Definition
A mathematical representation of the Earth's shape, size, and magnetic field used by avionics systems — particularly GPS, FMS, and inertial reference units — to compute position, track, and magnetic variation. The earth model includes a reference ellipsoid (such as WGS-84) for geographic position and a magnetic model (such as the World Magnetic Model) for converting between true and magnetic headings.
Plain English
A digital map of the Earth's shape and magnetic field built into avionics. The system uses it to figure out where you are and how true north relates to magnetic north at that location.
Context Anchor
Seen in instrument procedure, GPS, RNAV, database, and magnetic variation discussions, especially where true direction, magnetic direction, and stored position data must match correctly.
Derivation
“Earth” means the planet. “Model” comes from the idea of a pattern or representation of something real. Here, it does not mean a small physical globe; it means a calculated representation of the Earth that navigation systems can use.
Why Pilots Care
The Earth's magnetic field drifts over time, so the magnetic portion of the earth model must be periodically updated. If your avionics carry an outdated earth model, the magnetic variation it applies can be wrong, causing small but real differences between your displayed heading or course and what's published on current charts.
Grounding Statement
A navigation system needs a clean mathematical version of the Earth before it can turn real-world locations into usable courses and distances.
Intuition Check
Do not read “earth model” as a physical model of the planet. Here it means the mathematical Earth reference used for navigation calculations.
Example Sentence 1
When the FMS database was updated, the new earth model corrected a small magnetic variation error that had crept in over the past few years.
Example Sentence 2
When the earth model changes, the computed magnetic variation at a given airport can shift by several degrees.