DJI GO does not always show a warning when the compass is being negatively affected by some type of external magnetic force. Before taking off, you should always do the following:
1) Find a takeoff spot that you know is not near any magnetic metal objects (e.g. rebar in concrete sidewalks).
2) Make sure your drone is never near magnetic metal objects after it has been powered on. It’s best to power on your drone in the exact location where you plan to take off.
3) Check the status message at the top of DJI GO and make sure it’s not reporting a magnetic interference error (like in the screenshot below). If you see this error, that’s a sign that your drone is near some type of magnetic metal object. Move your drone to a different location.
4) When your drone is on the ground at the takeoff spot, look at the map in DJI GO and verify that the red aircraft symbol is pointing in the same direction as the drone. If the drone and arrow are not both pointing in the same direction, that’s a sign that your drone is near some type of magnetic metal object.
5) Make sure the red aircraft symbol in DJI GO is not slowly rotating as your drone is sitting on the ground. If the red aircraft symbol is rotating, that’s a sign that your drone is near some type of magnetic metal object.