北磁極
Another major difference between the NDA and SDA is that magnetic declination is not used in the NDA. Because the North Magnetic Pole is close to the NDA (and was formerly within it), magnetic declinations can be very large and changeable from year to year. This is further complicated by the fact that magnetic north wobbles every day in an elliptical path as much as . For these reasons, "true" tracks are always used in the NDA while magnetic tracks are frequently used in the SDA for convenience.