Bearings in Surveying - Meridian & Types of Bearings

Bearings are direction of a line measured in terms of horizontal angle from a reference line called as meridian. In this article, the details on meridian and types of bearings are measured in detail.

What is a Meridian in Surveying?

Meridian is defined as a fixed line of reference that can be any line in the survey or an imaginary line. Meridians used in surveying can either be:

  1. Arbitrary Meridian/ Assumed Meridian
  2. True Meridian/Astronomic Meridian
  3. Grid Meridian
  4. Magnetic meridian

1. Arbitrary Meridian or Assumed Meridian

A meridian that is arbitrarily chosen is called as arbitrary or assumed meridian. This meridian cannot be reestablished, if the original points are lost.
The first line of a traverse is taken as arbitrary meridian in general situation. It helps to determine the relative direction of the traverse lines.

2. True Meridian/Astronomic Meridian

If the meridian is the north-south line passing through the geographical poles of the earth, then it is called as True Meridian/Astronomic Meridian. This meridian is used for astronomical observations.
The directions measured with respect to true meridian does not change as true meridian is a fixed direction.
For most surveys, true meridian is used, especially for demarcating property lines.
The convergence of true meridians at several locations is a disadvantages that causes errors and requires correction.

3. Grid Meridian

If a meridian lies parallel to the central true meridian for a specified area that is covered by a plane coordinate system, it is called as grid meridian. 
For example, in order to survey a large area of a state, the true meridian of the central location of the country is taken as reference meridian. This reference meridian is called as grid meridian.

4. Magnetic Meridian

If the meridian reference line lies parallel to the magnetic lines of force (magnetic poles) of the earth, i.e. passing through magnetic north and magnetic south of earth is called as as magnetic meridian. A magnetic meridian can be identified using a freely-suspended magnetic needle that is influenced by earth's magnetic needle. Hence, for each location, magnetic poles created changes. Hence a magnetic meridian is not a fixed direction.
All the magnetic meridians will converge to a central point, called as magnetic pole. Used for rough surveys and to check surveys.

The magnetic meridian and true meridian won't coincide with each other in place. The horizontal angle between these two meridians is known as magnetic declination. 

Read More On: Declination and Dip, Variations and Isogonic Chart

Types of Bearings

Bearing Based on Meridian

Bearings as mentioned before, is a system of designating the direction of lines. The bearings of lines can be measured with respect to any meridian. Based on the type of meridian that is employed, it can be:
  1. True Bearing
  2. Magnetic Bearing
  3. Grid Bearing
  4. Arbitrary Bearing

Bearings Based on Measuring System

Bearings are usually measured clockwise or anticlockwise direction from the reference meridian. This gives two measuring system namely quadrantal bearing system (QB) and whole-circle bearing system (WCB).

1. Reduced Bearing or Q.B

During the computation of bearings, it is always convenient to measure reduced bearing. Reduce bearing is an acute angle measured between the reference meridian and the line. Reduced bearing is measured either from  north or south towards east or west, whichever gives acute angle. This hence can be measured either in clockwise or anticlockwise direction.

The reduced bearing is represented by the quadrant in which the line is present by letter N or S followed by the bearing value and East or West following it. For example, a line OA making an angle α in the first quadrant (NE) is given as N α E. This designation is called as quadrantal system of bearings.

Reduced Bearing

2. Azimuths or WCB

When the bearing is measured in clockwise direction from any reference meridian, it is whole circle bearing or azimuth. This system is called as WCB system.
In plane surveying, the azimuths are measured from North. It is measured from south for astronomical and geodetic surveying. 
In WCB system, it is hence necessary to mention whether an azimuth is measured from north or south.
Based on the type of meridian chosen, azimuth are called magnetic, or grid, or true.

Generally, the term bearing is used in plane surveying and azimuth is used in astronomical surveying.

Back Bearings and Fore Bearing in Surveying

A bearing of a line AB measured from A to B is (α)  called as fore bearing or forward bearing as shown in the figure below. If the bearing is measured in reverse direction from B to A it is called as backward bearing or back bearing (α ')

Back Bearings and Fore Bearing in Surveying
Back Bearings and Fore Bearing in Surveying

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