Azimuths and bearings are horizontal angles measured to represent or locate a line with respect to a meridian. Here, the important features of azimuth and bearing in surveying along with their comparison are explained briefly.
What is Azimuth in Surveying?
Azimuths are defined as horizontal angles that are measured from the reference meridian in the clockwise direction. Azimuths are also called a whole circle bearing system(W.C.B). Azimuths are used in compass surveying, plane surveying, where it is generally measured from the north. But azimuths are measured from the south by astronomers and in the military.
The figure-1 above shows the measurement of azimuth from the north direction. It is clear from the figure that every line has an azimuth value ranging from 0 to 360 degrees.
Based on the meridian used, the azimuths can be either geodetic, astronomic, assumed, record or magnetic is nature. It is always recommended to state the reference meridian before starting the surveying work so that later confusions can be avoided.
The forward direction of the line is given by the forward azimuth, while the reverse direction of the line is given by the backward azimuth. The forward azimuth is converted into back azimuth by adding or subtracting 180 degrees.
If the line AB has an azimuth of ?1= 70 degrees which is the forward azimuth then the backward azimuth is 70+180=250 degrees. If the azimuth of the line AD is 230 degrees, then backward azimuth is 230-180=50 degrees (From Figure-1).
Azimuth is used in boundary, control survey, topographic survey and other kinds of surveys.
What is Bearing in Surveying?
Bearing is defined as the acute angle that is measured between the reference meridian and the given line. The line is measured either from the north or south towards east or west that would give an angle less than 360 degrees. The angle is represented by N or S first followed by angle value and E or W direction, For example, N60E.
From the figure-2, the bearing of line AB is ? which is less than 90 degrees as it lies in first quadrant NE.
A magnetic bearing is measured from the local magnetic meridian, grid bearing from an appropriate grid meridian, assumed bearings from an appropriate arbitrary meridian, geodetic bearing from a geodetic meridian and an astronomic bearing from an astronomic meridian. The magnetic meridian is obtained by observing the needle of the compass.
Difference between Azimuths and Bearings in Surveying
|1||The value varies from 0 to 3600||The value varies from 0 to 90 degrees|
|2||Represented by a numerical value||Represented by two letters and the numerical value|
|3||The angles are measured clockwise only||The angles are measured both clockwise and anticlockwise|
|4||A single set of measurement is taken either from North or South||The angles can be measured either from north or south for a single survey|
Computation of Azimuths and Bearing
Given the quadrant in which the line lies and provided the value of azimuth or bearing can help determine the other value.
|Quadrant||Bearing to Azimuth Conversion|
|I (NE)||Bearing = Azimuth|
|II(SE)||Bearing = 1800 – Azimuth|
|III(SW)||Bearing = Azimuth – 1800|
|IV (NW)||Bearing = 360 – Azimuth|