It is an area with a magnetic declination of 25 degrees east longitude, and the magnetic declination is between 1-2 degrees; In areas above 25 degrees north latitude, the magnetic declination is greater than 2 degrees; If it is in the low latitude area of west longitude, the magnetic declination is 5-20 degrees; Above 45 degrees west longitude, magnetic declination is 25-50 degrees. In China, under normal circumstances, the maximum magnetic declination can reach 6 degrees, generally 2-3 degrees.