Hi I have a phantom3 professional an the other day I had a compass error mid air about 4000 ft out an was wandering what would cause that

The first place to look is where you took off from. The most common source of compass problems while in the air were caused by taking off from a spot that had iron or steel in it. Favourites are concrete roads, concrete building roofs, hoods of cars.  The metal is usually too close to the compass in the aircraft. Did you know the compass on a P3 is in the bulge on the foot of the landing gear?

Best to take off from grass or some place with no metal in it.

Hi. I am sorry for the trouble this has cause. With regards to this concern. As Geebax advised at post 2 the issue might be caused by the interference in the environment. I would recommend to please try flying to different location after calibrating the compass to isolate the issue. Please keep us posted for further assistance. Thank you.

Thanks for the advice but I was taking off from grass in my backyard nothing steel around the compass I made sure of that an yes it’s on the leg I knew that

Racerman17 Posted at 1-23 09:59
Thanks for the advice but I was taking off from grass in my backyard nothing steel around the compass I made sure of that an yes it’s on the leg I knew that

Your compass could have been slightly out of whack before you launched.  You might consider giving it a calibration.  

I have tried that I can get it calibrated but sometimes  everything is fine an sometimes not I done a Imu calibration an seemed to help too an also I had a new compass put in it