Accuracy—The closeness of an estimated value (that is measured or computed) to a standard or accepted (true) value of a particular quantity. Strictly, it only applies to absolute physical quantities, such as distance between stations, but this report also uses it to mean accuracy of station position within a reference frame (internal accuracy). Precision contributes to accuracy, but accuracy also takes into account systematic biases arising from calibration errors or imperfect observation models. Accuracy can be assessed if there is a superior measurement technique that can be used as a standard, but since geodesy uses the highest-accuracy techniques, accuracy estimation is not straightforward for geodesy. Accuracy estimates for geodesy therefore typically involve an “error budget” analysis of systematic effects.
Horizontal accuracy—The positional accuracy of a dataset with respect to a specified horizontal datum (Maune, 2007).
Vertical accuracy—The positional accuracy of a data set with respect to a specified vertical datum (Maune, 2007).
Advanced Land Observing Satellite (ALOS)—A remote-sensing satellite of the Japanese Aerospace Exploration Agency.
Aircraft Laser Mapping (ALM)—Aircraft-borne laser instrumentation (such as LiDAR) for making maps of the Earth’s surface.
Altimetry—A technique for measuring the height of the Earth’s solid surface, oceans, or glaciers and ice sheets from space (satellite altimetry) or aircraft (airborne altimetry).
Aquifer—A large zone beneath the water table that stores groundwater.
Argo—An array of thousands of drifting floats for taking ocean temperature and salinity profiles.
Autonomous navigation—Vehicular navigation from one point to another without the assistance of a driver (for example, autopilot).
Base Flood Elevation (BFE)—The computed elevation to which floodwater is anticipated to rise during the base flood (also known as a one percent annual chance flood and a 100-year flood) (FEMA, 2003).
Bathymetry—The underwater depth of the ocean floor.
Bench mark—A permanent monument established by any federal, state, or local agency, whose location and/or elevation are referenced to a specified datum.
Carrier frequency—The frequency used by a radio signal to carry information and to which a receiver must be precisely tuned to isolate that signal from the radio signals at other frequencies.
Celestial reference frame—The inertial (un-accelerated) non-rotating reference frame associated with the distant stars.
Co-location—Two or more geodetic techniques or systems occupying simultaneously or subsequently very close locations.
Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC)—A joint Taiwan/U.S. mission providing atmosphere profiles using GPS occultation measurements.
Continuously Operating Reference Stations (CORS)—A NGS-coordinated network of GNSS receivers to support positioning activities throughout the United States and its territories.
Coordinates—A set of N numbers designating the location of a point in N-dimensional space. Horizontal coordinates are two-dimensional coordinates, normally expressed as x, y coordinates, eastings and northings, or longitude and latitude (geographic coordinates).
Coordinated Universal Time (UTC)—A modern continuation of Greenwich Mean Time, the standard “clock time.”
Corner cube—A combination of reflecting surfaces that always reflect light parallel to the incoming direction.
Crustal deformation—The deformation of the Earth’s crust in response to stress.
Crustal Dynamics Data Information System (CDDIS)—A NASA system for space geodetic data archiving and distribution.
Cryosphere—The Earth’s glaciers and ice sheets.
Datum—A set of constants specifying the coordinate system used for geodetic control (i.e., for calculating coordinates of points on the Earth).
Horizontal datum (geometric reference frame)—A geodetic datum specifying the coordinate system in which horizontal control points are located. The North American Datum of 1983 (NAD83) is the official horizontal datum of the United States. For horizontal datums, at least eight constants are needed to form a complete datum: three to specify the location of the origin of the coordinate system, three to specify the orientation of the coordinate system, and two to specify the dimensions of the reference ellipsoid (NRC, 2007b).
Mean sea level—A tidal datum computed as the arithmetic mean of hourly heights observed over a specific 19-year Metonic cycle. Shorter series are specified by name (for example, monthly mean sea level, yearly mean sea level).
