Magnetic Declination Calculator

What is magnetic declination

A compass needle does not point to the geographic North Pole. It aligns with the horizontal component of the Earth's magnetic field. The horizontal angle between true (geographic) north and magnetic north at a given place is called magnetic declination (also magnetic variation, D). When magnetic north lies east of true north the declination is positive (east); when it lies west, it is negative (west). Declination varies from place to place and changes over time. Flows in the Earth's outer core alter the field slowly but continuously, so the same point shows a slightly different value every year.

True north, magnetic north and grid north

• True north: the direction along the local meridian towards the geographic North Pole. Geodetic azimuths refer to it.
• Magnetic north: the direction a compass points to. Its angle from true north is the declination.
• Grid north: the northing axis of a map projection such as UTM or the 3-degree Transverse Mercator zones used in Turkey. Away from the central meridian it is not parallel to the meridians; the angle between them is the grid convergence (meridian convergence, γ), which grows with distance from the central meridian.

To transfer a compass bearing onto a plan drawn in projected coordinates, use the difference of the two angles, the grid-magnetic angle (GM = D − γ): grid bearing = compass reading + GM. The tool reports all of these angles for the selected point and converts a compass reading into true and grid bearings. For background on UTM zones, TM strips and EPSG codes, see our guide to coordinate systems and EPSG codes (in Turkish).

How to use it

1. Tap a point on the map or enter latitude and longitude in decimal degrees. If your coordinates are in UTM or TM, open the "UTM / TM coordinates" tab and enter the easting (E), northing (N) and the zone or central meridian (CM).
2. Choose the date. It defaults to today. To update the declination printed on an old topographic sheet, simply calculate it for today's date. The model is valid only for 2025-2030 and the tool warns you about dates outside that window.
3. Enter the ellipsoidal height in metres if needed. For work near the ground, leaving it at zero is sufficient.
4. Select the grid system (UTM 6° or TM 3°). The results list the declination, its annual change, inclination, total field, grid convergence and grid-magnetic angle. Type a compass reading into the converter below to get the true and grid bearings in degrees and gon.

Use in surveying and fieldwork

Although traverse and detail surveys now rely mostly on GNSS and total stations, the compass is still used for reconnaissance, preliminary site studies, forestry and mining fieldwork, and geological strike and dip measurements. Before a compass bearing is plotted, both the declination and the grid convergence have to be applied. Applying the declination alone gives a true bearing; when you work in projected coordinates you need the grid-magnetic angle. The declination and annual change printed on older topographic maps belong to the year the sheet was produced, so enter the coordinates of the sheet centre to get a current value. The same correction applies when solar panel rows are aligned to the south with a compass; for panel tilt and sun angles see the Sun Position and Panel Tilt tool, and to compute the bearing between two points from their coordinates use the Azimuth Calculator.

Data and method

The calculation uses the World Magnetic Model 2025 (WMM2025), produced by the US National Oceanic and Atmospheric Administration's National Centers for Environmental Information (NOAA NCEI) and the British Geological Survey (BGS). The model consists of spherical harmonic coefficients of degree and order 12 with epoch 2025.0, valid until 2030.0, and is in the public domain. The coefficients are taken unchanged from the official WMM.COF file published by NCEI. The implementation reproduces all 12 points of the official WMM2025 test value table within the table's resolution (0.01° and 0.1 nT). Declination uncertainty follows the error model in the WMM2025 report. Grid convergence and UTM/TM coordinates are computed on the WGS84 ellipsoid with the Krüger series (Karney, 2011). Icons come from Lucide (ISC licence); basemaps are provided by Esri and OpenStreetMap contributors. All calculations run in your browser and your coordinates are not sent to any server; only map tiles are loaded from their providers. Open-source libraries used and their licences: Data Sources.

Accuracy limits

The WMM represents only the main field generated in the Earth's core and its slow change. Magnetised crustal rocks, iron ore deposits and volcanic areas can change local declination noticeably, and such local anomalies are not in the model. External variations such as magnetic storms are not included either. According to the WMM2025 report, the global average declination error is about 0.36° at the start of 2025 and about 0.40° at the start of 2030; the tool also shows the model's own uncertainty estimate for each point. In the field, vehicles, steel structures, power lines, pipes and electronic devices disturb compass readings. Near the magnetic poles the horizontal field becomes weak and compasses are unreliable; the tool issues a caution when the horizontal field drops below 6000 nT and a blackout-zone warning below 2000 nT. For engineering work that needs a precise direction, use GNSS or an astronomical azimuth.