The Compass and Declination

There are many different types of compasses and I’m not going to pretend to show you all of them. My goal is to show you the basic compass types and how to best use them.

Lensatic Compass

The two most common types are the lensatic compass and the map compass. The lensatic compass will usually open and close and is designed with aiming lines and mirrors or magnifying lenses so you can get a very accurate direction measurement, even over long distances.

Map Compass

The Map compass is usually mounted on a protractor so it has a clear bottom. It also has directional lines and measuring tools around the edges. These make it deal for use along with a map.

There are also small compasses and electronic compasses such as those in a GPS unit or a smartphone. These I recommend only for quick reference and not for serious navigation. It’s not that they aren’t accurate (although some need regular calibration) it’s just that they aren’t designed for serious navigation tasks.

GPS and iPhone Compass

Small Reference Compass

I’m going to assume you know that all compasses point north. Or to be more accurate, they point to the magnetic north pole which is different from the physical north pole.

Understanding how this difference works is important before you can safely use a compass to navigate.

The difference in angle between the physical north pole and the magnetic north pole is called declination. Declination is different for every location on earth and it changes over time because the earths magnetic field is not fixed.

You can find the declination for anywhere on earth from NOAA at the website http://www.ngdc.noaa.gov/geomagmodels/Declination.jsp and it will be listed on the bottom of your USGS 1/24,000 topo map.

Declination Key

For this map, the line with the star at the end represents physical north, sometimes called true north. The line with GN at the end represents grid north, this is the orientation for the UTM grid on the map sheet. And the line with MN at the end represents magnetic north.

The declination listed here as 11°E. This means that if you wanted to walk towards true north and simply followed your compass you would actually be walking 11° to the east of where you wanted to go. This would get you very confused if not lost.

To correct for this adjust your compass 11° west by subtract 11° from your direction, which is 0° to get 349°. So to walk towards true north you would have to point your compass at the angle 349° instead of 0°.

Also notice that the difference between magnetic north and grid north is 11°17′. So to go from north on the UTM grid to North on your compass you would have to subtract 11°17′. Most compasses aren’t designed to measure anything below degrees so it’s best to just round it off. Since there are 60 minutes in one degree this measurement rounds down and for this map sheet we’ll leave it at 11°.

Find more in our Navigation Skills series here.

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