Vertical datum—A set of constants defining a height (elevation) system containing a coordinate system and points that have been consistently determined by observations, corrections, and computations. The North American Vertical Datum of 1988 (NAVD88) is the official vertical datum of the United States.
Decadal Survey—The common name for the National Research Council report Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond (NRC, 2007a).
Deformation, Ecosystem Structure and Dynamics of Ice (DESDynI)—A proposed NASA InSAR and LiDAR mission optimized for studying hazards and global environmental change.
Distributed Active Archive Centers (DAAC)—NASA centers for archiving, documenting, and distributing data from past and current Earth-observing satellites and field measurement programs.
Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS)—A French geodetic technique in which transmitters on the ground communicate with receivers on satellites to provide precise orbit determination required by ocean altimeter satellites.
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.
Floodplain—Any land area that is susceptible to being inundated by water from any source (FEMA, 2003).
Fundamental station—A core geodetic ground station with at least one geodetic VLBI telescope (ideally two), an SLR station (with some stations having LLR capability), at least three GNSS/GPS stations to provide local tie information and monitor site deformation, a DORIS beacon, terrestrial survey instruments to determine and monitor local ties to the millimeter level, a superconducting or, preferably, an absolute gravimeter, meteorological sensors, and a variety of other sensors such as seismometers, tiltmeters, and water vapor radiometers (Plag and Pearlman, 2009).
Galileo—A European Space Agency GNSS system in development.
Geodesy—The science of accurately measuring and understanding the Earth’s geometric shape, its orientation in space, its gravity field, and changes in these properties over time.
Geodynamics—The study of Earth’s internal forces (dynamics) and their impacts.
Geographic Information System (GIS)—A system of computer hardware, software, and procedures designed to support the capture, management, manipulation, analysis, modeling, and display of spatially referenced data for solving complex planning and management problems (FEMA, 2003).
Geoid—The equipotential (level) surface of the Earth’s gravity field, which is the best approximation to global mean sea level extended over the land. The geoid undulates up and down with local variations in the mass and density of the Earth (Maune, 2007).
Gigaton—One billion metric tons. One metric ton is 1000 kilograms, or approximately 2,205 pounds.
Glacial isostatic adjustment (also called post-glacial rebound)—The ongoing deformation of the Earth due to the rapid disappearance of the glaciers that built up during the last glacial cycle.
Glaciology—The science of glaciers.
Global Navigational Satellite System (GNSS)—General term for positioning systems like GPS, GLONASS, Galileo, and COMPASS.
Global Positioning System (GPS)—A satellite-based navigation and positioning system that enables horizontal and vertical positions to be determined (FEMA, 2003); a global navigation satellite system (GNSS) maintained and operated by the United States.
Gravimetry—The measurement of gravity.
Gravity (Normal)—Normal gravity is an idealized model of the Earth’s gravity, comprised of a simple latitude and height dependence.
Gravity anomaly—A measure of how actual gravity deviates from the idealized value of normal gravity. Maps of gravity anomalies reflect the changes in the Earth’s mass distribution, particularly in the crust.
Gravity for the Redefinition of the American Vertical Datum (GRAV-D)—A proposed NGS program to redefine the vertical datum of the United States with greater accuracy using airborne gravimetry.
Gravity Recovery and Climate Experiment (GRACE)—A NASA satellite system for measuring gravity changes on the Earth, caused mainly by the movement of water on or near the Earth’s surface.
Group on Earth Observations (GEO)—The intergovernmental coordinating effort to build a Global Earth Observation System of Systems (GEOSS).
Height—The distance, measured along a perpendicular, between a point and a reference surface (for example, the height of an airplane above the ground surface). Also, the distance, measured upward along a plumb line (line of force), between a point and a reference surface of constant geopotential. Height is often called elevation if the reference surface is the geoid.
Ellipsoidal height—The height above or below the reference ellipsoid (the distance between a point on the Earth’s surface and the ellipsoidal surface, measured perpendicular to the ellipsoid). Also called geodetic height (NRC, 2007b).
