The compass needle always points toward magnetic north; however, topographic maps are drawn in reference to true north (North Pole). The difference between magnetic north and true north is called the angle of magnetic declination, or simply, the declination. Therefore, when using a map and compass together, an adjustment has to be made for declination.
Magnetic declination not only changes with geographic location, but also changes slightly over time. In the contiguous U.S., the magnetic declination generally varies between zero and twenty degrees (as shown in the image below). Only along the zero declination line are true north and magnetic north the same, and therefore, no adjustment has to be made for declination (this is the dark, heavy line labeled as 0° in the image below).
When someone is using their compass in a location that is east of the zero declination line (for example Maine), the needle will point in a direction that is west of true north – this is referred to as westerly declination. When someone is using a compass in a location that is west of the zero declination line (for example Nevada), the needle will point in a direction that is east of true north – this is referred to as easterly declination.
Magnetic Declination on Topographic Maps
Magnetic declination is printed in the lower left hand corner on USGS topographic maps in the arrow diagram or in the information block. Since declination does slightly change over time, topographic maps of the same area can have different declinations if the maps were published on different dates. There are web sites that will provide declination when a zip code is entered.
You can also look up the magnetic declination on the National Oceanic and Atmospheric Administration (NOAA) National Geophysical Data Center website by Latitude and Longitude or by city and zip code.
Adjusting a Compass
Making an adjustment for declination is essential when using a map and compass together. Either the map or the compass needs to be adjusted for declination. It is generally easier to adjust the compass for declination, rather than the map. When the compass is adjusted then geographic north is the reference point for both the compass and the map.
The process for adjusting declination on a compass depends upon the type of compass; therefore, refer to the owner’s manual for specific instructions. Some compasses have a declination screw that can be turned which rotates the orienting arrow. Others require the rotation of the center of the case so the orienting arrow is offset by declination. Some compasses are automatically adjusted. It is important to be accurate when setting declination because 1 degree off can result in 920 feet off course in 10 miles.
**Information adapted from the National Interagency
Incident Management System Basic Land Navigation
Manual, PMS 475 dated June 2007.