DEFINITIONS OF COMPASS SURVEYING
1. Bearing:
The bearing of a line is the horizontal angle, established with reference to a some fixed line, called Meridian.
2. Meridian:
Meridian is a fixed direction on the surface of the earth. The bearings of the survey lines are measured with reference to the meridians. The following are the different types of meridians used under different conditions.
3. True Meridian:
It is a line or plane passing through the true north pole and south pole, any point on the surface of the earth . It is also known as Geographical meridian. The direction of it through a point can be established by astronomical observations.
4. True Bearing:
The horizontal angle made by a survey line with reference to the true meridian is called true bearing of a line . It is always remains constant.
5. Magnetic Meridian:
It is the direction established by a freely suspended magnetic needle, uneffected by magnetic substances. The direction of it through a point can be established by a compass .
6. Magnetic Bearing:
The horizontal angle made by a survey line with reference to magnetic meridian is called magnetic bearing of a line . It changes with place and time.
7.Arbitrary Meridian:
Some times for the survey of a small area, a convenient direction is assumed as a meridian, known as ‘Arbitrary meridian’. The direction of it is established by a line joining the survey station to some prominent objects such as chimney, Church spire etc.
8. Arbitrary Bearing:
The horizontal angle made by a survey line with reference to the arbitrary meridian is called arbitrary bearing of a line.
9. Dip of the Magnetic Needle:
It a needle is perfectly balanced before magnetisation, it does not remain in the balanced position after it is magnetised. This is due to the magnetic influence of the earth. The needle is found to be inclined towards the pole. This inclination of the needle with the horizontal is called as the ‘dip of the magnetic needle’
10. Theory of Magnetism:
A popular theory of magnetism considers the molecular alignment of the material. This is known as ‘Weber’s theory’. This theory states all the magnetic substances are composed of tiny molecular magnets. Any un magnetized material has the magnetic forces of its molecular magnets neutralized by the adjacent molecular magnets, there by eliminating any magnetic effected. A magnetize material will have most of the molecular magnets lined up. so that the north pole of each molecule points are to the one direction, and the south pole faces to the opposite direction. A material with its molecules thus aligned will then have one effective north pole, and one effective south pole.
As per Weber’s theory, when a steel bar is stroked several times in the same direction by a magnet, the magnetic force from the north pole of the magnet causes the molecules to align themselves.
Magnetic Fields: The space surrounded at a magnet where magnetic forces act is called as the magnetic field. The field affect neighbouring objects along things called magnetic field lines.
Properties of Magnets:
A magnet has three properties they are
1. Directional Property: A freely suspended bar magnet always align itself to geographical north, south direction of earths magnetic field.
2. Attractive Property: A magnet can attract, magnetic substances like iron, cobalt, nickel, etc.
3. Inductive Property: When an unmagnetised substance placed near to a magnet, then the opposite pole is induced in it. Magnet is a dipole, both the poles are equal in strength.