Magnetic declination, also called magnetic variation, is defined as the angle between compass north and true north at a point on the Earth. Compass north is the direction shown on the north end of a compass needle while true north is the actual direction on the Earth’s surface pointing toward the geographic North Pole. Magnetic declination changes based on one’s location on the globe and as a result it is very important to surveyors, map makers, navigators and anyone using a compass to find their direction such as hikers. Without adjusting for magnetic declination work done by surveyors could turn out wrong and people like hikers using a compass could easily get lost.
Earth’s Magnetic Field
Before learning about the essentials of magnetic declination it is important to first learn about the Earth’s magnetic field. The Earth is surrounded by a magnetic field that changes in time and location. According to the National Geophysical Data Center this field resembles the magnetic field generated by a dipole magnet (one that is straight with a north and south pole) that is located at the center of the Earth. In the case of the Earth’s magnetic field the axis of the dipole is offset from that of the Earth’s rotation by around 11 degrees.
Because the Earth’s magnetic axis is offset the geographic north and south poles and the magnetic north and south poles are not the same and the difference between these two is magnetic declination.
Magnetic Declination Around the World
The Earth’s magnetic field is very irregular and it changes with location and time. This irregularity is caused by variations and movement of material inside the Earth’s interior that occur over long periods. The Earth is made up of different types of rock and molten rock that have different magnetic properties and as they move around inside the Earth, so too does the magnetic field.
According to the Wisconsin State Cartographer’s Office, variation inside the Earth “causes a ‘drift’ of magnetic north and oscillations of the magnetic meridian.” The normal change of magnetic declination is called annual change and it is very difficult to predict over long periods.
Finding and Calculating Magnetic Declination
The only way to predict changes in magnetic declination is to take various measurements in many locations. This is typically done via satellite and maps are then created for reference. Most maps of magnetic declination (North American Magnetic Declination map and global map (PDF)) are made with isolines (lines representing points of equal value) and they have one line along which magnetic declination is zero. As one moves away from the zero line there are lines showing negative declination and positive declination. Positive declination is added to orient a compass with a map, while negative declination is subtracted. Most topographic maps also state the magnetic declination for the areas they show in their legend (at the time the map was published).
In addition to using a map to find magnetic declination, NOAA’s National Geophysical Data Center operates a website that allows users to calculate an estimate of an area’s declination via latitude and longitude on a specific date. For example, San Francisco, California, which has a latitude of 37.775̊°N and a longitude of 122.4183̊°W, had an estimated magnetic declination of 13.96̊°W on July 27, 2013. NOAA’s calculator also estimates that this value is changing by about 0.1̊°W per year.
When stating magnetic declination it is important to pay attention to whether the calculated declination is positive or negative. A positive declination shows an angle that is clockwise from true north and a negative is counter clockwise.
Using Magnetic Declination and a Compass
An easy and often inexpensive tool to use for navigation is a compass. Compasses operate by having a small magnetized needle that is placed on a pivot so that it can rotate. The Earth’s magnetic field places a force on the needle, causing it to move. The compass needle will rotate until it aligns itself with the Earth’s magnetic field. In some areas this alignment is the same as true north but in others magnetic declination causes the alignment to be off and the compass must be adjusted to avoid getting lost.
In order to adjust for magnetic declination with a map one must find the isoline corresponding with their location or look to the map’s legend for a statement of the declination. Magnetic declination calculators such as the one from NOAA’s National Geophysical Data Center can also provide this value. Positive declination is then added to orient a compass with a map, while negative declination is subtracted.