What Is Geodesy?

Geodesy is the science of measuring and understanding three fundamental properties of the Earth: its geometric shape, its orientation in space, and its gravity field, along with how those properties change over time. It provides the precise reference frameworks on which all geographic information, navigation systems, and spatial data products depend. Without a common geodetic reference, maps from different surveys cannot be overlaid, positioning signals from different satellite constellations cannot be combined, and physical measurements of sea level change, tectonic deformation, or ice sheet thickness cannot be compared across time.

NOAA defines geodesy as the discipline that assigns coordinate systems to Earth's surface, giving every location a unique, unambiguous reference address. Historically, geodesists worked with ground-based instruments such as theodolites, precise angle-measuring devices mounted on tripods, to triangulate positions between benchmarks across continents. Modern geodesy has shifted to space-based techniques that achieve global coverage and centimeter-level accuracy.

Reference Frames and Earth Models

Geodesy depends on mathematical models that represent the Earth's shape with sufficient accuracy for the application at hand. The ellipsoid is the basic model: an oblate spheroid flattened at the poles by a ratio of about 1:298 in the standard World Geodetic System 1984 (WGS84) reference ellipsoid used by GPS. The geoid, a more complex surface defined by gravitational equipotential corresponding to mean sea level, departs from the ellipsoid by up to 100 meters at specific locations. Heights above sea level are defined relative to the geoid, while GPS positions are reported relative to the ellipsoid, requiring geoid models to convert between the two.

The International Terrestrial Reference Frame (ITRF), maintained by the International Earth Rotation and Reference Systems Service, provides the most precise global coordinate system, with station coordinates determined to sub-centimeter accuracy and updated to account for tectonic plate motion and post-glacial rebound.

Measurement Techniques

Modern geodetic measurement uses four primary space techniques. The Global Navigation Satellite Systems (GNSS), which includes the US GPS, European Galileo, Russian GLONASS, and Chinese BeiDou constellations, determine positions by measuring the travel time of signals from four or more satellites simultaneously. Satellite Laser Ranging (SLR) bounces laser pulses off retroreflector-equipped satellites to measure orbital arcs with millimeter precision. Very Long Baseline Interferometry (VLBI) correlates radio signals from distant quasars received at widely separated antennas to establish a celestial reference frame and measure Earth's rotation. The DORIS system uses Doppler tracking of microwave beacons on the ground to determine satellite orbits.

NASA's Space Geodesy Project operates new geodetic stations that combine GNSS, SLR, VLBI, and DORIS receivers at the same site, enabling the four techniques to be tied together in a consistent frame.

Satellite Geodesy and Applications

Dedicated gravity-mapping satellite missions have transformed the measurement of Earth's gravity field. The GRACE (Gravity Recovery and Climate Experiment) mission tracked changes in the distance between two co-orbiting satellites to detect month-to-month variations in mass distribution caused by shifts in groundwater, ice sheets, and ocean currents. NASA's GRACE mission data have been used to document groundwater depletion in aquifers across the American West, northern India, and the Middle East.

Applications

Geodesy has applications across a wide range of disciplines and industries, including:

• Navigation and positioning systems, from aircraft guidance to autonomous vehicle operation
• Shoreline mapping and sea level monitoring for coastal management and flood risk assessment
• Tectonic monitoring to detect fault movement and assess earthquake and volcanic hazard
• Precision agriculture using GPS-guided equipment for field operations
• Infrastructure construction requiring precise alignment over long distances