i have done quite a few flights from around the same location and every time i fly i allways check the sensors and the compass is around 1500, sunday i went to a new location which is about 20 mile from the old location, i switched on checked the compass sensor and it was 1300 but the green safe to fly was on. i did a new calibration and it was back to 1500.. is the setting 1300 safe to fly ? even though the green safe to fly was on. what situations make the sensor lower or higher than the recommended 1500.