How to Use Magnetic Declination with a Compass
What is Declination
So what is magnetic declination and how does it affect wilderness navigation?
Quite simply, magnetic declination is the difference between the magnetic and geographic North Poles.
Okay, and we care because?
"We should have hit that mountain pond campsite by now." Frank said with a heavy breath of exhaustion.
Ed looked down at his compass, made sure the bearing was correct and nodded to acknowledge his frustration.
"You remembered to compensate for declination, right?" Frank asked with a knowing stare.
Ed looked at the map, then at his compass; a look of panic ensued.
He had forgotten to add the fifteen degrees of West declination to his compass. They had been hiking for a few hours and they were now over a mile from where they needed to be.
Looking at the setting sun, Frank slipped his pack off his shoulders and muttered, "time to break out the headlamps."
A compass points to the magnetic north; however most maps are based upon geographical true north. This variance between the norths can cause a lot of confusion and some extra miles if you are in the backcountry. This article explains how to find and adjust for magnetic declination while navigating by compass.
The Three Norths
True North - Geographically where north is.
Magnetic North - Where the compass points.
Grid or Mapmaker's North - How grid lines are oriented to correct for spherical distortion.
East or West Declination?
Unless you live on the agonic line (where declination is zero), your declination value will either be east or west. For geologists, west declination is a negative value and east declination is a positive value.
Just to add some confusion, you should also know that if you live east of the agonic line - you are dealing with west declination and if you live west of the line - that is east declination. If you are feeling lost, don't call search and rescue just yet.
If you have trouble remembering which one is positive try out one of these ditties:
"East is least and west is best."
"In the West the compass points toward the granddaddy of all trails, the Appalachian Trail in the East; In the East, the compass points west toward the Continental Divide and Pacific Crest trails." (it helps to remember that the two western trails are longer and therefore positive.)
Before starting a huge East Coast versus West Coast rap battle, let's dig a little further into what these positive and negative values actually mean. Basically, if you are using a compass east of the agonic line, your compass will point west of true north. If you are out orienteering west of the agonic line, your compass points east of true north.
Are you confused yet? It's okay as mastering declination can take a lifetime of bushcrafting to master. I'll try to make it less bewildering as we progress further into this subject.
How Do You Find The Declination on a Map?
The Declination value for most areas is listed in the legend. However, be advised that declination changes over time as the magnetic pole drifts.
- In my location the current declination is -10° 44'
- However, in 1970 the declination was -8° 18'
- In 1950 the declination value was -7° 51'
So what's the problem? Just imagine you are going out hiking with a map that was printed in 1950 and use the declination value found in the marginalia; you will be off 3°. Over a distance, the difference could be huge.
The most accurate way to find your current declination is to check out the Geomagnetic Calculator from the National Oceanic and Atmospheric Administration. Just enter the latitude and longitude, or zip code, of the area you are traveling and voila - instant declination values.
Set Your Compass to Find True North
Many high-end compasses like the Silva Ranger have settings to adjust the orienteering lines to face true North. This is usually done with a small screw on the baseplate. As most people tend not to venture between areas with different declination, this method of adjusting the compass is easiest for most people.
For many models, making this adjustment changes the angle between the orienteering lines and the orienteering arrow. This offset looks strange but you can get use to it.
Expert tip: Remember to reset the declination screw on your adjustable compass every time you venture into a different zone.
To see which way the needle moves
- East Declination
Turn screw so that the orienteering arrow points to the east side of the declination scale.
- West Declination
Turn screw so that the orienteering arrow points to the west side of the declination scale.
Do the Math to Figure Out Declination
So you have a geographic bearing to walk and a compass without a declination adjustment, so you'll just have to do the math. This is where English majors like myself tend to get a little nervous.
So remember this handy phrase:
Declination east - compass least. Declination west - compass best.
Explanation: In the East with west declination, the magnetic bearing will be a greater value than true north. However, in the West with east declination, the magnetic bearing will be a smaller value than true north.
- Example: From your campsite you have plotted an azimuth on your map of 120 degrees to the top of a mountain. Your declination is 10 degrees east, so what bearing do you dial in your compass to walk directly from your campsite to the mountain summit? Here, you want to subtract the declination value of 10 from 120 to arrive at 110 degrees.
- Example: You're canoeing on a lake and want to paddle from an island to a small inlet stream. You plot it on the map as a bearing of 120 degrees and your declination is 10 degrees west. What should the dial on your compass read, in order to get to the stream directly? In this example, you want to add the declination value of 10 to 120 to equal 130 degrees.
To make things really easy for when to add and when to subtract, follow the handy chart provided below.
Simple Declination Chart
Map to Compass (TN>MN)
Compass to Map (MN>TN)
Let the GPS Figure It Out
Using the settings of your GPS, you can set your navigational unit to find any of the norths. This is easily done, by choosing your selection within your set-up menu.
Choosing the correct north is especially important when using a navigation device with traditional map and compass. For example, after projecting a waypoint that I wish to hike to in my GPS, I'll dial in the plotted bearing into my compass and follow that.
There is one downfall to always depending on your GPS for wilderness navigation and that is - what happens when the batteries die?
Test your declination knowledge
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Other Options for Dealing with Declination
Besides adjusting your compass, mathematically correcting declination or using the GPS, you do have a few other options.
The first option is to simply ignore it. If you live close enough to the agonic line or if you don't need precise navigation, this is surely a viable option. For example, if you only carry a compass to walk on a consistent line of travel and do not use it in conjunction with a topographic map, declination isn't something you have to worry about.
Another option is to draw magnetic meridians on the map. Using a protractor and a straight edge, you'll draw lines across your map on the angle of declination. Your compass will be adjusted to true north by aligning the edge along of these drawn lines. In the interest of preserving friendships, I don't suggest drawing lines across maps that you borrowed from fellow outdoor enthusiasts.
"Magnetic Declination Demystified" is one of the best explanations of this variance that I have found
Do you calculate declination while out in the outdoors?
© 2014 Dan Human