I was flying this weekend in an open field with a small incline in elevation. In one area, there was a practice football field, and the particular field was approximately 5-7 feet higher than where I had launched my Phantom from. My question to the group relates to changing your landing area in respect to altitude. The football field was around 2000 ft from where I launched my Phantom, and again, around 5-7 feet from ground level than where I launched. I was just trying to practice flying some figure 8's, low level flights, and camera operations, when I noticed the height on my DJI app indicated on the football field that I was approximately 10 feet off of the ground. I could see from the camera though that I was looking from a horizontal position directly near the bottom of the football goal post, so I knew I wasn't 10 feet high. I went ahead and landed. After I took off again, the compass had recalibrated, and the new height was accurate while on the football field. At this point, I flew my Phantom back to my location, and sure enough, as soon as I began to descend, the height measurement was off again. I thought the sensors on the bottom of the aircraft were there to monitor that adjustment? I'm still learning, so please feel free to tell me I'm wrong, but with small of a difference in altitude in a semi-flat area, it makes me wonder how the same theory would pan out if I were in a mountain range, or area where there were multiple peaks/valleys? Any help and/or recommendations is greatly appreciated! Thank you!!