Declination at a glance
- Measures
- True north vs magnetic north
- Lower 48 range
- About 15° E to 15° W
- Cost of 1° error
- About 92 ft per mile
- Changes
- With place and over time
Why do true north and magnetic north differ?
They are two different places. True north is the geographic pole the Earth spins around, and every topographic map is drawn with it at the top. Magnetic north is where the planet's magnetic field points a needle, and that field is generated by moving iron in the outer core, so it neither lines up with the spin axis nor stays put.
The gap between the two directions, seen from where you stand, is the declination. It is called east when the needle points east of true north and west when it points west of it. Sailors and pilots call the same thing magnetic variation.
How much declination is there in the United States?
In the lower 48 it runs from roughly 15 degrees east in the Pacific Northwest to roughly 15 degrees west in northern Maine. Along a line that runs roughly through the Midwest and down toward the Gulf Coast it is close to zero, and a compass there points very nearly at true north.
The numbers drift. The line of zero declination moves west a little every year, and a map printed decades ago can show a figure that is a degree or more out of date. NOAA publishes a free calculator that gives the current value for any location and date.
What happens if you ignore declination?
You walk in a straight line to the wrong place. Each degree of error moves you about 92 feet sideways for every mile you travel, so an uncorrected 14 degrees puts you about a quarter mile from your target after one mile.
On a marked trail that rarely matters, because the trail does the navigating. It matters when you leave the trail, when snow or fog hides it, or when you use a bearing to find a specific point such as a lake outlet or a trail junction.
How do you correct for declination?
The simplest way is a compass with adjustable declination. You turn a small screw or ring until the orienting arrow is offset by the local figure, and from then on every bearing you read is a true bearing that matches the map.
With a fixed-scale compass you do it by arithmetic: for an east declination, subtract it when going from a map bearing to a compass bearing and add it when going the other way, and reverse that for a west declination. It works, and it is the step people get backwards when they are tired.
What this means when you're choosing
- A compass with adjustable declination, set once for the area you hike in
- The current declination for your trip from NOAA's calculator, not an old map margin
- Whether your GPS watch or phone app is set to show true or magnetic bearings
- A fresh check whenever you travel far east or west, since the figure changes with location
The common mistake
Adding when you should subtract
With a fixed-scale compass the correction is added in one direction and subtracted in the other, and reversing it doubles the error instead of removing it. An adjustable compass set to the local declination takes the arithmetic away entirely, which is why it is the first feature to pay for.
Common questions
Is declination the same everywhere?
No. It depends on where you are. In the lower 48 it ranges from roughly 15 degrees east in Washington to roughly 15 degrees west in Maine, and it is near zero along a line through the Midwest.
How often does declination change?
Continuously, but slowly. In most of the US it shifts by a small fraction of a degree a year, which adds up over the life of an old map. Check NOAA's calculator before a trip that depends on bearings.
Do phones and GPS watches correct for declination?
Most do it automatically and show true bearings by default, with a setting to switch to magnetic. What matters is that your device, your compass and your map all agree on which north you are using.
Where this is defined
- Magnetic Field Calculators, NOAA National Centers for Environmental Information
- World Magnetic Model, NOAA National Centers for Environmental Information