Ohmforge Magnetic Declination (WMM2025)

What it is for

A compass needle points to magnetic north, not to the geographic North Pole. The angle between them is the magnetic declination (sailors say variation): about 5° east in Germany, about 15° east in Seattle. If you follow a map bearing with a compass and ignore it, you drift about 90 m sideways per kilometre for every 5° of declination.

This tool evaluates the World Magnetic Model WMM2025, the model that NOAA and the British Geological Survey publish for navigation systems and phones. You give a latitude, a longitude, a height and a date. You get the declination, the inclination (how steeply the field dips), the field strength, how fast all of it changes per year, and the grid variation near the poles. The compass in Ohmforge's Pocket instruments uses the same model for true north.

The model describes the Earth's core field only. It does not know about the steel beam next to you, so it can't correct a compass for local disturbances.

Worked example

You're in Denver (latitude 39.74, longitude −104.99, height 1609 m) on 8 November 2026. Your compass shows a bearing of 270° to a peak. The result:

• Declination 7.49° E, changing by −0.080° per year.
• Inclination (dip) 65.84°, total field 50 950 nT, horizontal field 20 856 nT.
• True bearing 277.5°.

So the peak lies at 277.5° true, and a map line drawn at 270° would miss it. The yearly change tells you the value drifts by less than half a degree over the five years the model is valid.

A second run for Longyearbyen on Svalbard (78.22 N, 15.65 E) gives 12.95° E, a horizontal field of only 7228 nT and a grid variation of −2.70°. Up there, polar maps use a grid instead of meridians, and the grid variation is the angle a compass needs relative to grid north.

How to read the result

• Declination: east means the needle points east of true north. True bearing = magnetic bearing + declination (with west as a negative number).
• Yearly change: how much the declination moves per year. It matters for old maps, which print the declination of their survey year.
• Inclination and field strength: useful for checking a magnetometer or a phone sensor. Your reading should be close to the total field in nanotesla.
• Field components table: north, east and down components with their yearly change, in the form the WMM report uses.
• A caution or unreliable warning appears when the horizontal field drops below 6000 nT or 2000 nT.

Tips and common mistakes

• Leave the date empty for today. The model is only valid from 2025.0 to 2030.0; older dates are rejected rather than guessed.
• Height barely matters on the ground; it counts for aircraft.
• Watch the sign of the longitude: west is negative. Swapping it is the most common way to get a wildly wrong answer.
• A phone compass sits next to a battery, a speaker magnet and often a magnetic case. Calibrate it (figure-eight motion) and keep it away from cars, rebar and fridges before you trust a few degrees.
• The declination changes slowly with distance, so one value usually covers a whole hike.

FAQ

Is this the same value my phone shows?

Most phones and GPS units use the World Magnetic Model too, so the declination should agree to a few hundredths of a degree for the same place and date.

How accurate is WMM2025?

It reproduces the official test values exactly. Against the real field, the WMM report quotes a typical declination error of well under a degree at mid latitudes, larger near the magnetic poles and in areas with magnetic rocks.

What is the difference between declination and grid variation?

Declination is measured from true north. Grid variation is measured from grid north of a polar stereographic map and is only given for latitudes beyond 55°.

Can I use it for dates before 2025?

No. WMM2025 starts at 2025.0; earlier dates need the previous model, which this tool does not include.