes the P3 to react if you give it full throttle and full reverse at the same time when traveling at or near maximum speed like when trying to avoid an obstacle showing up on the video? To avoid that 120' pine tree I did give it full throttle to try to avoid the top of the tree but it happened so fast I don't think it rose in altitude happened before I hit it.
     
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  18. Octoruss

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    Here's a link to the original crash video. Fast forward to the end. I had plotted a course to go through/under the gate, but it flew too high, and over it:

     
  19. NormanNormal

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    Sorry about your crashes. I too made the assumption that the altimeter was very accurate and I was running some questionable missions based on that data. During my research and one observation I noticed how off it was and how off it is known to be, which is very unfortunate. Do not run any waypoints within 30ft above or below any obstacles. Check the official litchi documentation about this issue:

    Important Notes

    Important Notes

    • 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) above any obstacles. 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.


    • It's recommended to set Finish to "RTH" in case of lost control signal during the mission, while aircraft will RTH and auto-land upon completion of mission.

    • For safety reason, you might consider to fly a pre-mission flight with Heading set to RC/Forward (and turn off "Auto-Tilt" in Target-WP mission) as a preview to ensure clearing any obstacles. You may control aircraft yaw and gimbal tilting with RC to preview the upcoming mission flight path, and make necessary adjustments. After pre-mission flight, you can switch Heading back to "WP" (and turn "Auto-Tilt" back on in Target-WP mission) for shooting footage.
     
  20. isopro

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    I would never use any automated flight, at an altitude lower then the highest obstacle.