Magnetic Declination can be a very confusing topic when it comes to Land Navigation. I will try to simplify it in this article. You have three North’s on a Topo map that you have to understand:
- True North: The geographical direction that points to the North Pole.
- Grid North: The vertical Grid lines on a Topo map which point to your Grid North.
- Magnetic North: This is the direction that your compass points.
Magnetic Declination is the difference, (in degrees), between your True North and the Magnetic North (Compass Bearing). On some maps you will see both Grid and True North and the difference between them is very small. Some Topo maps only have True North in the Magnetic Declination Diagram.
If your map does not have a Magnetic Declination Diagram go to this website, NOAA website, to find out the Magnetic Declination in the Wilderness Area you plan on exploring
Magnetic Declination is either Westerly or Easterly. To determine which one you have is very simple. You look at what direction your Magnetic North arrow is pointing. If the arrow is pointing east then it is an Easterly Declination. If the arrow is pointing west then it is a Westerly Declination. Below are pictures showing you an Easterly Declination and a Westerly Declination.
*The arrow that looks like a harpoon is designated as the Magnetic North. In some Topo maps the arrow has a symbol MN. The arrow with the star is your True North.
If you have an Easterly Magnetic Declination then you subtract your Magnetic Declination. To remember this here is a saying you can use:
“EAST IS LEAST” (Least signifies subtracting)
So if your map bearing/azimuth is 180 degrees and you have an Easterly Magnetic Declination of 6 degrees you subtract 6 from 180.
180 degrees (Map) – 6 degrees = 174 degrees (Compass)
Now your compass is synced with your Topo map. You set your compass to 174 degrees and head to your destination. To convert your compass back to your map bearing/azimuth you add your declination in an Easterly Declination. REMEMBER, the above saying, “East is Least” is to convert Map to a Compass reading.
If you have a Westerly Magnetic Declination then you add your Magnetic Declination. To remember this use the below saying:
“WEST IS BEST” (Best signifies adding)
*So if your map bearing/azimuth is 180 degrees and you have a Westerly Magnetic Declination of 6 degrees you Add 6 to 180.
180 degrees (Map)+ 6 degrees = 186 degrees (Compass)
Now your compass is synced with your Topo map. To convert your compass back to map bearing you subtract your declination in a Westerly Declination. REMEMBER the above saying, “West is Best” is to convert Map to Compass.
You should always check your magnetic declination to ensure that it is correct. You may have a Topo map that is several years old and the Magnetic Declination can be incorrect. Magnetic Declination changes slightly every year.
To find out the current Magnetic Declination in the area you plan on exploring you can visit the NOAA website. This calculator will give you the recent declination that may be different from your Topo map. Below is a short video I posted on my You Tube Channel for further information.