After reading this article you will learn about the meaning and variation of magnetic declination, with some examples.
Meaning of Magnetic Declination:
Except in few places, the magnetic meridian at a place does not coincide with the true meridian at that place. The horizontal angle which the magnetic mariotion makes with the true meridian is known as the magnetic declination or simply declination of the needle at that place.
When the needle is deflected towards east of the true meridian, it is said east declination, and west declination when it is deflected towards west of the true meridian. Since the magnetic meridian varies from place to place and from time to time on the earth’s surface, the amount and direction of the declination is different at different places and at different times.
If the true bearing of a line is determined by the astronomical method, the declination at that place can be found by observing the magnetic bearing of that line and finding their difference. The declination can also be obtained approximately from isogonic charts published from time to time through government agencies.
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The lines joining the places of equal declination at the same time are called isogonic lines, and the charts showing these lines are called isogonic charts. The lines joining the places of zero declination are known as agonic lines.
Variation of Magnetic Declination:
The declination at any place is not constant, but is subjected to fluctuations or variations which may be regular or irregular.
1. Regular or Periodic Variations:
This class of variation may itself be analysed into several components of different periods and amplitudes.
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They are:
(i) Secular,
(ii) Annual, and
(iii) Diurnal or Daily.
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(i) Secular Variation:
The magnetic meridian swings like a pendulum. It swings in one direction for a long time (100 to 200 years) and gradually comes to rest and then swings in the opposite direction.
(ii) Annual Variation:
The change produced annually by secular variation amounts in different places from 0 to ± 12 minutes but does not remain constant at any place.
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(iii) Diurnal Variation:
It is an oscillation of’ the needle from its mean position during the day. The amount of this variation varies from 1 minute to about 12 minutes at different places. This is greater in high latitudes than near the equator, and more in summer than in the winter at the same place.
2. Irregular Variations:
These are caused by magnetic storms such as earthquakes or volcanic eruptions and their amount may be even 1° or 2° at a time.
ADVERTISEMENTS:
Of the above variations, secular and diurnal are sufficiently pronounced and be kept in view by the surveyor. When magnetic bearings are taken in a survey, it is always desirable to note on the plan, the date of the survey and the magnetic declination on that date and its annual variation. Any survey line can easily be retraced if magnetic declination when the survey, was made and the present declination are known.
Calculation of True Bearings:
All survey maps which are to form a permanent record such as revenue survey maps are plotted with reference to the true meridian. And if the survey is made with a compass, the observed magnetic bearings shall have to be converted to the true bearings.
The magnetic bearings can be converted to the true bearings by the following rule:
Rule 1:
True bearing of a line = Magnetic bearing of the line ± declination.
Use (+) sign, when the declination is east as in fig. 5.13. (a) and (-) sign when it is west as in fig. 5.13 (b).
On the other hand, the magnetic bearing of a line can be deduced from its bearing by the following rule:
Rule 2:
Magnetic bearing of a line = True bearing of the line ± declination. Use (□-) sign, when the declination is east, and (+) sign then it is west.
Note:
The above rules apply only to the W.C.B.
Examples on Declination:
Example 1:
The magnetic bearing of a line is 197°. Find its true bearing, it the magnetic declination is 3° W.
Solution:
Since the magnetic meridian is deflected towards west of the true meridian, true bearing of the line, = the magnetic bearing – declination = 197° – 3° = 194° (Ans.)
Example 2:
If the magnetic hearing of a line is N 37° W and the magnetic declination is 2° E find the true bearing.
Solution:
(Fig. 5.14) Magnetic, bearing of the line = N 37° W
Magnetic declination = 2° E
True bearing = Magnetic bearing-Declination.
= N (37° – 2°) W = N 35° W (Ans.)
Example 3:
True bearing of a line is 217° and magnetic declination is 2°W. Find the magnetic bearing.
Solution:
Since the magnetic meridian is deflected towards west of the true meridian, magnetic bearing of the line
= True bearing of the line + declination
= 217°+ 2° = 219° (Ans.)