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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.

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4 thoughts on “Land Navigation: Declination

  1. Nice job on post. Navigation is one of the most underestimated and least taught field craft by far now that we have the handy GPS. You are right on the mark with your statement “Declination is a very real thing”.
    However, I think you missed the fact that declination is also a constantly changing thing as well. The annual declination variance is 13.2’ (minutes). Not accounting for this change can lead to some serious frustration to navigators. So it is very important to note the date of the map being used and make the adjustments for your figures. I found this out the hard way. In early 2000’s I was navigating in the Sierras using an already out dated map and noticed some deviations in map plotting. I really didn’t give much attention as I knew the area and had ample terrain features to navigate by. Later on in 2009 I was back with another group and I noticed even more variations to the legs we shot. It was at that time the notion donned on me that I forgot to change the numbers on the declination diagram to the current Magnetic Declination – Magnetic Inclination.
    Your right in short distances and small deviations most people will drift and get lucky. Most people drift to the dominate leg which usually is the right leg. It would be my luck to get the left legged navigator.
    Below is from GeoKov website and the link is below.
    Declination Diagram
    Topographic maps usually contain a declination diagram in their margin. The year for which the declination was measured and the annual rate of change are stated in the diagram. Remember that the rate of change of the magnetic field and therefore the declination is not constant with time. As a result calculating the present day declination using the annual change from older maps is not going to be very accurate.
    On the declination diagram typically there are three lines, one denoting the direction to true north, one for magnetic north, and one for grid north (parallel to grid lines on the map). Also measurement of two angles (if all three lines present) are given. Depending on the relative position of true north, grid north and magnetic north lines, the angles may represent: true north declination – the angle between true north and magnetic north; grid declination – the angle between grid north and magnetic north; convergence angle – angle between true north and grid north. Again depending on the relative position of these lines (see Fig. 20, Geological Survery of Canada tutorial) to each other, you will need to add or subtract the convergence angle from either grid north or true north to find the desired declination (grid or true north). Grid declination is probably more useful when using maps with gridlines since bearings are measured relative to grid lines.
    In order to calculate declination for a specific date from this diagram, first the date of publication of the map needs to be noted (i.e. 2009 for this map). Assuming the present year is 2013, next step is counting the number of years that have elapsed since publication of the map (2013 – 2009 = 4 years). Total change in declination is found by multiplying the annual change by the number of years elapsed, annual change is 13.2 minutes (13.2′ x 4 = 52.8′). Adding or subtracting this value (depending on whether declination is decreasing or increasing) from the original declination (true north or grid declination) will result in the desired declination value. Here declination value is decreasing by 13.2′ per year. Therefore the total change in declination needs to be subtracted from original declination.

    http://geokov.com/education/magnetic-declination-inclination.aspx

  2. Pingback: Declination Angle for Fort Bragg | Carrying the Gun

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