So,  I've been recently using my iPad Mini 2 as my device to use for my DJI Phantom 3 4K, It's been going well, but after a few times, I've been getting an error on the screen saying Compass Error Exit P-GPS Mode, and it's saying atti mode. I was at a paddock at my parents house when all this happened. So, I used my iPad again at a football oval, and it happened again... Do I need to calibrate it in an open area before I fly to prevent this? So. I went to another football oval, decided to use my iPhone 6s this time, I didn't get any Compass Errors. But I like using my iPad. Because it's bigger than my iPhone. Can I still use my iPad without getting a Compass Error? Thanks. -Josh

Which device you are using has no bearing on whether you get a compass error. A compass error in flight usually means you did not do a good compass calibration on the ground. It is not necessary to calibrate the compass before every flight, just do a good one and leave it at that.

"Do I need to calibrate it in an open area before I fly to prevent this?"

That would be a good idea.
Get a good compass calibration in an area clear of steel and reinforced concrete and stick with it.

"I like using my iPad. Because it's bigger than my iPhone. Can I still use my iPad without getting a Compass Error?"

The compass error comes from the compass in your Phantom.
It is unrelated to the device you use to view the app.

Hello Josh, the mobile devices that you use have nothing to do with the drone's compass. Also, it is recommended to re-calibrate the compass in an open area free of steel materials