Dealing with Declination: Why your compass doesn’t point North

Most people would agree that compasses point north. However, this is only partially true. Modern compasses point towards the Earth’s magnetic north pole while most maps are aligned with grid north, along the Universal Transverse Mercator (UTM) grid lines (for the purpose of this article we are assuming grid north is equivalent to true north). The variation between grid north and magnetic north is what’s known as declination (dec.). The declination value varies based on your location. If you are navigating with map and compass but not factoring in declination, at best you will be slightly off your target and at worst you may find yourself completely lost.

In order to gain a deeper understanding of declination we must first understand what’s known as the agonic line and the Earth’s magnetic field. As stated previously, compasses point along the Earth’s magnetic field towards the magnetic north pole, which as of 2015 is located over Ellesmere Island, Canada. The agonic line is an imaginary geographical line connecting Earth’s north and south magnetic poles along which the declination value is zero. This means magnetic north is directly in line with true north (or grid north for most navigational purposes).

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The further you are from the agonic line (dec. 0°), the greater the angle between magnetic north and grid north. For example, if you are navigating in Raleigh N.C. your compass needle will be drawn roughly 8° west from grid north due to the pull from Earth’s magnetic field. This means that if you are ignoring declination and assuming grid north and magnetic north are the same, you will be 8° off even if you followed your azimuth perfectly. This may not sound like much but if you traveled for just one mile while 8° off of the correct azimuth, you will be roughly 738 feet away from your intended target.

Luckily, the intrepid navigator has a few methods at their disposal to account for declination and travel with precision. Before you step off on your next adventure ensure the map you will be carrying displays a declination diagram. If your map does not include this, a quick search online can tell you the declination value in your area.

Method 1: Adjust for Declination Mathematically

While this method may sound intimidating to those of us not mathematically inclined, it’s fairly simple and requires only basic addition and subtraction. It is important to note that an easterly declination simply means that the agonic line (dec. 0°) is east of grid north and a westerly declination means the agonic line is west of grid north.

A key phrase to help remember these rules is “East is least, West is best”, meaning easterly magnetic azimuths will always be less than grid azimuths and westerly magnetic azimuths will always be greater than grid azimuths.

For example, let’s assume that we are converting a magnetic azimuth (azimuth from your compass) of 70° to a grid azimuth (azimuth on your map) and have an easterly declination of 10°. To account for declination and determine an azimuth that can be applied to your map, you must add 10° to the magnetic bearing. Such that:

70° magnetic + 10° E dec. = 80° Grid

In another example, we have a grid azimuth of 160° with the same easterly declination of 10o. In order to convert the grid azimuth to a magnetic azimuth you must subtract 10° from the grid azimuth. Such that:

160° grid - 10° E dec. = 150° magnetic

Although this method requires more mental work than the rest, it is important to understand how to apply this technique. This is especially important for new navigators since not all compasses can be adjusted for declination.

Method 2: Adjust for Declination on the Compass

Certain compasses such as the Suunto MC-2 or Silva Ranger 2.0 allow you to manually adjust the compass for declination. This offsets the orienting arrow in the rotating bezel so that the compass is now properly aligned with grid north and you no longer need to convert mathematically between grid and magnetic azimuths. This is a quick and easy way to adjust for declination but you must ensure that you adjust to the correct declination value for your current location.

Method 3: Adjust for Declination by Drawing Magnetic North/South lines on the Map

By drawing the correct magnetic north-south lines on the map at the angle of magnetic declination, you may now determine azimuths from your map that may be directly indexed in your compass and vice-versa. However, there are several drawbacks to this. If you cannot have the magnetic north-south lines drawn by a computer generated topographical map, you will have to draw them by hand with a protractor and ruler, possibly introducing error. This can decrease precision and obscure important details on the map or routes of travel if the drawn lines are too thick. Magnetic declination is also changing every year as the Earth’s magnetic north pole shifts so these drawn lines will eventually become obsolete.

Method 4: Ignore it

If you are traveling very short distances or you are navigation along the agonic line where declination is 0°, then you may be able to ignore declination. I do not recommend this method but at very short distances the variation will be relatively insignificant especially if your intended destination is something large or a long linear feature like a backstop or handrail.

For those new to land navigation, declination can seem confusing. The key to mastering land navigation is just like mastering any other skill, dedication and practice! So get out there, be safe, and have fun exploring new places!

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Tyler Bryant

Wilderness Survival Instructor

Woodsrunner School, LLC