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How do geodesists determine the position of a control point on Earth?

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Diagram showing the deflection of the vertical from the geoid to the ellipsoid or spheroid, with labeled lines for geoid, spheroid, rock surface, ocean, and geoid height in a geology context. — geoid Deflection of the vertical from the geoid to the spheroid.

Geodesists determine the position of a control point on Earth by integrating classical land-based surveying techniques with modern satellite-based methods to account for the planet's complex shape and gravitational irregularities.

Classical Methods

Historically, geodesists relied on triangulation, a method involving the measurement of angles and base lines to establish horizontal positions. Because the geoid—the equipotential surface representing mean sea level—is irregular due to gravity anomalies, surveyors use a regular mathematical ellipsoid as a reference surface. Vertical control is established through geodetic leveling, where spirit levels are used to determine elevations relative to the geoid.12345

Modern Positioning

Today, these networks are refined using satellite geodesy:
* Satellite Ranging: Laser stations measure travel times to satellites to fix ground positions and track continental drift.4
* Doppler Effect: Radio signals from satellites are used to calculate an observer's position based on frequency shifts.6
* Global Integration: These methods allow geodesists to tie local networks into a single world datum, providing precise latitude, longitude, and elevation for control points.42