There are three kinds of the north. They are True north, Grid north, and Magnetic north. The American practical navigator, Nathaniel Bowditch defines the magnetic declination as the angle between the magnetic and geographical meridians at any place which we express in degrees and minutes east or west to specify the direction of magnetic north from true north.
Introduction to Magnetic Declination
Magnetic declination is the angle between magnetic north and true north on the horizontal plane. It is not continuous and keeps changing as per the position on the surface and time of the earth.
We use the Greek letter δ as the symbol for magnetic declination and we also refer to is as a magnetic variation. Further, when the magnetic north is east of true north, the declination is positive and when the magnetic north is west of true north, the declination is negative.
The other terms which we use are isogonic lines. This is when the lines along the declination are constant. Next, we have agonic lines which mean the lines along the declination are zero.
True north is the direction along the surface of the earth towards the true North Pole or the geographical North Pole. We also call it the geodetic north and is different from the magnetic north which is the direction pointed by the compass and from the grid north which is in the direction along the grid lines towards the north.
Grid north is the direction which is in northwards along the grid lines on a map projection. We use this term for navigation and the deviation of the grid north from the true north is very less.
Magnetic north is the direction which the compass needle points in response to the magnetic field of the earth. The deviation between the true north and the magnetic north differs from place to place as the magnetic poles of the earth are not fixed with respect to its axis.
Difference between Magnetic North and True North
The difference between magnetic north and true north is that magnetic north is the north direction which the compass needle points that is along the magnetic field of the earth. On the other hand, true north is the geographical north which points towards the North Pole.
How to Calculate Magnetic Declination?
We can use the following ways to calculate the magnetic declination:
It is an easy way of calculating the declination of any location on the earth. By giving the year, latitude, and longitude of a particular location, the calculator provides the declination on the basis of magnetic reference field models.
Magnetic Declination Chart
A magnetic declination chart is a map which has the magnetic fields of the earth available on it.
There are three kinds of bearing which are true, magnetic, and compass bearing. We use a compass for calculating the declination as it is one of the errors of the compass and the other is magnetic variation. These three are related by:
T = M + V
M = C + D
T = C + V + D (it is a general equation relating compass and true bearings)
C = compass bearing
M = magnetic bearing
T = true bearing
V = variation
D = compass deviation
V < 0, D < 0 for westerly variation and deviation
V > 0, D > 0 for easterly variation and deviation
To calculate compass bearing from true bearing, we use:
True bearing – variation = magnetic bearing
Magnetic bearing – deviation = compass bearing
To calculate true bearing from compass bearing:
Compass bearing + deviation = magnetic bearing
Magnetic bearing + variation = true bearing
Thus, always remember when checking for north direction using a compass, the needle is actually pointing towards the magnetic north of the earth and not the true north.
FAQ on Magnetic Declination
Question 1: What Is Grid North?
Answer 1: Grid north is referred to as the direction which is in northwards along the grid lines on a map projection. We use this term for navigation and the deviation of the grid north from the true north is very less.
Question 2: What Is Magnetic Dip?
Answer 2: The magnetic dip is the angle made with the horizontal by the magnetic field lines of the earth. We also refer to it as the dip angle or magnetic inclination. Georg Hartmann discovered it in the year 1544.