Magnetic Declination

Magnetic declination is the angle between true north and magnetic north, and it changes over time

A compass points toward magnetic north, while maps and geodesic bearings usually reference true geographic north. Declination provides the correction between those directions at a particular place and date.

The magnetic field changes over time and local anomalies can affect compasses. Critical navigation should use a current geomagnetic model, chart or official navigation source rather than an undated planning estimate.

When this tool is useful

Use Magnetic Declination when the question matches the method the page actually uses. Typical workflows include:

• Run the same workflow for locations in different countries while keeping units and assumptions consistent.
• Answer a one-off question about magnetic declination and keep the result tied to a visible map or explicit input.
• Compare two or more scenarios involving true north without creating an account or installing desktop software.

Worked examples

These examples are meant to show how to interpret this kind of output, not to replace the values produced from your own inputs.

• Eye height near sea level — With eyes about 1.7 m above sea level, the refracted-horizon estimate is roughly 5 km.
• 100 m viewpoint — At 100 m elevation above a clear horizon, the estimate increases to roughly 39 km.
• Antipode example — New York's antipodal coordinate is approximately -40.71, 105.99.

How to interpret the result

The most common errors come from using a correct result to answer the wrong geographic question. Keep these points in mind for Magnetic Declination:

• Include your height inputs explicitly (eye height, mast height, deck level) when sharing visibility results: the same hill answers differently for a child, an adult and a 10 m antenna, and unstated heights make numbers unreproducible.
• Treat climate and hardiness outputs as 30-year character, not this year's weather: a zone tells you what winters usually allow, and microclimates — walls, slopes, urban heat — shift a real garden half a zone either way.

Limits & appropriate use

Terrain intelligence carries one dominant limit: the model sees bare earth at ~90 m spacing with metres of vertical error, and is blind to canopies, walls and wires. Every downstream answer inherits it — profiles show terrain truth with noisy amplitude, line-of-sight verdicts are pending trees, horizons assume open ground, and solar radiation averages assume an unshaded panel. The second limit is temporal: climate letters and hardiness zones are thirty-year character, not this year's behaviour, and seismicity is a mood, not a hazard model. Each result here is printed with its recipe precisely so these boundaries travel with the number.

The value proposition is screening-grade truth at zero cost: orientation for sites, gardens, antennas, shoots and curiosity, with the upgrade points clearly marked. When money or safety attaches to the answer — tower siting, flood insurance, structural shading, avalanche terrain — the escalation is professional: licensed survey, certified shade studies, regulatory flood determinations and geological hazard services. A good free tool makes that ladder visible instead of pretending to be the top of it.

• Construction and tower siting → licensed topographic survey.
• Flood decisions → regulatory determinations (FEMA/FIRM in the US).
• Solar finance → certified shade and production studies.

Data & methodology

Elevation and radiation use Open-Meteo's open API on Copernicus/ERA5 datasets; visibility math adds standard refraction (k = 0.13). US hydrology/flood layers come from USGS and FEMA public services.

For source details, licences and model limitations, see Data Sources and Methodology.

Key terms worth knowing

Refraction coefficient

The standard 0.13 factor bending sightlines over the curvature.

Antipode

The diametrically opposite point; latitude flips, longitude shifts 180°.

Go deeper: guides & original studies

Use these resources when you need the reasoning behind the calculation, a comparison with another geographic method, or an example of the same approach applied across a dataset.

Related tools

If this page answers only part of your question, these tools use a nearby but different method or dataset.