The Three Norths

DIRECTION

TRUE NORTH - GEOGRAPHIC NORTH

True North is the direction that points to the North Pole from any location on the surface of the Earth.

The North Pole is the point where the Earth’s axis of rotation intersects the surface of the Earth in the Northern Hemisphere.
True North is a fixed point on the Earth’s surface where lines of longitude converge on maps.

True North is also called Geographic North or Geodetic North.

MAGNETIC NORTH

The Magnetic North Pole is a point on the surface of the Earth where the magnetic field lines point vertically directly into the ground.
The Magnetic North Pole is a NOT a fixed point on the Earth’s surface: it moves over time in response to changes in the Earth’s magnetic core.

MAGNETIC NORTH

A compass needle aligns itself with the horizontal component of the local magnetic field in a direction that is called Magnetic North which is not necessarily the Magnetic North Pole! So what to do?

MAGNETIC DECLINATION - MAGNETIC VARIATION

It is easy to measure and calculate the angle between the Magnetic North and True North: this is called the Magnetic Declination. This angle varies from place to place, changes over time and can be positive or negative.

Magnetic North is east of True North: the Magnetic Declination (D) is positive.

Magnetic North is west of True North: the Magnetic Declination (D) is negative.

The Magnetic Declination is called Magnetic Variation by pilots and mariners.

GRID NORTH

Some type of projection is required to represent the curved 3D surface of an area on Earth onto a 2D map. Typically this is done using a Transverse Mercator projection. The vertical grid lines on the map points to a Grid North. The Grid North for a particular map is chosen so that the difference between Grid North and True North is small (within 2° east or west of True North).

GRID CONVERGENCE

The Transverse Mercator projection has a central meridian line which matches one chosen longitude. Elsewhere the longitude lines curve away from the straight grid lines. The angle between Grid North and True North is called Grid Convergence. The angle between Grid North and Magnetic North is the Grid/Magnetic Angle and it can be positive or negative.

RELATION BETWEEN THE THREE NORTHS

True North is a direction to a fixed point on the surface of the Earth. The Magnetic Declination is the angle between True North and Magnetic North. Convergence is the angle between Grid North and True North.

These relations are encapsulated into the Declination Diagram which is found on topographic maps.

In this diagram, the Magnetic Declination is positive as Magnetic North is to the east of True North.The Grid Convergence is also positive as Grid North is to the east of True North.

From the diagram, you can deduce that the Magnetic Declination is the sum of the Grid/Magnetic Angle and Grid Convergence, which is for the year of 2000, 10.9° + 1.5°= 12.4°.

WHICH NORTH TO USE?

It depends!

Grid North is easy to use on maps that use a grid coordinates such as 1:25000 topographic maps.

Small scale maps may not have grid coordinates. Lines of longitude can be used as True North reference lines.

Magnetic North is the natural choice when using a compass.

IN SUMMARY

True North is the direction that points towards the North Pole.

Magnetic North is the direction the compass needle points to.

Grid North is the direction northwards along the vertical grid lines north on a map.

Magnetic Declination is the angle between True North and Magnetic North. This angle can be positive (east of True North) or negative (west of True North)!

Grid Convergence is the angle between True North and Grid North: this angle can be positive or negative.

The Grid/Magnetic Angle is the angle between Grid North and Magnetic North: this angle can be positive or negative.