Orthometric height (Elevation)—The height above the geoid as measured along the plumb line between the geoid and a point on the Earth’s surface, taken positive from the geoid.
Horizontal Datum—See Datum.
Hydrology—Study of the global, regional, or local movement, distribution, and quality of water.
Ice, Cloud, and Land Elevation Satellite (IceSAT)—A NASA satellite for measuring the elevation of the surface of glaciers and ice sheets.
Ice sheet—A very large body of ice extending across the land surface.
Interferometer—An instrument that measures differences between the phases of two electromagnetic signals originating from a common source that have traversed different paths. The phase differences are measured by combining the two signals. The amplitude of the combined signal is a function of the phase difference between the two signals. The phenomenon of fluctuations in the amplitude of the combined signals in response to phase changes in the input signals is sometimes referred to as interference (NRC, 2007b).
Interferometric Synthetic Aperture Radar (InSAR)—An airborne or spaceborne interferometer radar system, flown aboard rotary or fixed-wing aircraft or space-based platforms, used to acquire 3-dimensional coordinates of terrain and terrain features that are both man-made and naturally occurring. InSAR systems form synthetic aperture images of terrain surfaces from two spatially separated antennae over an imaged swath that may be located to the left, right, or both sides of the imaging platform (NRC, 2007b).
Interlacing—An agricultural technique whereby multiple crops are planted in the same field.
International Atomic Time (TAI)—A high-precision measurement of time as measured by atomic clocks. From the French, Temps Atomique International.
International DORIS Service (IDS)—An international service under the IAG for coordinating analysis and distribution of DORIS data and data products.
International Earth Rotation Service (IERS)—Serves the astronomical, geodetic and geophysical communities by providing data and standards related to Earth rotation and reference frames.
International GNSS Service (IGS)—An international service under the IAG for coordinating analysis and distribution of GNSS data and data products.
International Laser Ranging Service (ILRS)—International service under the IAG for coordinating analysis and distribution of laser ranging (LLR and SLR) data and data products.
International service—One of the geodetic services under the IAG: IAS, IDEMS, IDS, IERS, IGeS, IGFS, IGS, ILRS, IVS, and PSML.
International Terrestrial Reference Frame (ITRF)—The most accurate global reference frame for scientific and other applications.
International VLBI Service (IVS)—An international service under the IAG for coordinating analysis and distribution of VLBI data and data products.
Ionosphere—The layer of charged particles (ions) surrounding the Earth that can affect radio communications.
Ionospheric refraction—The delay and bending of a radiometric signal caused by free electrons in the ionosphere. Similar to the tropospheric refraction caused by the neutral atmosphere, except that the delay and bending are strongly frequency-dependent and are negligible at optical frequencies.
Jason—A series of joint NASA/CNES ocean altimetry missions (Jason-1 and Jason-2).
Laser Geodynamics Satellites (LAGEOS)—A series of NASA and joint NASA/ASI (Italian Space Agency) spherical laser reflecting satellites (LAGEOS 1 and LAGEOS 2).
Leap second—A one-second adjustment to international atomic time (TAI) (to produce Coordinated Universal Time (UTC)) to maintain its synchronization with the solar day.
Length of day—The exact amount of time (nominally 24 hours) it takes the Earth to rotate on its rotation axis; due to motions of mass on and within the Earth, the length of day continuously varies.
Leveling—The process of finding differences of elevation (NRC, 2007b).
Light Detection and Ranging (LiDAR)—An instrument that measures distance to a reflecting object by emitting timed pulses of light and measuring the time between emission and reception of reflected pulses. The measured time interval is converted to distance (NRC, 2007b).
Location-based services—Services (such as might be delivered to a mobile device) that are aware of and utilize the location of the user.
Low-Earth orbiting (LEO)—LEO satellites are used for radio occultation measurements of the atmosphere using GNSS.
Lunar Laser Ranging (LLR)—A geodetic technique in which a laser signal is transmitted from a ground-based station, reflects off specially designed mirrors (retro-reflectors) placed on the moon, and is received back at the station. LLR provides information about the moon’s orbit and rotation.
