Bearings and Angles in Compass Surveying

Bearings and Angles:

The direction of a survey line can either be established

  1. with relation to each other, or
  2. With relation to any meridian.

The first will give the angle between two lines while the second will give the bearing of the line.

Bearing:

Bearing of a line is its direction relative to a given meridian. A meridian is any direction such as

  1. True Meridian
  2. Magnetic Meridian
  3. Arbitrary Meridian

1) True Meridian:

True meridian through a point is the line in which a plane, passing that point and the north and south poles, intersects with surface of the earth. It thus, passes through the true north and south. The direction of true meridian through a point can be established by astronomical observations.

True Bearing:

True bearing of a line is the horizontal angle which it makes with the true meridian through one of the extremities of the line. Since the direction of the true meridian through a point remains fixed, the true bearing of a line is a constant quantity.

2) Magnetic Meridian:

Magnetic meridian through a point is the direction shown by a freely floating and balanced magnetic needle free from all other attractive forces. The direction of magnetic meridian can be established with the help of a magnetic compass.

Magnetic Bearing:

Magnetic bearing of a line is the horizontal angle which it makes with the magnetic meridian passing through one of the extremities of the line. A magnetic compass is used to measure it.

3) Arbitrary Meridian:

Arbitrary meridian is any convenient direction towards a permanent and prominent mark or signal, such as a church spire or top of a chimney. Such meridians are used to determine the relative position of line in a small area.

Arbitrary Bearing:

Arbitrary bearing of a line is the horizontal angle which it makes with the any arbitrary meridian passing through one of the extremities of the line. A theodolite or sextant is used to measure it.

Traverse Survey:

Traversing is that type of survey in which a number of connected survey lines form the framework and the directions and lengths of the survey line are measured with the help of an angle (or direction)measuring instrument and a tape(or chain)respectively. When the lines form a circuit which ends at the starting point, it is known as a closed traverse. If the circuit ends elsewhere, it is said to be an open traverse.

Fore Bearing and Back Bearing:

The bearing of line whether express in WCB/QB system differs accordingly as the observation are made from one end of the line and the other.

FB & BB

If the bearing of a line AB in fig is measured from A towards B ,it is known as forward bearing/fore bearing is observed as F.S.If the bearing at AB in fig is measured from B towards A is known as Backward / Back bearing (BB).

Units of angle measurements:

An angle is the difference indirection of two intersecting lines. There are three popular systems of angular measurements.

a) Sexagesimal System: 1 circumference =360 º(degrees of arc)

1 degree =60’ (minutes of arc)

1 minute =60” (second of arc)

b) Centesimal System: 1 circumference =400g(grads)

1 grad =100 c (centigrads)

1 centigrad =100 cc (centicentigrads)

c) Hours System: 1 circumference =24h(hours of time)

1 grad =60 m (minutes of time)

1 centigrad =60 s (seconds of time)

The sexagesimal system is widely used in US, Great Britain, India and other parts of the world. Most of the surveying instruments are graduated according to this system. However, due to facility in computation and interpolation, the centesimal system is gaining more favour in Europe. The hours system is mostly used in astronomy and navigation.

 

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