The geomagnetic pole is close to the south pole and the north pole, and does not coincide with the south pole and the north pole. One is about 72 north latitude and 96 west longitude. One is about 70 south latitude and 0/50 east longitude. The magnetic north pole is about 1500km from the geographical north pole. During the day, the position of the magnetic north pole is constantly changing, and its trajectory is roughly elliptical. The data measured by the above method are the magnetic south and magnetic north of the earth, that is, magnetic meridian. Is not really the north and south of the earth. There is a certain angle between the magnetic meridian and the meridian of the earth, which is called "magnetic declination" in geodesy. In China, the magnetic declination increases from south to north, and the difference between north and south is above 13 degrees. Moreover, the magnetic declination in most areas is westward, and only a few areas are eastward.
At this stage, many people in the industry are arguing endlessly about whether to choose magnetic declination, both for and against it.
The reasons for opposing the use of magnetic declination are:
1. The establishment of heaven and earth is influenced by the interaction of Yin and Yang, and geomagnetism is one of them. The north pole of the earth is used to determine the position of stars, so don't confuse the two. The geomantic orientation should still be based on the magnetic north pole, and should not be based on the adjusted earth north pole. The compass was invented in Song Dynasty. Before the Tang Dynasty, sages, Yang Gong and other geomancers could not have known about the magnetic declination. Even if the problem of magnetic declination was found, the measurement at that time was not as accurate as it is now, and it should be done according to the ancient method.
3. In the south, the magnetic declination is relatively small, for example, Nanning is the smallest at 0050 ′ west, Haikou is at 10 west, and Kunming is at 10 west. Such a small magnetic declination can be ignored, and the error is very small.
4. It is complicated to calculate the magnetic declination, and it is also difficult to obtain the local magnetic declination.
In favor of the use of magnetic declination reason is:
1. On the Chinese mainland, such as Nanning, the magnetic declination is the smallest, only 0.50 points to the west, while the magnetic declination of Mohe River adjacent to Russia in the north exceeds 1 1 minute to the west. China has a vast territory, such as Urumqi, where the magnetic declination is 2 degrees 40 minutes east, indicating that the magnetic declination of Urumqi is 13 degrees 44 minutes different from Mohe.
2. With the passage of time, the geomagnetic declination changes constantly. It only happens once every 200,000-300,000 years, and the 24 mountain directions measured in different years in the same area will not be the same. That is a direction of a mountain carved on the tablet 30 years ago, but it is not the original picture 30 years ago. Therefore, the magnetic declination is definitely not a small problem that can be ignored, because it changes greatly in China, for example, Beijing is 5 degrees 50 minutes west longitude, Harbin is 9 degrees 39 minutes west longitude, and Shanghai is 4 degrees 26 minutes west longitude. This deviation angle is the real difference between the north and south of that area and the magnetic south pole, and the position obtained after the conversion of this difference is the real geographical position we require to measure.
2. Before the compass was invented, it was measured with a soil gauge. It is also recorded in China's historical classics. In ancient times, orientation was mainly measured by soil gauge. For example, in Zhou Li during the Warring States Period, there are the following records: "Situ, Di Gong, Zhou Li": "Only Wang Jianguo is on the right, and the country is in the wild ... Where the country is founded, its city is made of a land of expensive soil." "Zhou Li Geological Map": "Measure the depth of the soil by the method of soil rules, and look at the sun (shadow) to enter the ground." "Zhou Li's Big Bo" says: Measure the depth of the soil by the method of soil rules, and look at the sun (shadow) to find the land. ..... The scenery (shadow) from the sun to the sun is five inches long, which means the place where heaven and earth meet, four seasons meet, wind and rain meet, and yin and yang meet. However, everything is safe, it is a matter of building a kingdom, a thousand miles away, and sealing trees. Guan Chundian Rui: "When the land returns to the sun and the moon at four o'clock, the country will seal the land." Zhang Heng (78- 139), the inventor of the famous armillary sphere and seismograph in the Han Dynasty, said in his works that Luoyang, the imperial capital, was established in this way: "In the past, the king city was also a city, and the nine Shaanxi provinces were hidden, instead of camping in the wilderness, the king city was built after returning to the land to explore the scenery." Guo Pu, the founding emperor of the Jin Dynasty (276-324), said in his Book of Burials: "The earth is expensive to measure its position, and the jade measures its distance. Those who regard branches as dragons and tigers will stop at hills, such as elbows and arms, which is called embracing. Those who take water as a rosefinch are prosperous and tight, and should avoid turmoil, which is called weeping. Rosefinch was born in anger, and was born in the king, so it was peaceful. Therefore, before the Tang and Song Dynasties, the method of returning to the earth was used to measure the positions of the celestial bodies, the sun, the moon and the stars, so it was natural to adopt true north. From the above ancient books, we can know that in the past, the twelve directions were mainly determined according to the soil rules.
3. The development history of the three needles of the compass. When you see the compass, you will find that the compass is the innermost part, followed by the innate gossip, the ground needle, the human needle and the sky needle, which are distributed on the compass from the inside out. It is said that the three-needle compass handed down from ancient times first invented the ground needle for the dragon, and measured the human disk of the star position to remove sand; The celestial disk understands water by measuring the gravity of planets. These arguments have misgivings that mislead the geomantic circle. If these arguments are not corrected, the intention and background of the compass needle will only be misinformed and have a great impact. In fact, in the Tang Dynasty, Yang Gong, as the founder of geomantic omen in southern Jiangxi, established a complete geomantic omen theory, but he still used Tugui method in geomantic omen positioning. By the middle Tang dynasty, compass orientation had been widely used, and the magnetic south and magnetic north were taken as the standard when measuring. At the end of the Tang Dynasty, in the process of using the compass, Yang Junsong discovered the so-called "magnetic declination" phenomenon in modern geomagnetism. He found that there was an angle between the geomagnetic meridian and the direction of the north and south poles (geographical meridian) determined by the shadow of the sun, so he added a layer of azimuth circle to the compass. At this time, the magnetic declination is 7.50 northeast, and this azimuth circle is called "sewing needle". Lai Wenjun, a Feng Shui master in the Song Dynasty, found that the magnetic declination changed from northwest to 7.500, so he added a layer of azimuth circle to the compass, which was called the "middle needle". From the development history of the three needles of the compass, it is proved that the understanding of "site needle is used for dragon orientation, human disc needle is used for sand removal, and sky disc needle is used for water collection" is wrong. The real meaning of compass three needles is the application process of magnetic declination.
The formula for calculating the magnetic declination is:
Compass scales are arranged counterclockwise, and the formula is:
Geographical orientation = magnetic orientation reading-magnetic declination (West)
Compass scales are arranged clockwise, and the formula is:
Geographical orientation = magnetic orientation reading+magnetic declination (west)
In practice, first adjust the compass pointer to the north-south direction, and then find out the magnetic declination in this area. If the magnetic declination is 4 degrees west longitude, point the north needle of the pointer to 4 degrees west longitude (that is, 4 degrees counterclockwise and 356 degrees clockwise). If the magnetic declination is 2 degrees east, point the north needle of the pointer to 2 degrees east (that is, 358 degrees counterclockwise and 2 degrees clockwise). At this time, the scale indicated on the compass is the earth meridian scale, and the mountain direction determined by the aiming tool is the geophysical mountain direction.