Introduction

When working in the field with a map, GPS and compass, there are usually three different norths to be considered: Grid North, True North and Magnetic North.

Grid North is the direction of a grid line which is parallel to the central meridian on the National Grid, i.e. the north to which grid lines on a map point (the top of the map).

True North is the direction of a line of longitude which converges on the North Pole.

Magnetic North is the direction indicated by a magnetic compass. Magnetic North moves slowly with a variable rate and currently is west of Grid North in Great Britain.

Declination is the horizontal angular difference between True North and Magnetic North. This is also known as the Magnetic Variation. Magnetic declination will vary from place to place depending on the intensity of the Earth’s magnetic field and can be positive (east) or negative (west). The horizontal angular difference between Grid North and Magnetic North is called Grid Magnetic Angle.

For deployment of seismic equipment, you should take the following steps outlined in this FAQ section:

1. Find true north with a compass, find true north with a GPS, or find true north indoors.
2. Transfer true north to a sensor pit
3. Align the sensor with true north