Geodesy is the scientific discipline concerned with the study of the Earth's geometric shape, dimensions, gravity field, and their temporal variations. In its classical form, geodesy was defined by F.R. Helmert in 1880 as the "science of the measurement and mapping of the Earth’s surface." This definition, which also includes the Earth's external gravity field and the ocean floor, has retained its relevance. In modern terms, it has been extended to include the temporal variations of the Earth and its gravity field. The primary objectives of geodesy are, on the one hand, to determine the positions of points on the Earth’s surface and their changes with high precision, and on the other hand, to study the Earth’s shape, size, and gravity field, as well as geodynamic phenomena.

Classifications

By Disciplinary Focus

Ellipsoidal Geodesy (Theoretical Geodesy): Studies the Earth as a whole, its shape, dimensions, and gravity field, and establishes geodetic reference systems.
Geodetic Control Surveying: Measures the coordinates and elevations of surface points across one or more countries to establish national geodetic networks.
Marine Geodesy: Establishes geodetic control networks over oceanic areas to enable surface and subsurface positioning, and to measure the marine gravity field, sea surface topography, and marine geoid.
Engineering Geodesy: Conducts detailed surveys over localized areas, typically referencing the horizontal plane.

By Spatial–Temporal Characteristics

Geometric Geodesy: Investigates the Earth's shape and size using geometric methods and projects terrestrial networks onto a reference ellipsoid.
Physical Geodesy: Focuses on the Earth’s external gravity field and establishes physical models of the geoid.
Dynamic Geodesy: Studies Earth’s regional and global motion and interprets them through time-varying positional and gravity measurements.
Integrated Geodesy: Unifies geometric and physical measurements within a single spatio-temporal reference model.

By Technical Method

Terrestrial Geodesy: Uses optical and electronic instruments for triangulation, leveling, and astronomical measurements over short distances.
Space Geodesy: Employs satellite and extraterrestrial observations for absolute and relative positioning and gravity measurements.
Inertial Geodesy: Uses the principles of inertia for relative positioning and gravity field analysis.

A modern geodetic system has emerged centered on space geodesy, providing more accurate and comprehensive data than traditional methods. It has expanded geodesy's role in socio-economic applications and solidified its foundation in the geosciences.

Applications

Topographic Mapping, Engineering, and Transportation

Geodetic networks ensure control over mapping errors, maintain unified coordinate systems, and enable the projection of ellipsoidal positions onto planar surfaces. Modern GNSS systems such as GPS allow for point positioning within minutes with centimeter-level accuracy, facilitating rapid and precise mapping. In engineering, geodesy provides the control frameworks necessary for design, construction, and post-construction deformation monitoring. In transportation, geodetic technologies improve navigational accuracy and safety, reducing traffic accidents and improving efficiency.

Space Technology

Geodesy supports space missions through the provision of precise geodetic coordinate systems and gravity models. The positions of ground stations (e.g., TT&C stations) and the orbital determination of spacecraft rely on high-precision geodetic data. The Earth’s gravity model, particularly for low Earth orbit satellites, is essential for accurate orbit prediction and control.

Geoscience

Modern geodesy contributes significantly to plate tectonics and crustal deformation research. Techniques such as VLBI, SLR, and GPS enable direct observation of plate movements and the validation of tectonic models. Geodesy also aids in understanding variations in Earth rotation and polar motion, which are related to the planet's internal structure and dynamics. These data help refine models of Earth’s elasticity, fluid core dynamics, and angular momentum exchanges.

Resource Development and Environmental Monitoring

Geodesy plays a vital role in resource exploration and exploitation, especially for offshore oil and gas fields. GPS and sonar techniques enable 3D marine control networks and subsea topographic mapping. Satellite altimetry and gravity measurements contribute to the creation of marine geoid models. Furthermore, geodetic techniques are crucial for monitoring global environmental changes such as sea-level rise and glacial melt, which are linked to climate change. Programs like GOCE and CryoSat-2 provide critical data for assessing these phenomena.

Disaster Prevention and Mitigation

Geodesy supports disaster management by enabling monitoring of crustal deformations and early warning of natural hazards such as earthquakes and subsidence. Accurate geodetic observations contribute to risk assessment and the implementation of protective measures.

Geodesy