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Waypoint Accuracy, Use Caution
4370 7 2016-11-25
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Wingsy
lvl.3

United States
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Hopefully this post will save someone from a crash. Using Litchi, I programmed the waypoints you see in the picture. I knew it was risky flying so close to solid objects because I have seen waypoints drift before. But this wasn't so much drift as my method of setting waypoints. My P4 wound up crashing into the upper right corner of the house at waypoint #5. Hit the porch column and fell about 10 feet onto rocks. Broke 3 props and knocked out the battery. No other damage. Lucky. Here is what I did wrong:

Rather than setting the waypoints by flying to the point and recording the location, I tapped on the map to set them. That's not nearly as accurate as recording the position where your drone is hovering. I had to abort the flight and move a waypoint a couple of times before I thought it was good enough.

I lost sight of the drone when close to the crash point. Bad thing to do. I was trying to hide from the camera so I wouldn't be in the movie, but I could have taken the time to hide somewhere else so I could maintain a visual.

I'm determined to do this flight, but today I will trash the current waypoints and set them by flying to each point and recording it, I'll fly the route before the GPS position has a chance to drift much (which it will do as the satellites' geometry changes over time), and I will hide from camera view in a place where I can see it at all times.

Waypoints.jpg
2016-11-25
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Labroides
Captain
Flight distance : 9991457 ft
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Australia
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The problem with this is the level of accuracy in consumer GPS.
It is not centimetre accurate and can be +/- 2 metres or sometimes more.
Whether you place waypoints on a map or fly them first won't make much difference.
When you go to fly them they can be out.
And if there are obstacles to hit ......
Plan your waypoint route to keep you cler of potential collisions.
2016-11-25
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Nigel_
Captain
Flight distance : 388642 ft
United Kingdom
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There is likely a lot more inaccuracy in the "satellite" photo position on the map than in the aircraft GPS position.   

A good idea to pick a "calibration point" on the satellite photo as the first waypoint, eg directly above the chimney, then compare the position of the chimney looking directly down on the live view with where it is on the satellite photo, if you can see that it is 10m out then you know to abort and adjust the waypoints.  That doesn't tell you if it is the GPS or the map that is wrong, but it does give some confidence that there wont be a crash, with 2 calibration points you can also check the angle of the photo against reality, but that is unlikely to be far out.
2016-11-25
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Labroides
Captain
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Australia
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Nigel_ Posted at 2016-11-26 00:22
There is likely a lot more inaccuracy in the "satellite" photo position on the map than in the aircr ...

Google Earth is usually pretty good but consumer GPS isn't.
And GPS (in)accuracy is variable.
Trusting GPS close to obstacles is asking for trouble.
2016-11-25
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fansa84fe8a4
First Officer
Flight distance : 3 ft
United States
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Good idea about setting a reference point first and verifying that is where the aircraft will go to first before proceeding at a new location.  These satellite maps seem to differ as we have Google Maps, Apple maps, and Here maps.  I set one in Google and it was inside a mountain instead of over the top of it even with a +50 foot clearance set over it on the waypoint, so best to be quick on the PAS switch and cancel - and do a pre-fly checkout maybe.
2016-11-25
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Wingsy
lvl.3

United States
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Nigel_ Posted at 2016-11-25 08:22
There is likely a lot more inaccuracy in the "satellite" photo position on the map than in the aircr ...

I dunno about that. Twice now I have used the map to set waypoints, flown the mission, then trashed it and repeated using aircraft positions. My experience tells me the recorded aircraft position flies a more accurate route. For a while anyway. Come back a few hours later and it's way off, or not.

Very good idea about referencing the chimney. I'll experiment with that. Right now my neighbor is feeding his cows and I can't fly where I need to. Later.
2016-11-25
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FuZZyPiLOT
First Officer
Flight distance : 1353215 ft
Belgium
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Good reading and advice, thanks guys. I just bought the Litchi app and was planning on testing it this weekend.
2016-11-25
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Labroides
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Flight distance : 9991457 ft
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Australia
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Wingsy Posted at 2016-11-26 01:55
I dunno about that. Twice now I have used the map to set waypoints, flown the mission, then trashe ...

This is the nature of consumer GPS.
It is variable in its accuracy and won't position your Phantom in exactly the same place every time.
It cannot be trusted for close in precision as the location it gives can commonly be plus or minus 2 metres and sometimes even more.

Read p21-22 of this to get an understanding of the limitations of GPS for accuracy: http://www.gps.gov/systems/gps/performance/accuracy/
2016-11-25
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