True azimuth and magnetic azimuth: what is the difference?

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. Therefore, orientation on the ground should be made taking into account their difference. This is necessary in order not to encounter the difficulty when access to the intended point on the map of the area is indicated by different compass readings and azimuth measurements on the map.

Difference between two azimuths

Difference between two azimuths

The azimuth denotes the angle between two directions: the north and the one where the necessary point is located on a particular area. In this case, the direction to the North Pole is considered northern. But at the same time, the compass northern direction is, in fact, an indication of the magnetic pole, and not the geographical position. Thus, speaking of the true pole, we have in mind the geographical one associated with the shape of the Earth, as well as its axis of rotation passing through two poles - the north and south. The definition of the magnetic pole is connected with the existence of a magnetic field around the Earth, which has two poles that do not coincide with the true earthly ones, as well as the corresponding true and magnetic meridians.

If between these meridians we postpone a certain direction relative to the subject of the area, then the angle formed between each of these meridians will be the true and magnetic azimuth, respectively. The difference between the two azimuths is magnetic declination. Depending on the side of the world to which the compass needle is leaning from the true meridian, the declination is accordingly called: east when deviating to the east, west when deviating to the west. In this case, the declination to the east is indicated by a plus sign, and to the west by a minus sign. Depending on the terrain, 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, you need to multiply its annual change by the number of years that have passed since the map was compiled, adding to it the value indicated at the time of compilation.

The relationship between true and magnetic azimuths

To determine the magnetic meridian, a compass is used.

To determine the magnetic meridian, a compass is used. This is a special round box, with a ring inside it with divisions indicating degrees every 10 values. The divisions with degrees from 0 to 360 in counterclockwise direction count the azimuths, and the divisions from 0 to 90 of each quarter of the round field count the so-called rumbas. The box is closed by glass, and on the spire in the middle of it an arrow freely rotates, showing the magnetic meridian with its direction. Knowing the rumba, you can calculate the azimuths and vice versa. Thus, a connection is formed between true and magnetic azimuths.

Determination of true and magnetic azimuth

Determination of true and magnetic azimuth

To know the azimuths, you must first determine the direction of the meridians. When it is found, you need to measure the angle between the direction of the meridian and the one that indicates the desired point in 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 spire. A device for determining the magnetic azimuth and direction of the magnetic meridian using the magnetic arrow is called the 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 the declination of the arrow at a specific point in the terrain are known. A more accurate way to determine the true meridian is to observe the celestial bodies. This is done using special goniometer tools placed horizontally.

Determination of the geographic meridian with azimuth by the corresponding stellar heights

Use an ocular prism theodolite to observe tall stars

To do this, a theodolite with an ocular prism is used to observe tall stars, as well as special devices for illuminating the net of threads inside the pipe with reflected light. Theodolite is set in such a way that the northern part of the sky with a specific object, which determines the true azimuth, is clearly visible. The instrument must have vertical and horizontal limbs. When installing the theodolite strictly horizontally, a limb is fixed, and the pipe must be directed to the illuminated point of a particular area. Observation of the stars is as follows: a cross of threads is induced on the stellar body closest to the passage of the meridian, after which it is necessary to make accurate readings on the horizontal and vertical limbs. When a stellar body passes through the meridian, several similar observations must be made. After the stellar body passes through the meridian, theodolite is set to such a height that it occupied at the last moment of observation until the culmination of the stellar body. When the star approaches the horizontal thread of the pipe, you need to point it vertically at it, and then count the values ‚Äč‚Äčalong the horizontal limb. Thus, readings on the horizontal limb are made every time the tube occupied the position until the culmination of the star. After that, the meridian count is determined according to the arithmetic mean value of two symmetric samples of the horizontal limb. From the resulting definitions of the true meridian directions, which correspond to the number of star positions before passing through the meridian, as well as after it, you need to take the average value and subtract it from the reference for the terrain.

Determining true azimuth from the sun

Determining true azimuth from the sun

This method is more accurate and simple: it is necessary to observe the sun at the same heights. If you go from the point of 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

Orientation of directions: true and magnetic azimuths

Knowing the magnetic and true azimuths, you need to know the definition of directions, having a geographical map and a compass. That is, in this case, it is necessary to calculate the relationship between the true and magnetic azimuths and calculate the right direction and further orientation. To do this, you need to translate one value into another and vice versa. So, in order to translate the true azimuth into magnetic, knowing the magnetic declination, it is necessary to reduce the first by the magnitude of this declination in the case of a deviation to the east, and, conversely, to increase in the case of a deviation to the west, in order to eventually arrive at the right place on the map. To convert magnetic to true, you need to do the calculations the other way around, i.e. in the western declination, its value is taken away, and in the east - it is added. This method can be used when you need an object invisible from the point of view, found and determined from the map, can also be determined from the point of view of magnetic declination when moving around the compass. If it is necessary, on the contrary, to transfer the object found on the ground to the map, you need to determine its location by the compass, respectively, and its magnetic azimuth. In order not to make mistakes when postponing a landmark to the map, you need to determine the true azimuth, and then postpone this angle on the map.

In addition to the above two directions, there is a third, called the directional angle. This is an angle whose degree value can be any degree value from 0 to 360, which is plotted from the vertical coordinate grid in the direction of clockwise movement between the north direction and the one that indicates the desired object on the ground. Postponing 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 the turning points of the topographic map.
  • Calculate the directional angle with the length of each section of movement in a straight line.
  • Next, you need to translate the value of the directional angles in magnetic azimuths. This is carried out using a formula in which they are found by subtracting the declination from the direction angle and adding the meridian convergence index. The approach 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 converted into pairs of steps if movement is planned to be carried out on foot. After that, all data is applied to a topographic map or a detailed route diagram is drawn up in order to carry out orientation.

Working with a compass: laying the right route

Watch the video: True Bearing and Magnetic Bearing. Surveying (April 2020).