Magma—Molten (melted) rock that is generally beneath the Earth’s surface and is occasionally released by volcanoes.
Mass transport—The movement of mass within the Earth systems, including the atmosphere, oceans, cryosphere, and solid Earth.
Mean High Water (MHW)—Mean high water is a tidal datum computed as the arithmetic mean of the high-water heights observed over a specific 19-year Metonic cycle. For stations with shorter series, a comparison of simultaneous observations is made with a primary control tide station in order to derive the equivalent of the 19-year value (see Datums).
Mean sea level—See Datums.
Milligal—A milligal is about one millionth of the standard gravity acceleration on the Earth’s surface (one “g” or approximately 9.8 meters per second squared).
Monument or control monument (also called reference mark)—A structure that marks the location of a corner or point determined by surveying; generally, any material, object, or collection of objects that indicates the ground location of a survey station or corner (http://www.ngs.noaa.gov/CORS-Proxy/Glossary/xml/NGS_Glossary.xml).
Monumentation—The practice of marking known horizontal, vertical, gravity, or other control points with permanent structures, such as concrete pedestals and metal plaques. Once surveyed and marked, these monuments can be used for further surveying and for the alignment of land-parcel boundaries and infrastructure. Good monumentation for a geodetic observing site is where the antenna mounting is durable and stable, the site environment has minimal impact on the measurement signal, and the location of the reference point for each instrument can be precisely determined.
National Flood Insurance Program (NFIP)—The federal program under which flood-prone areas are identified and flood insurance is made available to property owners in participating communities (FEMA, 2003).
Navigation—The science of directing or commanding the movement of a vehicle or craft. U.S. government policies dealing with navigation distinguish it from real-time positioning in that navigation also encompasses a safety-of-life component.
Nutation—Small nodding oscillations of the Earth’s rotation axis in space.
Occultation—Passing behind another object. COSMIC uses GPS systems to measure the change in radio waves as a satellite sets behind the Earth from the perspective of another satellite, thereby yielding information on the Earth’s atmosphere.
One percent annual chance flood—A flood that has a one percent chance of being equaled or exceeded in any given year; also known as a 100-year flood and a base flood (FEMA, 2003).
Orthometric height—See Height.
Plate tectonics—The theory that explains many geophysical phenomena in terms of the motions of plates that cover the surface of the Earth.
Polar motion—The movement of Earth’s rotation axis relative to the crust.
Position—The location of a point on the surface of the Earth, expressed in terms of one of several coordinate systems. Examples are geographic position (latitude, longitude, and altitude); Universal Transverse Mercator (UTM) northing, easting, and height; or State Plane northing, easting, and height (NRC, 2007b).
Postglacial rebound—See glacial isostatic adjustment.
Postseismic—Occurring after an earthquake.
Precise orbit determination—The precise determination of the orbital position of a satellite by geodetic methods.
Precipitable water vapor (PWV)—A measure of the total amount of water in the atmosphere.
Precision—A measure of the repeatability of a measurement. In the context of this report, precision quantifies the ability to repeat the determination of a position within a reference frame (internal precision), and can be measured using various statistical methods on samples of estimated positions. Although precision does not imply accuracy, high precision is a prerequisite for consistently high accuracy, and is necessary to resolve changes in position over time. The precision of a reference frame itself (external precision) refers to the variation in the reference frame parameters (origin, orientation, and scale) that arise from statistical variation in the data used to define the frame.
Precision agriculture—Application of geodetic, remote-sensing, and geographical information management technologies to farming.
Preseismic—Occurring before an earthquake.
Quasars—The most distant and luminous objects in the universe; quasars emit radio waves that are used in the geodetic technique of VLBI.
Radar—Radio detection and ranging. An instrument for determining the distance and direction to an object by measuring the time needed for radio signals to travel from the instrument to the object and back, and by measuring the angle through which the instrument’s antenna has traveled (NRC, 2007b).
