Magnetic declination is the angle between the magnetic north line and the true north line. If you already know the reference value A of the magnetic declination a few years ago, and you also know the annual change value C of the magnetic declination, let The number of years between intervals is n. Then the current magnetic declination angle á is equal to the magnetic declination angle a few years ago plus the interval number of years n multiplied by the annual change value C . That is: á = A + n C
For example: The annual change value of Beijing area C = - 3 ′ 5 6 ″
The magnetic declination angle in 1 9 6 4 years = 5 ° 1 7 ′ 2 3 ″ , find the magnetic declination in 2 0 0 3 years after 3 9 years:
á = A + n c = 5 . 2 9 ° - 3 9 * 0 . 0 6 5 ° = 2. 7 6 °
That is, the geographical north-south direction is 2. 7 6 ° west than the direction pointed by the compass.
When we use a compass to measure direction, if we do not calculate this magnetic declination, we cannot find the true north-south direction. There is a saying in Feng Shui that "there is a thin line between gold and wealth, and wealth and honor cannot meet", which illustrates the importance of using a compass to determine the direction. Therefore, before going to various places to survey Feng Shui, you must calculate the local magnetic declination, and then adjust the compass pointer to the degree of magnetic declination for use. Otherwise, it will be a slight difference and a thousand miles lost. Be careful, be careful.
Table: Annual changes in longitude, latitude and magnetic declination of major cities in China:
North latitude
East longitude
Annual changes
1964 reference value
Shanghai
31°11′
121°26′
-3′57″
3°53′18″ West
Nanjing
32°04′
118°47′
-3′45″
3°43′03″ West
Hangzhou
30°15′
120°10′
-3′45″
3°51′30″ West
Changsha
28°12′
112°56′
-3′00″
1°54′00″ West
Chongqing
29°32 ′
106°34′
-2′30″
1°30′00″ West
Chengdu
30°40′
104°05′
-2′20″
1°15′00″ West
Fuzhou
26°05′
117°18′
-3′20″
2°04′00″ west
Guangzhou
23°15′
113°18′
-2′45″
0 °52′30″ West
Kunming
25°04′
102°44′
-2′00″
0°56′00″ West
Beijing
39°55′
116°27′
- 3′56″
5°17′23″ West
Tianjin
39°08′
116°12′
p>-3′55″
5°23′30″ West
Qingdao
36°04′
120°19′
-4′02″
5°15′00″ West
Xi’an
34°15′
108°56′
-3′00″
2°14′00″ West
Urumqi
44°46′
87°34′
+2′05″
2°53′24″ West
Lhasa
28°40′
91°05′
-1′15″
0°42′30″ West
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Magnetic declination in major cities in my country
Chengdu 1 degree 16 minutes west
Chongqing 1 degree 34 minutes west
Guilin 1 degree 22 minutes West
Nanjing 4 degrees 03 minutes west
Kunming 1 degrees 00 minutes west
Lhasa 0 degrees 21 minutes west
Mohe 11° 00'(West)
Xuchang 3°40'(West)
Urumqi 2°44'(West)
Qiqihar 9°54'(West)
Tianjin 5°30'(west)
Yinchuan 2°35'(west)
Harbin 9°39'(west)
Jinan 5°01'(west)
Xi'an 2°29'(west)
Changchun 8°53'(west)
Hohhot 4° 36'(West)
Lanzhou 1°44'(West)
Manzhouli 8°40'(West)
Xuzhou 4°27'(West )
Xining 1°22'(west)
Shenyang 7°44'(west)
Taiyuan 4°11'(west)
Lhasa 0°21'(west)
Dalian 6°35'(west)
Baotou 4°03'(west)
Beijing 5°50' (West)
Zhengzhou 3°50' (West)