Magnetic Declination: True North vs. Magnetic North

What Is Magnetic Declination

Your compass needle points to magnetic north — the location of Earth's geomagnetic pole, currently in the Canadian Arctic. Your map is oriented to true (geographic) north — the actual North Pole. These two norths are not the same.

The angle between them at any given location is magnetic declination. It varies from nearly 0 degrees near the Great Plains to more than 20 degrees in parts of the Pacific Northwest and New England.

If you take a compass bearing and use it on a map without correcting for declination, you'll be traveling at an angle to your intended direction. The larger the declination, the larger the error.

How to Apply Declination

There are three ways to handle declination:

Method 1: Compass Declination Adjustment (Easiest)

Many quality compasses have a small adjustment screw that rotates the orienting arrow relative to the bezel. You set the declination once and forget it — the compass automatically corrects.

Set it by turning the adjustment screw the correct number of degrees in the correct direction per the compass manual. After setting, the compass reads grid bearings directly. No mental math needed on any subsequent bearing.

Method 2: Mental Arithmetic (Most Common in the Field)

Going from map bearing to field (magnetic) bearing:

• Declination East: subtract. Map bearing 040° - 15° = magnetic 025°.
• Declination West: add. Map bearing 040° + 10° = magnetic 050°.

Going from field bearing (magnetic) to map bearing:

• Declination East: add back. Field bearing 025° + 15° = map bearing 040°.
• Declination West: subtract back. Field bearing 050° - 10° = map bearing 040°.

Memory aid: The phrase "East is least" (subtract for east declination) and "West is best" (add for west declination) works for converting from magnetic to grid. Reverse the operations for grid to magnetic.

Method 3: Pre-Drawn Magnetic North Lines on Map

Draw a series of north-south lines on your map slightly offset from the grid north lines by your local declination angle. When orienting the map with a compass, align the needle to these magnetic north lines rather than the grid lines. No arithmetic required during navigation.

This method works well for multi-day trips in one area: prepare the map once with the correct lines and navigate without repeated calculation.

Checking Your Current Local Value

Declination values printed on USGS maps include the year of the data. For maps older than 5-10 years, look up the current value at:

• Online: ngdc.noaa.gov/geomag/calculators/magcalc.shtml
• Offline app: NOAA Declination app (download before emergencies)

Input your zip code or lat/long and get the current declination to 0.1-degree precision with the annual rate of change.

Grid North vs. True North vs. Magnetic North

Most US maps use UTM grid north, which is aligned to longitude lines in the center of each UTM zone. At the zone edges, grid north diverges slightly from true north — but by less than 3 degrees across all zones. For most practical navigation, treat grid north and true north as equivalent.

The relevant distinction for day-to-day compass work is magnetic north (where your compass points) vs. grid north (what your map uses). That's the declination you need to apply.