Compass declination: why your compass and map disagree, and how to correct for it

A phone map is the first thing most of us reach for, until the battery dies or there is no signal. A paper map and a baseplate compass keep working. But the needle and the map do not agree on where north is, and the gap is large enough to put you on the wrong ridge. This article explains that gap, called declination, using NOAA, USGS and Japan's national mapping agency, and shows how to correct for it.

What declination is

NOAA's National Centers for Environmental Information (NCEI) define magnetic declination, sometimes called magnetic variation, as the angle between magnetic north and true north. By convention it is positive when magnetic north is east of true north and negative when it is west. It changes with location and over time.

The USGS puts it plainly: at most places on the Earth's surface, a compass doesn't point exactly toward geographic north. At very high latitudes, it notes, a compass can even point south.

One common belief is wrong. NOAA says a compass needle does not point toward the magnetic pole; it lines up with the horizontal part of the magnetic field where you are standing. That is why declination has to be looked up for your own location: NOAA warns that "a mile or two away the magnetic declination may be considerably different."

How big the error gets

Each degree you ignore moves you sideways by about 17 meters for every kilometer you walk, or about 92 feet per mile. At 10 degrees that is about 175 meters per kilometer, or roughly 930 feet per mile (our calculation). In fog or thick forest, that can be the difference between the trail and the next drainage over.

Where to find your declination

• NOAA's online calculator. NCEI's declination calculator gives the value for a latitude and longitude on a given date, and for U.S. locations you can enter a ZIP code. NOAA's calculators use the World Magnetic Model (WMM) and the International Geomagnetic Reference Field. The current WMM2025 was released on December 17, 2024 and is valid until late 2029; a new version comes out every five years.
• Your map's margin, with care. The collar of a USGS topographic map shows the declination at the center of the map for the year the map was made. On an older map that number may be out of date, so check it against the calculator.
• Japan. Japan's Geospatial Information Authority (GSI) publishes its own declination data and an online calculator. Everywhere in Japan the declination is west: GSI gives about 10 degrees west for Sapporo and about 6 degrees west for Naha (2020.0 values). Tokyo is about 7.6 degrees west. GSI also found that, on average across Japan, westward declination grew by about 1.4 degrees over the past 50 years, and it notes that only topographic maps published from March 2022 onward carry the 2020.0 value.

Correcting for it

The rest of this section is our explanation of the standard methods for a baseplate compass with a rotating housing. Pick one method and stick with it.

Method 1: Set the declination on the compass

Many compasses have an adjustable declination scale. Set it once for the area you are visiting, following the maker's instructions, and the compass will then read true bearings that match the map.

Method 2: Draw magnetic north lines on the map

Before the trip, draw parallel lines across the map, rotated from true north by your declination: to the east for east declination, to the west for west declination (as in all of Japan). Then, in the field:

1. Lay the edge of the baseplate on the map so it joins where you are to where you want to go, with the direction-of-travel arrow pointing toward the destination.
2. Turn the housing until its orienting lines are parallel to your magnetic north lines, with "N" toward magnetic north.
3. Take the compass off the map, hold it level, and turn your whole body until the red end of the needle sits inside the orienting arrow.
4. The direction-of-travel arrow now points where you need to go. Pick a landmark, such as a tree or a rock, in that direction, walk to it, and repeat.

Method 3: Do the arithmetic

If you measure a bearing from the map (a true bearing) and want to follow it with an uncorrected compass: subtract east declination, or add west declination. Going the other way, from a compass bearing to the map, reverse the sign. Example: in Tokyo, a map bearing of 90 degrees (due east) is about 98 degrees on an uncorrected compass, because the declination there is about 7.6 degrees west (our calculation).

Keep the compass away from steel and magnets while you read it: knives, phones, magnetic clasps and vehicles can all pull the needle (our suggestion).

Traveling with a compass

• Southern Hemisphere. NOAA explains that a compass needle needs a small balance weight to stay level, because the magnetic field dips into the Earth. Compasses for the Northern Hemisphere carry the weight on the south end of the needle; in the Southern Hemisphere it needs to be on the north end. A compass balanced for the wrong hemisphere can tilt and drag. Some compasses are made to work worldwide, so check before a trip south.
• Near the magnetic poles. NOAA defines "blackout zones" around both magnetic poles where compass accuracy is badly degraded, and a wider "caution zone" around them. This matters for high-Arctic and Antarctic trips, not for ordinary hiking.
• Phones. NOAA notes that electronic compasses and the World Magnetic Model commonly co-exist in GPS receivers. Many phone compass apps can show either true or magnetic north; check which one yours is set to before comparing it with a paper map (our suggestion).

Practice before you need it

A compass is easy to understand and hard to use under stress. Practice in a park or on a familiar trail, walking from one known point to another, before you rely on it in fog (our suggestion). Orienteering clubs and events are a good way to learn.

For what to carry and what to do if you get lost, see The Ten Essentials and a trip plan and Getting found. For hiking in Japan, see Hiking in Japan.

Our Japanese edition has a step-by-step guide with GSI's declination values for Japanese cities: 地図とコンパスの基本.