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Precise Geodetic Infrastructure: National Requirements for a Shared Resource
Drift (1)—Drift refers to effects on the measurements that change with time in a detectable pattern, for reasons unrelated to the phenomenon under study. Detecting a drift is often a useful way to identify a source of systematic errors.
Drift (2)—Drift also refers to relative rotation, translation, and scale between different reference frames resulting in different velocities between stations given in each frame.
Earth gravitational model, 2008 (EGM2008)—The latest high-resolution global geoid height model, released by the NGA in 2008.
Earth orientation—Wobble and nutation of the Earth’s rotation axis.
Earth rotation—The rotation of the Earth on its rotation axis. In geodesy, Earth rotation refers specifically to the perturbation of the rotation rate, which leads to variations in the length of day.
Earth tide—Tides in the solid Earth that are analogous to ocean tides, but of smaller amplitude.
Earthquake cycle—The cycle of strain accumulation on faults followed by rapid release during an earthquake.
EarthScope—An NSF program (with the USGS and NASA as partners) aimed at understanding the structure and evolution of the North American continent.
Elevation—The height of a location above some reference surface (such as the geoid). The elevation of a point is normally the same as its orthometric height (see Height).
Ellipsoid, reference—A reference ellipsoid is an ellipsoid of specified dimensions that is associated with a geodetic reference system or a geodetic datum. Coordinates given in this system are said to be “with respect to the reference ellipsoid” (NGS, 2010). Detailed definitions of ellipsoid can be found on the National Geodetic Survey Website: http://www.ngs.noaa.gov/CORS-Proxy/Glossary/xml/NGS_Glossary.xml
Ellipsoid height—See Height.
El Niño-Southern Oscillation (ENSO)—Disturbances in the ocean temperature (El Niño) and atmospheric pressure (Southern Oscillation) with a 3–7 year cycle, having important consequences for global weather and climate.
Envisat—A European Space Agency Earth-observing satellite for gathering information about the Earth’s land, water, ice, and atmosphere.
Ephemeris—A table of values, relative to a specified coordinate system, giving the position of objects in orbit as a function of time (plural: ephemerides).
Epoch—A moment in time used as a reference for a model that has time dependence.
Error—In general, the scientific term “error” (as opposed to a simple “mistake”) is intended to measure, and sometimes explain, the difference between an observed or calculated estimate of a quantity and the (usually unknown) true value of that quantity. Related terms are provided below. A detailed definition and descriptions of “error analysis” can be found, for instance, on the National Geodetic Survey Website: http://www.ngs.noaa.gov/CORS-Proxy/Glossary/xml/NGS_Glossary.xml
Random error—a statistical quantity that measures how repeated measurements of the same quantity by a single observer or multiple observers yield slightly different results.
Systematic error—the effect on the result of a measurement caused by a flaw in the measuring instrument or the measuring procedure. Systematic errors (labeled as “biases”) can be detected by comparing the outcome of measurements made using completely different instruments or experimental procedures. In that case, they usually can be eliminated by applying a correction procedure to the measured values.
Etalon—A Russian family of passive geodetic satellites (Etalon-I and Etalon-II) dedicated to satellite laser ranging.
European Remote Sensing (ERS)—Satellites (ERS-1 and -2) of the European Space Agency that perform a variety of measurements for Earth monitoring.