Grid North, Magnetic North and True North

There is no single north. There are three, they point in three different directions from wherever you are standing, and a bearing is meaningless until you say which one it was measured from.

Confusing them is not a rounding error. Depending on where you are, the gap between grid north and magnetic north can exceed 20°, which is about 350 m of lateral error over a kilometre of movement.

The three norths

True north

Toward the geographic North Pole, along a meridian

Grid north

Straight up the vertical grid lines on the map

Magnetic north

Where the compass needle actually points

True north

The direction of the Earth's axis of rotation. Every line of longitude runs to it. It is fixed, it is what latitude and longitude are referenced to, and it is the reference for astronomical navigation. Symbol on a map sheet: a star.

Grid north

The direction of the vertical grid lines printed on the map. Because a map is a flat projection of a curved surface, those lines are parallel to each other but not to the meridians. They are parallel to the central meridian of the UTM zone, so grid north equals true north only along that one line.

Anywhere else in the zone, the two differ by the grid convergence, which grows as you move away from the central meridian and as you move toward the poles. Roughly:

convergence ≈ (your longitude - central meridian) × sin(latitude)

At the edge of a zone, 3° from the central meridian, that is about 2.1° at 45° latitude and about 2.6° at 60°. In the tropics it is well under a degree. It is the smallest of the three corrections and the one people most often ignore, which is fine at low latitudes and not fine in Alaska or Scandinavia.

Symbol on a map sheet: the letters GN.

Magnetic north

Where a compass needle settles, following the Earth's magnetic field lines. This is not a fixed geographic point and it is not the magnetic pole in any simple sense: the needle follows the local field, which is shaped by the whole planet's core and crust.

Critically, it moves. The magnetic north pole has migrated hundreds of kilometres across the Arctic in recent decades, at times faster than 50 km per year, which is why declination values carry a date and a rate of annual change. Symbol on a map sheet: a half arrowhead.

The declination diagram

Every topographic sheet carries a small diagram in the margin showing the three norths and the angles between them, usually near the scale bar. It gives you the two numbers that matter:

• The G-M angle, between grid north and magnetic north. This is the one you use for converting compass bearings to map bearings.
• The grid convergence, between grid north and true north.

Converting between them

The rule is easier to hold onto as a picture than as a mnemonic. Draw the two norths you are converting between, look at which side the target is on, and adjust in that direction.

Written out, for the true-to-magnetic conversion:

The traditional phrasing is "east is least, west is best", meaning east declination is subtracted and west declination is added, when going from true to magnetic. Reverse it going the other way.

A worked example. You are somewhere with 12° east declination, and the map gives you a true azimuth of 085° to your objective:

Magnetic azimuth = 085° - 12° = 073°

So you set 073° on the compass and walk it. Set 085° instead and you are 12° off, which puts you roughly 210 m to the side after a kilometre.

Going the other way, you shoot a magnetic bearing of 200° to a landmark and want to plot it:

True azimuth = 200° + 12° = 212°

For grid rather than true, use the G-M angle from the declination diagram in place of the declination, and the same add/subtract logic applies.

Which north do your instruments give you?

This is where it gets practical, because the answer is not consistent.

• A magnetic compass gives magnetic. Always. That is the only thing it can measure.
• A paper map gives grid, if you are measuring against the grid lines, or true if you are measuring against the graticule ticks in the margin.
• A GPS receiver computes bearings from position data, so it natively works in true north. Most units offer a setting to display magnetic instead, applying a built-in model.
• A phone compass app reads the magnetometer, which is magnetic, but most apps apply a declination model and display true north by default. Some let you choose. Many do not say which they are showing.

Practical discipline

1. Label every bearing with its reference. "085 true" or "073 magnetic". An unlabelled azimuth is an invitation to a navigation error.
2. Convert once, at a known point, rather than repeatedly in the field. Every conversion is a chance to add instead of subtract.
3. Work in one reference for a whole leg. Mixing grid bearings off the map with magnetic bearings off the compass within the same route is how people end up applying the correction twice, or not at all.
4. Sanity check the size of the error. One degree of error is about 17 m per kilometre. If you are 100 m off after a kilometre, you are about 6° out, which smells like an uncorrected or double-corrected declination.
5. Update old map declinations using the annual change rate on the diagram.

The declination value itself, why it varies so much by location, and how to find a current figure are covered in what is magnetic declination.