I've had my P4 for a month and it's been flying great. I took it out  today and flew about 1 1/2 miles round trip over water and everything  went fine.

I relocated to another lake and was planning on flying around the  perimeter. I got everything linked up, but showed a compass error and  interference warning, so I re-caled the compass. It took 2 trys to get  the compass calibrated, but when it passed I had the ready to fly GPS mode.

I went up to an altitude of 100', was ready to start flying the  perimeter when suddenly it started to move on its own. It made a half  moon loop passing through some trees heading straight for the water. I  was in a gimbal down at this time and it slowly descended, hit a clump  of grass and flipped over a foot away from the water!

I just looked at the flight log and had 41 warnings. It indicates that  the mode went to GPS ATTI and assisted takeoff before I even lifted off.

The log file doesn't show any + altitude readings. The video showed the aircraft spinning as it descended.

"I relocated to another lake and was planning on flying around the  perimeter. I got everything linked up, but showed a compass error and  interference warning, so I re-caled the compass. It took 2 trys to get  the compass calibrated, but when it passed I had the ready to fly GPS mode."

There's your problem.
The compass was telling you that you had magnetic interference where you wanted to launch.
The correct action would have been to move away from the magnetic effect.
Instead you recalibrated to compensate for the interference and then flew away so that your compass now was compensating for something that wan't there.

Just like this guy:

Don't recalibrate your compass where it is telling you there is a problem.

"It indicates that  the mode went to GPS ATTI and assisted takeoff before I even lifted off"
btw .. GPS-atti is just another word for GPS mode .. it is not atti.

labroides@yahoo Posted at 2016-6-17 19:58
"I relocated to another lake and was planning on flying around the  perimeter. I got everything link ...

You're exactly right. My flight time was so short that I didn't have time to look at the screen to see compass error. I was just trying to get to down before it made it to the water. Other than the lake and a nearby hill, I'm not sure what the interference could have been.

The compass readings were off after I returned home. I did a compass cal and it flew good. I also read to do the compass cal away from the ground.

Thanks for the video post, Laboides. A learning experience.

Roadstar1700 Posted at 2016-6-18 08:37
You're exactly right. My flight time was so short that I didn't have time to look at the screen to  ...

There are magnetic anomalies in the wild. we have a mountain around here that's notorious for messing with compasses, the handheld type as well.

treeboyy Posted at 2016-6-18 00:27
There are magnetic anomalies in the wild. we have a mountain around here that's notorious for mess ...

If the compass reading gets irratic and the P4 starts to drift and go out of control, would it help to switch to ATTI mode and regain control?

Roadstar1700 Posted at 2016-6-18 12:58
If the compass reading gets irratic and the P4 starts to drift and go out of control, would it hel ...

Yes that is the advised practice.

Roadstar1700 Posted at 2016-6-18 21:58
If the compass reading gets irratic and the P4 starts to drift and go out of control, would it hel ...

If the compass is giving crazy numbers, your Phantom will probably switch to aitti itself to deal with the confliicting data.
But if the compass is the real problem being in atti mode won't make much difference.
It's still going to be  tricky.

labroides@yahoo Posted at 2016-6-18 08:57
If the compass is giving crazy numbers, your Phantom will probably switch to aitti itself to deal  ...

Here is my video from that very short flight.

Roadstar1700 Posted at 2016-6-18 21:32
Here is my video from that very short flight.Phantom 4 Compass Error

That's a close one, phew!

labroides@yahoo Posted at 2016-6-17 17:58
"I relocated to another lake and was planning on flying around the  perimeter. I got everything link ...

How did u make that video with all the app overlays?

labroides@yahoo Posted at 2016-6-18 06:57
If the compass is giving crazy numbers, your Phantom will probably switch to aitti itself to deal  ...

If u switch to ATT in a case like that, you do have to take full control, and use video feed for orientation..  isn't that the best way? I asume flying FPV with a possibility of being pushed by wind, would not be too wise....?

Roadstar1700 Posted at 2016-6-18 07:32
Here is my video from that very short flight.Phantom 4 Compass Error

Damn! Now, if you did not do the comass dance, I'd understand, but you did.. perhaps your p4 decided it liked dancing so much it wanted more! You were so lucky to miss the trees and the bench! Not  to mention water! So you were not lowering it down? It did on it's own? That is wild! Damage report?

