Declination and Angle of Dip in Surveying

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. The magnetic meridian takes the direction of magnetic poles. 

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Magnetic poles are not points. They are oval areas located about 2000 kms away from the actual location of the geographic poles of the earth. These pole direction are shown by the needle of a magnetic compass which is used to represent magnetic meridian. Hence, it is obvious that the magnetic and true meridian will not be parallel to each other. 

They are only few locations on the surface that would coincide at some point ( as they are not parallel, they will coincide).

Hence, magnetic declination is the horizontal angle and direction by which the needle of the compass deflects from the true meridian at any particular locality. This deflection of needle can be eastward or westward of the true meridian as shown in figure-1 below.

Dip and Declination Eastwards and Westwards

In the figure TN represent true north of true meridian and MN represent magnetic north. If the magnetic declination is on the left side of true meridian , the angle formed between the two meridians is declination west (-ve) and that formed on right is called as declination east (+ve).

1. Declination is used to determine the true bearing
2. True bearing = Compass bearing + declination
3. If the declination is declination east, then it is added ( compass bearing + declination)
4. If the declination is declination west, then it is substracted ( compass bearing - declination)

Isogonic Chart

A chart or a map that shows lines connecting points where the magnetic declination of the compass needle is the same at a given point of time is called as Isogonic Chart or isogons. A line passing through points of zero declination are called as agonic lines. The chart helps to predict the magnetic declination at any place on a particular date.

Variations in Magnetic Declination

The changes happening to magnetic meridian at a given place are not constant. They can be categorized as daily, annual, secular and irregular.

Angle of Dip

Angle dip gives the direction of earth' magnetic field. Earth is like a large magnet with large magnetic field around it. A dip meter is a vertically mounted compass that when placed in various location of the earth will give various angles of earth's magnetic field with the vertical plane. This angle is called as angle of dip. Its value is 0° at equator and 90° at magnetic poles. 

Angle of Dip

The imaginary lines joining the points on the earth's surface with equal dip are called as isoclinic lines.

Also Read

Bearings in Surveying - Meridian & Types of Bearings