Yesterday I was shooting a property for a client! 40 acres on the side of a gentle slope! There was a nice convient flat boulder to fly from by my truck so I did d! I expected some compass interference and there was some! Calibrate compass or move it said! Well it got gps lock easy enough so I took off and after a bit the compass problem cleared! But throughout the shoot I would get the compass error, even at several hundred feet in the air! Looking at the pictures last night it was plain to see that the hill was a solid chunk of granite! Must be like a giant magnet!