Phantomski Posted at 2016-6-19 15:53
If u switch to ATT in a case like that, you do have to take full control, and use video feed for o ...

Under normal flying circiumstances you can ignore the GPS and fly in atti which still uses the compass.
But if the compass is supplying crazy data, crazy is how the Phantom will fly in atti or any other mode.
You need the compass to be able to fly straight.

Phantomski Posted at 2016-6-19 01:55
Damn! Now, if you did not do the comass dance, I'd understand, but you did.. perhaps your p4 decide ...

It could have been a lot worse. When I saw it was moving on its own and arcing back around towards the lake, I went left stick down and held it trying to get it down onto land. The only damage was a nicked up blade, so I replaced all 4. I plan on going back with my tablet and running a compass app to see what kind of interference there was in the area. There are metal ore deposits all around where I live. The towns sprung up from the mining of the ore.

labroides@yahoo Posted at 2016-6-19 00:19
Under normal flying circiumstances you can ignore the GPS and fly in atti which still uses the com ...

Actually in ATTI mode you will fly with no issue, compass is not used for flying in that mode. It may be used for orientation on the map, but not actual flying of the aircraft, so in ATTI mode you should be quite safe. That is based on P3, but I cannot imagine it being diff in p4.

Phantomski Posted at 2016-6-20 14:53
Actually in ATTI mode you will fly with no issue, compass is not used for flying in that mode. It  ...

You have it all mixed up.
Atti mode ignores GPS input but you still need the compass.
If your compass calibration is screwed up, your flying will be exciting  .. at least.

labroides@yahoo Posted at 2016-6-20 00:19
You have it all mixed up.
Atti mode ignores GPS input but you still need the compass.
If your comp ...

Why do you think that?
ATTI flies like any quad, of course it has some smarts for stabilization, but not for actually holding heading and such?
I am unclear why you think compass is actually still needed? Where did you get that info? I DO want to know details, as it's not easy to simulate a compass failure, and testing it may be risky, I want to understand as much as possible - just in case..... so any specific info you can give (or anyone else) would be great!

Phantomski Posted at 2016-6-20 11:41
Why do you think that?
ATTI flies like any quad, of course it has some smarts for stabilization, b ...

I went back to the lake that I tried to fly at yesterday with a compass app loaded on my tablet. At home the compass was working fine. It also shows the magnetic strength in uT (micro Telsa's). I was getting a reading of 50 uT at home which is a normal reading from what I've read online. I was reading double the norm ~ 100uT at the lake and the compass continually moved around and not once pointed North.

I only wished the flight was longer where I could have thought about going to ATTI mode to see if I could have stabilized it. The mis-calibrated compass is definitely what caused my problem. I'm now more cautious before each flight and look at the compass readings first.

Phantomski Posted at 2016-6-21 01:41
Why do you think that?
ATTI flies like any quad, of course it has some smarts for stabilization, b ...

"Why do you think that?
ATTI flies like any quad, of course it has some smarts for stabilization, but not for actually holding heading and such? "

Because atti uses the compass and all the other sensors except GPS and VPS.

labroides@yahoo Posted at 2016-6-20 19:22
"Why do you think that?
ATTI flies like any quad, of course it has some smarts for stabilization,  ...

That worries me.. I know that phantom have dual compass, but that implies that if u have a magnetic field issue, even being in ATTI may still cause an issue.. interesting, and.. a bit disconcerting....

Phantomski Posted at 2016-6-21 13:01
That worries me.. I know that phantom have dual compass, but that implies that if u have a magneti ...

Don't worry about imaginary scenarios.
Keep a good compass calibration and don't fly in problem areas.

Well, I do like to understand things, and it seems that really not flying in questionable areas is the only solution...  it really does seem like the compass will screw you over in any flight mode, if there are magnetic field issues..  that is so weird! But then again, if it's a problem for .01% of users.. and why do I say this? Well... my local Sam's club has DJI drones in stock! Actually in stock! Not internet hop, but the local Sam's club! Now, everyone, and their aunt, and their dog, and the dogs, friends, will all have one!