Magnetic Elements of a Place

What are the Magnetic Elements of a Place?

The magnetic elements of a place are the quantities that define the Earth’s magnetic field, and these are the angle of declination, the angle of dip and the horizontal component of the Earth’s magnetic field.

When a magnet is freely suspended, the magnetic poles of the earth attract the opposite poles of the magnet and the magnet comes to rest with its axis pointing in the North-South direction since it is under the influence of the Earth’s magnetic field.

Angle of Declination or Variation (δ):

The vertical plane through the magnetic axis of a magnet is called Magnetic Meridian.

The Geographic Meridian is the vertical plane passing through the Geographic North and South Poles of the Earth.

The Angle of Declination is the angle between the magnetic and the geographic meridian, it varies from place to place.

• Places on the Earth’s surface with the same angle of declination are joined by a line called an isogonal line. 
• Magnetic and Geographic Meridian coincide at a place where the angle of declination is zero (0°).
• The isogonal of places with zero declination is called an agonic line.
• Declination varies by location and time and is positive when it’s East of True North and Negative when it’s West.

Application of Declination or Variation (δ):

It is important to know the area’s angle of declination or magnetic declination because it helps you accurately interpret the compass readings and navigate correctly.

Sailors and mariners use the angle of declination to determine true north which is the direction indicated on a map. Because compasses point to magnetic north, the angle of declination is needed to navigate using a compass. 

How to Use:

To use a compass, you can add the declination value to the compass reading to determine true north. Declination values that are east of true north are positive, while those that are west are negative. 

True Bearing = Magnetic Bearing \( \scriptsize \pm \) Angle of declination

• Use positive when declination is East
• Use positive when declination is West

Example 2.4.1:

If the angle of declination in a place is 10°E, calculate the true geographic bearing if the compass reads N40°E

Solution

True Bearing = Magnetic Bearing \( \scriptsize \pm \) Angle of declination

Angle of declination is East so we use the positive sign

True Bearing = 40° + 10° = N50E°

Example 2.4.2:

The True bearing of a place is N34°E and the magnetic declination is 2°W. The magnetic bearing is:

Solution

True Bearing = Magnetic Bearing \( \scriptsize \pm \) Angle of declination

Angle of declination is West so we use the negative sign

34° = Magnetic bearing – 2°

Magnetic bearing = 34° + 2° = N36°E

Angle of Dip or Inclination:

This is the angle between the direction of the Earth’s resultant magnetic field and the horizontal.

Horizontal and Vertical Components of the Earth’s Magnetic Field:

The Earth’s magnetic field can be resolved into two components, Horizontal and Vertical Components since it is a vector quantityVector quantities are quantities with magnitude and direction. Examples of vector quantities include displacement, velocity, position, force, and torque. More.

The Horizontal Component (H) of the earth field is the magnetic force acting in a horizontal direction.

The Vertical Component (V) of the earth field at a point is the magnetic force acting in a vertical direction.

∴ If the angle of dip at a place is Φ then:

tan Φ = \( \frac{V}{H}\)