Geodesy in the 21 st Century

From flat Earth, to round Earth, to a rough and oblate Earth, people's understanding of the shape of our planet and its landscapes has changed dramatically over the course of history. These advances in geodesy— the study of Earth's size, shape, orientation, and gravitational field, and the variations of these quantities over time—developed because of humans' curiosity about the Earth and because of geodesy's application to navigation, surveying, and mapping, all of which were very practical areas that benefited society. Today, geodesy is no different. The field is now concerned with changes in the shape of Earth's surface, because small detectable changes are associated with issues of great societal impact such as ice melting, sea level rise, land subsidence, and aquifer depletion. For example, the rate of polar ice melt may be estimated from combined satellite gravity and ground Global Positioning System (GPS) measurements. Global estimates of sea level changes are measured by altimetry satellites within entire ocean basins. Human-induced depletion of aquifers is reflected in subsidence measured by synthetic aperture radar (SAR) satellites. Twenty-first-century geodetic studies are dominated by geodetic measurements from space. Space geodesy uses a set of techniques relying on precise distance or phase measurements transmitted or reflected from extraterrestrial objects, such as quasars, the Moon, or artificial satellites. Early space geo-detic measurements, beginning in the 1980s, had accuracy levels between 5 and 10 centimeters. These measurements were conducted across the entire globe and yielded the first direct observations of tectonic plate motion. Further improvements to space geo-detic technologies have increased the accuracy to subcentimeter levels. Today, space geodetic observations can detect small movements of the Earth's solid and fluid surfaces as well as changes in the atmosphere and ionosphere. Hence, geodesy has many applications in a variety of fields extending well beyond its traditional role in solid Earth sciences (Figures 1 and 2). Geodesy, like many scientific fields, is technology driven. Over the centuries, it has developed as an engineering discipline because of its practical applications. By the early 1900s, scientists and cartographers began to use triangulation and leveling measurements to record surface deformation associated with earthquakes and volcanoes. For example, one of the most important geophysical discoveries, the basic understanding of earthquake mechanics known as the " elastic rebound theory " [Reid, 1910], was established by analyzing geodetic measurements before and after the 1906 San Francisco earthquakes. In 1957, the Soviet Union launched the artificial …