3.01 – Overview

Geodesy is the science of measuring the size and shape of the Earth. Its history dates back to Eratosthenes (276–196 BC) who first determined the first radius of the Earth. In the same era Hipparchus (190–120 BC) introduced the concepts of lines of latitude and longitude and started one of the three pillars of modern geodesy namely the measurement of the geometric shape of the Earth. The second pillar of geodesy, the gravity field of the Earth, was not measured quantitatively until the 1600s with Galileo Galilei (1564–1642) taking up the challenge of measuring gravity. The third pillar of geodesy, the rotation of the Earth, has been measured since ancient times through the precession of the equinoxes, which occurs because the direction of rotation axis in space changes. The measurement of changes in the rotation axis relative to the crust of the Earth did not happen until much later (1800s). Measurements of changes in the rate of Earth’s rotation were also made in these later times. The chapters in this volume cover all aspects of these pillars of geodesy. Despite the long history of geodesy, two major developments in the mid-1900s thrust geodesy into new measurement accuracies and capabilities: the development of methods that allowed distance measurements to be measured using light traveltimes, and the launch of Earth-orbiting satellites. Both of these developments allowed a truly global view on measurement of the Earth. Perturbations in the orbits of the satellites provide a method for determining the Earth’s gravity field and its time variations. Measurement of distances and their time derivatives from and to these satellites allow global positions and rotation vector measurements. The measurements can also be applied to bodies outside of near-Earth orbit giving direct measurement of solar system bodies and extragalactic objects both of which allow measurements in an inertial reference frame. This volume of the treatise explores and describes the theory, instrumentation, and results from modern geodetic systems. The chapters in this volume cover the mathematics of the gravity field of the Earth and instrumentation for measuring the gravity field that are so sensitive that people moving near the instruments affect the results. The measurement and the results obtained from variations in the rotation of the Earth that have led to measurement of the flattening of the fluid core, and the influence of the world’s ocean and atmosphere on the rotation of the Earth are covered in sections on short- (nearly diurnal) and long-period rotation changes. Temporal variations in the gravity field are covered in sections that discuss tidal analysis, intermediate periods of months to years that can sense changes in water table levels around the world, and long-period changes that reveal residual effects of the ice age that ended tens to thousands of years ago. Position variations that reveal, among other things, plate tectonic motions, earthquake motions, and loading effects from the atmosphere and hydrosphere are covered in chapters that examine space geodetic methods such as very long baseline interferometry, satellite laser ranging, the global positioning system, and interferometric synthetic aperture radar.