AI Agent for QGIS
Ask AI Agent in QGIS to turn the compass bearings of a field survey into true bearings, point by point.
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AI AgentFrequently asked questions
Do I add or subtract magnetic declination?
To turn a compass bearing into a true bearing, add an east declination and subtract a west one. To set a true bearing from a map on your compass, do the opposite. Example: with 15° E, a compass bearing of 30° is a true bearing of 45°. The card writes the rule for the place you chose.
How do I find the magnetic declination for my location?
Tap My location, search your address or ZIP code, or click the map. The declination comes from the World Magnetic Model for that exact point and today's date; topographic maps print it too, but the printed value ages by a fraction of a degree each year.
How much does magnetic declination change each year?
Usually less than 0.2° a year, but it varies by place: the card gives the yearly change where you are. Over a decade that adds up to a degree or two, which is why the declination printed on an old map should be updated before you rely on it.
Where is magnetic declination the largest?
Near the magnetic poles, in northern Canada, Greenland, Siberia and around Antarctica, where it passes 30° and changes fast over short distances. The agonic line, where declination is zero and a compass points true north, runs in 2026 from Minnesota to the Gulf of Mexico, through western Britain and Spain, through central Asia and down the west of South America.
About this tool
Magnetic declination is the angle between true north and the north a compass points to. This calculator gives it for any place and any date from 2025 to 2029 from the World Magnetic Model WMM2025, the model of the US and UK geological agencies that phones and navigation systems use. Search an address, type coordinates, use your location or click the map: the card gives the declination in degrees and minutes, east or west, whether to add or subtract it to turn a compass bearing into a true bearing, how fast it changes each year, the angle between grid north and magnetic north for UTM and MGRS maps, and the field's inclination and strength. The map draws the isogonic lines, which join places with the same declination, every 5 degrees, and at the pin the true north and the compass north. Copy the value or a link that reopens the same place and date.
How the declination is computed
- The World Magnetic Model describes the Earth's main field with 90 pairs of coefficients and their yearly change. The calculator evaluates it in your browser for the point, at sea level, on the chosen date.
- Declination is the angle from true north to the horizontal field, east positive. The yearly change comes from the model's secular variation, so a value for 2028 already includes three years of drift.
- Grid to magnetic is the declination minus the UTM grid convergence at the point: the angle to set between the grid lines of a UTM or MGRS map and your compass.
Data and limits
World Magnetic Model WMM2025, by NOAA NCEI and the British Geological Survey, public domain, valid from 2025.0 to 2030.0; this calculator matches all 100 official test values to within 0.005°. Nothing you search leaves the page. The model leaves out local anomalies from rocks, steel and power lines, which can reach several degrees, and the field changes from day to day during magnetic storms. Around the magnetic poles, where the horizontal field falls below 2,000 nT, the compass is not reliable.