Land Navigation: Declination

Declination: “It’s a real thing.”

There are some things in life I feel strongly about. No Super Bowl will be greater than Super Bowl 42. Reality television is simultaneously everything that is great and terrible about America. And you must adhere to the declination diagram of a given map! Here at Fort Benning, declination is usually glossed over as unimportant.

“It’s only 4˚ gentlemen, you don’t even have to use it.”

At its worst, I sat dumbfounded in a land navigation class as the instructor said that to get a magnetic azimuth you SUBTRACT the G-M angle from the grid azimuth. After the class, I spoke with him, confident that to get a magnetic azimuth at Fort Benning you add 4˚ to the grid azimuth. I was told I was wrong, because “General (Grid) to Major (Magnetic) is a demotion, so you subtract.” I’m sure that he learned that somewhere, at another post, where that mnemonic worked. It doesn’t work at Fort Benning, and if you did indeed subtract, you would be off azimuth by 8˚, which is certainly not negligible (double the numbers at the diagram I have at the bottom).

What is declination? From FM 3-25.26 (Map Reading and Land Navigation):

Declination is the angular difference between any two norths. If you have a map and a  compass, the one of most interest to you will be between magnetic and grid north. The declination diagram shows the angular relationship, represented by prongs, among grid, magnetic and true norths. While the relative positions of the prongs are correct, they are seldom plotted to scale. Do not use the diagram to measure a numerical value,. This value will be written in the map margin (in both degrees and mils) beside the diagram.

Fort Benning Declination Diagram.

In more basic terms, any azimuth you get using a protractor is not useable on the ground until it is converted using the declination diagram. At Fort Benning, to get a magnetic azimuth from a grid azimuth, you add the G-M angle which is 4˚ (70 mils). If, for example, you plotted an azimuth of 90˚ to a point, you would have to shoot a magnetic azimuth of 94˚ in order to walk the actual azimuth you plotted.

I’m assuming that most instructors advise students to ignore the G-M angle for simplicity. It might be too confusing to add 4˚ to a grid azimuth.

I’m a firm believer in using the G-M angle because it is the actual correct azimuth. To ignore it is accepting that you will not walk exactly where you intend to. When navigating, it seems most people tend to drift to the right. That might explain why so many people swear by ignoring the G-M angle – their drifting right actually puts them on the right azimuth!

From Left to Right (top): Start Point, 50m, 90m, 130m, 200m.
From Left to Right (bottom): Start Point, 300m, 600m, 1000m, 1500m.

The map above shows how declination works at Fort Benning. From the start point (SP) at the road on the left I plotted a 90˚ azimuth to the road on the right. If you added the G-M angle (4˚) and walked a perfectly straight azimuth of 94˚, you would walk along the bottom line. If you did not add the G-M angle and instead walked a perfectly straight azimuth of 90˚, you would walk along the top line. The numbers on the bottom line are the distances in meters and the numbers on the top line are the approximate distances off azimuth a navigator would be at the given ranges.

So, for example, by ignoring the G-M angle, you would be off by approximately 50 meters after walking 300 meters. Not a big deal if you are looking for something big, like a house. But if you’re looking for a small orange and white box on a six foot stake in the woods, obscured by foliage and sadistically placed in the most out-of-sight-spot, at night, it might be hard to see that from half a football field away.

As you move further along your un-declinated azimuth, the distance only widens. At 600 meters, you are just under 100 meters off azimuth. At 1 kilometer you would be about 130 meters off. 1500 meters: 200 meters off.

Of course, a good way to compensate for this is to understand the terrain you will be traversing. If I was walking the 94˚ azimuth in the diagram, I would know that to get from one road to the other I would be crossing the creek at just over 1000 meters and then crossing a second creek at about 1500 meters. If I chose not to add the G-M angle, I would still cross the creek, but that would happen at about 600 meters. Coming up to the creek 400 meters too soon should give the navigator pause and he should stop to figure out what is going on.

The “oh by the way” of this is I have plenty of friends who have successfully completed land navigation courses here without using the declination diagram. They may have drifted into their correct azimuth or used a combination of land navigation techniques to improve their chances of finding their points. The point is, at Fort Benning it is possible to ignore the G-M angle and still do well. But why knowingly handicap yourself when all you have to do is add 4˚?

Declination. It’s a real thing. When it comes to land navigation, I’ll take any advantage I can get it.

Enjoy these posts? Follow me on Twitter and sign up for the monthly newsletter.

Leave a comment

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s