What is the relationship between true and magnetic azimuths?

Surely, everyone knows about the existence of true azimuth and magnetic azimuth. They do not coincide in the same way as the geographic pole of the Earth with the magnetic one. Therefore, orientation on the terrain should be made taking into account their differences. This is necessary in order not to encounter the difficulty when the exit to the intended point on the map of the area is indicated by different compass readings and measurements of the azimuth on the map.

  • Difference between two azimuths
  • 2Connection between true and magnetic azimuths
  • 3 Determination of true and magnetic azimuth
    • 3.1. Determination of the geographical meridian with azimuth from the respective stellar heights
    • 3.2 Determination of true azimuth of the Sun
  • 4 Orientation of directions: true and magnetic azimuths
  • 5Working with a compass: laying the right route

Difference between two azimuths

connection between true and magnetic azimuthsThe azimuth is the angle between two directions: the north and the one where the required point is located on a specific location. The north direction is considered to be the direction to the North Pole. But at the same time, the compass north direction, in fact, is an indication of the magnetic pole, and not its geographical position. Thus, speaking of the true pole, we mean the geographical, associated with the shape of the Earth, as well as its axis of rotation passing through two poles – north and south. The definition of a magnetic pole is associated with the existence of a magnetic field around the Earth, which has two poles that do not coincide with the true terrestrial, as do the corresponding true and magnetic meridians.

If a certain direction is set aside between these meridians relative to the object of the terrain, then the angle formed between each of these meridians will be the true and magnetic azimuth, respectively. The difference between these two azimuths is magnetic declination. Depending on the direction of the world to which the compass needle is leaning from the true meridian, the declination is respectively called: eastern when it deviates to the east, western when it deviates to the west. At the same time, declination to the east is indicated by a plus sign, and to the west – by a minus sign. Depending on the location, the magnitude of the declination varies, as well as over time. To determine the magnitude of the magnetic declination indicated on the topographic map at the moment of calculation, it is necessary to multiply its annual change by the number of years elapsed since the compilation of the map, adding to it the value indicated at the time of the compilation year.

The relationship between true and magnetic azimuths

true and magnetic azimuth detectionTo determine the magnetic meridian, the compass is used. This is a special box of a round shape, with a ring inside it with divisions denoting degrees every 10 values. The divisions with the designation of degrees from 0 to 360 in the counterclockwise movement count the azimuths, and the divisions from 0 to 90 of each quarter of the round field count the so-called rumba. The box is covered with glass, and on the center of the steeple, the arrow freely rotates, showing the magnetic meridian with its direction. Knowing the points, you can calculate the azimuths and vice versa. Thus, a connection is formed between true and magnetic azimuths.

Determination of true and magnetic azimuth

the difference between true and magnetic azimuthsTo learn azimuths, you must first determine the direction of the meridians. When it is found, it is necessary to measure the angle between the direction of the meridian and that which indicates the desired point of the terrain. The desired angle is measured clockwise to the right of the north direction. To calculate the magnetic azimuth, you can use the value shown by the magnetic arrow, free from the influence of terrestrial magnetism, metal objects, electrical networks. It should be well magnetized, rotate freely on the steeple. A device for determining the magnetic azimuth and direction of the magnetic meridian using a magnetic needle is called a compass.

Determining the true azimuth using a compass is not entirely accurate, but you can find out its value with a difference of 1-2 degrees, provided that the magnitude and direction of declination of the arrow at a particular point of the terrain is known. A more accurate way to determine the true meridian is to observe the heavenly bodies. This is done using special goniometer tools placed horizontally.

Determination of the geographical meridian with azimuth from the respective stellar heights

orientation of the directions of true and magnetic azimuthsFor this, a theodolite with an ocular prism is used to observe high stars, as well as special devices for illuminating the grid of filaments inside the tube with reflected light. The theodolite is set in such a way that the northern part of the sky can be clearly seen with a specific object, which determines the true azimuth. The measurement tool must have vertical and horizontal limbs. When installing the theodolite horizontally, the limb is fixed, and the pipe must be directed to the illuminated point of a specific area. Observation of the stars is as follows: a cross of filaments is induced on the starry body, closest to the passage of the meridian, after which you need to make accurate readings on the horizontal and vertical limb. When a stellar body passes through the meridian, several similar observations need to be made. After the stellar body passes through the meridian, the theodolite is set to such a height, which it occupied at the last moment of observation before the culmination of the stellar body. When the star approaches the horizontal thread of the pipe, it is necessary to put a vertical one on it, and then count the values ​​along the horizontal limb. Thus, readings on the horizontal limb are performed each time the pipe took up the position until the star culminated. After that, the countdown of the Meridian is determined according to the arithmetic average of two symmetrical readings of the horizontal limb. From the resulting definitions of the directions of the true meridian, which correspond to the number of positions of the star before passing through the meridian, as well as after it, you need to take the average value and subtract from the reference for the area.

Determining the true azimuth of the Sun

relationship between true and magnetic azimuthsThis method is more accurate and simple: it is necessary to observe the sun at the same height. If you go from a point of a terrain to the highest point occupied by the Sun, you can find the southern direction of the true meridian.

Orientation of directions: true and magnetic azimuths

relationship between true and magnetic azimuthsKnowing the magnetic and true azimuths, you need to know the definition of directions, having a geographic map and a compass. That is, at the same time it is necessary to calculate the relationship between the true and magnetic azimuths and calculate the right direction and further orientation. To do this, translate one value to another and vice versa. So, to translate the true azimuth into magnetic, knowing the magnetic declination, it is necessary first to reduce this declination in case of deviation to the east, and, conversely, to increase in case of deviation to the west in order to arrive at the right place on the map. To translate magnetic into true ones, you need to do the calculations in reverse, i.e. in the western declination, its value is subtracted, and in the east it is added. This method can be used when an object invisible from the point of view that was found and defined on the map is necessary to determine also from the point of view of the magnetic declination when moving along the compass. If, on the contrary, it is necessary to transfer the item found on the ground to the map, you need to determine its location and, accordingly, its magnetic azimuth using a compass. In order not to make mistakes when putting the landmark on the map, you need to determine the true azimuth, and then put this angle on the map.

In addition to the two directions mentioned above, there is a third, called a directional angle. This is an angle, the degree of which can be any degree value from 0 to 360, which is set aside from the vertical grid of coordinates in the direction of the movement clockwise between the north direction and the one that points to the desired object on the ground. Delaying the directional angle allows orientation on a topographic map. To prepare the route of movement in magnetic azimuths on the map, you must:

  • Mark landmarks on topographic map turning points.
  • Calculate the value of the directional angles with the length of each movement in a straight line.
  • Next, you need to translate the value of the directional angles into magnetic azimuths. This is done using a formula in which they are found by subtracting the declination value from the value of the direction angle and adding the meridians convergence indicator. In this case, the convergence of the meridians is determined by the angle between the true meridian and the vertical grid line of the topographic map.
  • The resulting distance must be translated into a couple of steps if the movement is planned to be done on foot. After that, all data is plotted on a topographic map or a detailed route map is drawn up to guide you.

Working with a compass: plotting the correct route

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