North Reference

Which Way Is Up?

Navigating with a traditional compass and paper map requires the compass be adjusted to the magnetic declination diagram for the map being used. An incorrectly adjusted compass is useless for anything beyond a general sense for where north is and may be worse than no compass at all. When using a GPSr in combination with a magnetic compass and paper map, the GPSr north reference should also be adjusted. The choice of adjusting the north reference or not will affect how you work with your compass and should be considered carefully.

True North

The True North Pole is the axis of the earth’s rotation. The North Star is used as a true north reference. It’s position in the sky causes it to appear almost stationary with the other stars rotating around it. Lines or meridians of longitude can also be used as true north reference lines. Meridians of longitude converge at the True North and South Poles. The vertical edges of many maps are defined by a meridian of longitude, and can be used as a true north reference.

Magnetic North

The magnetic poles are aligned with the earth’s magnetic field. A free floating magnetic needle in a compass will align itself with the magnetic field and thus points to the magnetic poles. Declination is the angular difference between true north and magnetic north for a given location. The Magnetic Pole may appear to be either east or west of the True North Pole. Declination changes depending on your position relative to the two poles. Declination also changes over time, because the location of the magnetic poles changes with time. Most GPS receivers will calculate the declination for their current position. This is often referred to as “automatic” north reference.

Grid North

Grid north is useful because it allows you to use the UTM grid lines on your map as your north reference. Grid north is typically within 2° east or west of true north and varies with your position within a UTM zone. When minimal accuracy is all that is required, it is common to treat grid north lines as true north lines without accounting for the small difference.