Essential Skills: Knowing the Impact of Magnetic Declination on Compass Accuracy

Posted on AP 23 March 19.

This was originally published on the DTG blog under a similar name; it’s still there if you want to look it up.  The subject is so important, however, I updated it for consumption here.  Enjoy, and please add your two cents in the comments.

land nav map 1

In the past when discussing the subject of Magnetic Declination Compass Adjustment and/or Azimuth Calculations, from time to time, the comments seem to minimize the importance of A:  Knowing what your local Magnetic Declination is, B: How to adjust for it both on a compass or a map, and C: the minimal amount of extra walking won’t kill the party doing the walking.

Now, for every general example and subject description on land navigation there are people out there who don’t use it because they are so well versed in their AO they literally know every tree, stream, draw, ridge, and hill and don’t need the information presented. AO familiarity is a wonderful thing, and we should all work to improve on our local topographical knowledge.  However often the map & compass and associated skills, such as knowing and adjusting for magnetic declination are considered ‘back up,’ it’s wise to keep those skills very sharp, if for nothing more than showing the newly aware ‘trad’ or ‘normie’ how to do it.  Or grand kids.  Or friends and their kids.

USMC Land Nav

For discussion’s sake, let’s get into a SHTF scenario or some other situation where the person is using a new map and is unfamiliar with the territory.  Say, for example, they’re the team leader charged with running a security patrol with your NPT and have the task of linking up with a neighboring NPT at a particular location at a particular time.  If the NPT’s in the scenario don’t concern themselves with accurate grid azimuth conversion to magnetic conversion, and then ‘shooting’ an adjusted azimuth, lives could be at stake, and the link up will most likely not occur.

Experienced navigators backed up by the facts regarding magnetic compasses and the magnetic ‘North Pole’ will quickly tell you that if you don’t account for the local declination, you stand a great chance of not reaching your objective (which may be getting back to your truck or home or reaching and injured person or whatever you can think of) or becoming lost yourself.  Let’s look at the impact failing to adjust for local declination will have on your route planning and actual arrival.  Below is a graphic that shows both East & West declination differences.

US Magnetic Declination Map

                                                              US Magnetic Declination Map

Let’s say you’re studying your map to navigate the link up patrol, and find by using your protractor you need to take a 78 degree azimuth.  So, you set your compass accordingly BUT you haven’t converted the grid azimuth to a magnetic azimuth or corrected the declination reading on your declination adjustable compass for the local declination ( Declination is defined as the difference between Magnetic North and True North, either East or West – see the graphic below).   When using a grid map, you’ll use Grid North instead of True North – there is a minor difference.  Either way, you are starting your link up mission with a built in proportionate error when you shoot your azimuth (bearing)  to start your navigation.  Here’s the error factor of being off by various degrees computed to distance from the target/link up point (if on a straight line from your start point:

declination map

We’re using one kilometer (‘click) which is just under 1,100 yards (1,093.61 to be exact) for our arbitrary example.  The degrees of error are declination degrees.  If your declination is off by the below examples, the distance off target is the minimum error you can expect even if you walk in a perfect azimuth with no drift, deviation, or pace count error.

1 degree of error at 1,000 meters from start point = 17.5 meters off target (or 19 yards)

5 degrees of error at 1,000 meters from start point = 87.5 meters off target (95 yards)

8 degrees of error at 1,000 meters (my AO) start point = 140 meters off target (153 yards)

10 degrees of error at 1,000 meters start point = 175 meters off target (191 yards)

16 degrees of error at 1,000 meters start point = 280 meters off target (306 yards)

21 degrees of error at 1,000 meters start point = 367.5 meters off target (401 yards)

That’s for a 1 click leg.  Now, let’s multiply that to, say, a 7 click (4.35 miles) straight line walk.   Drum roll:  2,572.5 meters ( off target at the end of that little 7 click jaunt from the start point, not taking into account any additional anomalies you may encounter while trying to walk that perfectly straight 7 kilometer line you drew on your map.

My AO has an 8 degree West Declination for example.  I know; I check it frequently.  I shudder to think of what would happen to my ability to get from ‘A’ to ‘B’ without properly adjusting my compass or converting the grid azimuth to magnetic azimuth by adding the 8 degrees declination to my grid azimuth (I’m West, so I add to get a MAZ (Magnetic Azimuth) and subtract to take a MAZ and convert to a GAZ (Grid Azimuth).  So, in my case, before I even start, I’m going to be about 140 meters off for a .  This can be deadly in a SHTF scenario.

But for discussion’s sake, let’s make our walk shorter.  We’ll use the maximum variation in the US – 21 degrees East (Washington State) for a short, 3 click walk.  Drum roll:  1,102.5 meters/1,206 yards (even backing it down to the declination in my AO, 8 degrees, it comes out to 420 meters/459 yards – almost half a click – which is a LOT in a rural/wilderness environment).  Over a click off your target from the get go!  Want to gamble the success of your link up mission on ‘personal AO knowledge’ or the ‘noob’s’ knowledge?

That’s where you are before you start; imagine where you will be when you think you’re at the end of your first leg.  The mind boggles.  For fun in this mental exercise, let’s add a little thing called, “night” to the equation.  Now, for added flavor, consider when the map isn’t matching up to the terrain and you’re positive that your on the right azimuth, the disorientation (something that occurs with little notice) that can add to all the things you’re dealing with, and oh yes, human error in our calculations.

Good luck.

So, if you’re serious about learning or improving your land navigation skills, find a good course and attend it.  Or join a local orienteering club.   If you decide you want to have personalized instruction from AP staff, send the administrator a note with how many people, when you’d like to take the course, and we’ll see what’s possible.

ADDED:  A good place to get your updated Magnetic Declination for your own little corner of the world::

Feel free to comment! Debates are welcome, so long as they add to the discussion. Ad hominem attacks, accusations, uncontrolled vitriol, thread hijacks, personal threats, or any comment that otherwise detracts from DTG's stated mission will not be approved or posted. Repeat violators will be banned.

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