Written by 3:20 pm Wilderness Navigation Masters

22) Magnetic declination

Hello and welcome to this video of the online course Wilderness Navigation Masters, where you will learn multiple things about the magnetic declination, like :

  • How much you will deviate from your route if you forget your magnetic declination?
  • When you can ignore your magnetic declination?
  • How to get this year’s magnetic declination of your topographic map, even if it’s 10 years old. Yes, it’s possible.
  • How to use the magnetic declination online calculator to get the declination for every place on earth.
  • And the little trick to avoid forgetting the magnetic declination when you use your compass.

When you can ignore the map magnetic declination

First thing first, how much you will deviate from your route if you forget your magnetic declination?

  • Map magnetic declination of 2°

If your map has a magnetic declination of 2°, and you forgot to take it into consideration, walking for half a mile you will be deviated by 100 feet.

This is not a big deal, because, easy to see landmarks like rivers or mountains can be seen from 100 feet.

  • Map magnetic declination of 20°

But if you forgot a magnetic declination of 20°, you will arrive at 1000 feet away from your destination, and this is a very important difference, and most of the time you will be lost.

This is why it’s important to know the magnetic declination of your map and take it into consideration.

  • Good news:

But I have good news for you, when the magnetic declination is below 3°, you can neglect it.


Actual magnetic declination from an old map

Let’s take a look at a magnetic declination diagram, and how you can determine the current year magnetic declination of your topographic map even if it’s 10 years old.

This is the magnetic declination diagram from my National Geographic Trails Illustrated Map of Rocky Mountain National Park in Colorado.

  • The approximate declination at this place is 9° East at 2016
  • And the approximate annual change is 00°06’ to the West.

If this topographic map have the magnetic declination value of 09°00’E in 2016, now at 2019, we have a difference of 3 years.

So, 00°06’ times 3 equal 00°18’.

If the magnetic declination at 2016 was East and the change is to the West, that means we should subtract 00°16’ from the 09°00’.

So, 08°44’E is the current approximate magnetic declination for this map in this year of 2019.

Magnetic declination online calculator

Now that you’ve seen how much you will deviate from your route if you forgot the magnetic declination, and how to determine the approximate current year magnetic declination from an old map, let’s see how to get the magnetic declination value for every place on earth.

Yes, you can know the magnetic declination from your map, but what to do if you want to know about the magnetic declination when you don’t have the topographic map of that place?

You can use the magnetic declination online calculator from the website of the National Oceanic and Atmospheric Administration.

It’s a very useful online tool, easy to use, and available online 247 unless the website is under maintenance.

Before I show you how to use this tool, I want you to know that, for precision, the magnetic declination in this tool is represented using seconds after the degrees.

For example, 1°55’ (‘=seconds).

And each degree, equal 60 seconds.

Now let’s see how to use this tool.

[Skip to 3:52]

  • Because it’s hard to arrive here from the website homepage. To arrive to this page,  I did leave the direct link below the lesson, or you can google Magnetic Field Calculator NOAA.
  • The first thing you have to do is to search for your desired place here.
  • You can use the street address, name, or intersection. And for better results, you can include the city, the state, or the zip code.
  • For me, I want to know the magnetic declination for Rocky Mountain in Colorado, so I will use the city name which is Estes Park.
  • I don’t know if you did notice that I did get the Latitude and the longitude here.
  • WMM stands for World Magnetic Model, IGRF for International Geomagnetic Reference Field, and EMM for Enhanced Magnetic Model.
  • The first two, are both estimated from the most recent data and are of comparable quality.
  • But, if you want to search for an old declination, it’s better to choose the IGRF. For me, because I want the declination of just 2016, I will choose WMM.
  • For the year, I will select 2016.
  • The result format doesn’t matter to me.
  • Click on calculate.
  • A new window will pop up.
  • You can see the model used, latitude, and longitude.
  • What matters to us is the date and the declination.
  • For Estes Park, in 2016, the declination was 8° 44′ E, plus or minus 21 minutes, with an annual change of  0° 6′ W.
  • As you see, it’s pretty much the same thing between my National Geographic Trails illustrated topo map and this tool. Especially, when I add 21 minutes of error margin because I will get exactly 9° 5’.
  • Another way to use this tool is to enter your latitude and longitude directly here.
  • Google maps, is the tool that can help you to know the latitude and longitude of every place on earth.
  • All you have to do is to open google maps.
  • I will search for a rocky mountain National Park.
  • With my computer mouse right-click, I get these options.
  • Click on What’s here, and on the bottom of this page, you will get the latitude and the longitude like that.
  • In this case, the first value is positive and the second one is negative.
  • Copy them like that.
  • For latitude, the positive is north and the negative is south.
  • For Longitude, what’s positive is East and what’s negative is West.
  • Click on calculate.
  • As you see, the more we give this tool the exact place, the more the results get closer to the National Geographic declination value.
  • To be sure of this coordinates search method when I zoom out I see Estes Park here.
  • That’s all for how to use this online tool. Like I’ve said before, the direct link to this page is below this lesson


How to apply the compass magnetic declination duct-tape trick

Because adjusting the magnetic declination each time you want to use your compass can frustrate you, or, you can forget it, especially when you are tired outdoor.

In order to not forget it, the duct tape trick consists of marking, your magnetic declination position, on the back of your compass using an easy to see duct tape like this.

For example, if your map magnetic declination is 20° to the West, you can mark it like that, and every time you are supposed to put the red needle in the red orientation arrow or the red in the shed, you place it on this new mark.

[Skip video to 7:30]

That’s all for this trick, I’ve wanted to show you how to apply the duct tape on the back of your compass, but because it’s simple and I’m writing this video script at 5 AM, after just 2 hours of sleep, I’ve chosen to explain using a photo only.

But if you want me to show you how-to, let me know in the comments section below, and I will be happy to do it.

Now let’s go for the first exercise.

Exercise #1

In the first exercise, I want you to search for at least 2 years old topographic map, and calculate this year’s magnetic declination.

Let me know what you’ve found in the comments section below, to check it after you.

Exercise #2

In the second exercise, I want you to check your result using the magnetic declination online calculator we’ve just seen.

That’s all for this lesson, thank you for watching, and see you in the next one.

Updated on June 7, 2021 by Ben

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