RADARSAT—A series of Canadian remote sensing satellites (RADARSAT-1 and RADARSAT-2).
Radio telescope—Parabolic radio dishes that are used in VLBI.
Reference frame—A set of three-dimensional Cartesian coordinates (x, y, z), and the rates of change of these coordinates over time, for a network of points on the Earth’s surface that defines the coordinates for other sites.
Reference system—The theories, models, and physical constants underlying a reference frame.
Remote sensing—A general term for systems that remotely collect data from an aircraft, spacecraft, satellite, buoy, or ship about an object or phenomenon on the surface of the Earth.
Retroreflector—An array of optical corner cubes.
Satellite Laser Ranging (SLR)—A geodetic technique in which a laser signal is transmitted from a ground-based station, reflects off specially designed mirrors (retro-reflectors) placed on satellites, and is received back at the station. SLR provides range tracking data for precision orbit determination of geodetic satellites.
Scale—A parameter that controls the distance between points in a network. In the context of mapping, scale is a number, constant for a given map, which represents the ratios of small distances on the map to the corresponding actual distances.
Sea level—In general, the reference elevation of the surface of the sea from which elevations are measured. This term is used as a curtailed form of mean sea level (see Datum) (NRC, 2007b).
Sea surface height—The spatially and temporally variable height of the sea surface.
Sea surface topography—Sea surface height.
SEASAT—The first satellite designed for remote sensing of the Earth’s oceans with synthetic aperture radar.
Shoreline—The boundary line between a body of water and the land, in particular, the boundary line between the water and the line marking the extent of high water or mean high water (Datum) (NRC, 2007b).
Space weather—The environmental conditions in near-Earth space, including the ionosphere.
Stability—Reference frame and station position predictability through time.
Standard—An agreed-upon procedure in a particular industry or profession that is to be followed in producing a particular product or result (NRC, 2007b). Alternatively, a number, or set of numbers, established in an industry, a science, or a technology, setting limits on the precision or accuracy with which operations, measurements, or products are to be made.
Starlette—A passive French satellite, launched in 1975, used in Satellite Laser Ranging, predominantly to measure the gravity field.
Stick-slip—Behavior (often of a fault) characterized by periods of sticking followed by periods of slipping.
Strain—A measure of deformation that occurs (in the Earth’s crust, for example) in response to applied forces.
Subduction—The process whereby one of the Earth’s tectonic plates flows beneath another plate.
Subsidence—Downward vertical motion of land.
Surface Water Ocean Topography (SWOT)—A proposed NASA/CNES satellite mission to make the first complete survey of Earth’s oceans and freshwater bodies.
Synthetic Aperture Radar—A radar containing a moving or scanning antenna; the signals received are combined to produce a signal equivalent to that which would have been received by a larger, stationary antenna (NRC, 2007b).
TOPEX-Poseidon—Joint NASA/CNES satellite altimeter for mapping ocean surface topography.
Topography—The form of the features of the actual surface of the Earth in a particular region, considered collectively; also called terrain (Maune, 2007).
Total Electron Content (TEC)—A measure of the density of free electrons per square meter as integrated along a path traced through the ionosphere, usually measured in TEC units, where 1 TEC = 1016 electrons / m2.
Troposphere—The lowest region of the atmosphere, containing almost all the water vapor.
UNAVCO—NSF- and NASA-sponsored organization that supports geodetic research in the United States.
Universal Time 1 (UT1)—Used to represent the Earth’s rotation, this nomenclature is left over from when time was determined by the Earth’s rotation rather than by atomic clocks.
Vertical Datum—See Datum.
Very Long Baseline Interferometry (VLBI)—A geodetic technique using large, ground-based, parabolic-dish radio telescopes to observe quasars (the most distant objects in the cosmos). VLBI sites provide information on the Earth’s rotation and the direction of the Earth’s spin axis.
Wobble—See polar motion.
World Geodetic System 1984 (WGS-84)—The latest version of the DoD World Geodetic System, which is consistent with ITRF at the centimeter level (but the ITRF is more accurate).