Bob, this will come up if you traveled a certain number of miles/meters from your last flight location. It's really just the aircraft telling you to re-calibrate the compass, but they didn't bother coming up with a message for that (so they used the mag interference message).
Of course, that error could also mean actual magnetic interference, but as you say, you were in a rural environment.
Check the app: all this means is that DJI Go will give you more information that the limited LCD display on the remote. For instance, the app might just say "Calibrate compass", so then you know it's a compass issue. But if the app also says mag interference, then it's still possible there's something about your environment that's causing that (even in rural areas -- power lines?).
Chris