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.

Radiosonde—Weather balloon.

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.

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