Glossary
Geodetic Coordinates (Geodetic Position)
Geodetic Height (Ellipsoidal Height, h)
An accuracy figure representing the stated percentage of probability that any point expressed as a function of two linear components (e.g., horizontal position) will be within the given circle. Commonly used are CEP (50 percent), CE 1-sigma (39.35 percent), and CE (90 percent).
A statement of accuracy based on a statistic whose distribution function is known; e.g., the normal distribution function or bivariate normal distribution function. Errors are stated as some percentage of the total probability of 100 percent; e.g., a 90 percent assurance level. Also called assurance level; error interval; probability interval.
Linear or angular quantities that designate the position that a point occupies in a given reference frame or system. Also used as a general term to designate the particular kind of reference frame or system, such as Cartesian coordinates or spherical coordinates.
Any numerical or geometrical quantity or set of such quantities specifying the reference coordinate system used for geodetic control in the calculation of coordinates of points on the earth. Datums may be either global or local in extent. A local datum defines a coordinate system that is used only over a region of limited extent. A global datum specifies the center of the reference ellipsoid to be located at the earth's center of mass and defines a coordinate system used for the entire earth.
Vertical distance measured along the local plumb line from a vertical datum, usually mean sea level or the geoid, to a point on the earth.
The surface generated by an ellipse rotating about one of its axes. Also called ellipsoid of revolution.
The line of zero geodetic latitude; the great circle described by the semi-major axis of the reference ellipsoid as it is rotated about the semi-minor axis.
A variation of a particular map projection where the center of the projection is located on the equator.
A surface with the same potential, usually gravitational potential, at every point; a level surface.
Cartesian coordinates (X, Y, Z) that define the position of a point with respect to the center of mass of the earth.
Geodetic Coordinates
(Geodetic Position)
The quantities of latitude, longitude, and geodetic height (f, l, h) that define the position of a point on the surface of the earth with respect to the reference ellipsoid.
Geodetic Height (Ellipsoidal
Height, h)
The height above the reference ellipsoid, measured along the ellipsoidal normal through the point in question. The geodetic height is positive if the point is outside the ellipsoid.
The angle between the plane of the Equator and the normal to the ellipsoid through the point in question. Geodetic latitude is positive north of the equator and negative south of the Equator.
The angle between the plane of a meridian and the plane of the prime meridian. A longitude can be measured from the angle formed between the local and prime meridians at the pole of rotation of the reference ellipsoid, or by the arc along the Equator intercepted by these meridians.
The equipotential surface of the earth's gravity field approximated by undisturbed mean sea level of the oceans. The direction of gravity passing through a given point on the geoid is perpendicular to this equipotential surface.
The distance between the geoid and the mathematical reference ellipsoid as measured along the ellipsoidal normal. This distance is positive outside, or negative inside, the reference ellipsoid. Also called geoidal height; undulation of the geoid.
A plane-rectangular coordinate system usually based on, and mathematically adjusted to, a map projection in order that geodetic positions (latitudes and longitudes) may be readily transformed into plane coordinates and the computations relating to them may be made by the ordinary methods of plane surveying.
A horizontal datum specifies the coordinate system in which latitude and longitude of points are located. The latitude and longitude of an initial point, the azimuth of a line from that point, and the semi-major axis and flattening of the ellipsoid that approximates the surface of the earth in the region of interest define a horizontal datum.
A one-dimensional error (such as an error in elevation) defined by the normal distribution function. Stated as a percentage of the probability that any point expressed as a function of a single linear component will be along the given line. Commonly used are Linear Standard Error or 1-sigma Error (68.27 percent), and LE (90 percent).
A function relating coordinates of points on a curved surface (usually an ellipsoid or sphere) to coordinates of points on a plane. A map projection may be established by analytical computation or, less commonly, may be constructed geometrically.
The ratio between a distance on a map and the corresponding actual distance on the earth's surface.
The average height for the surface of
the sea for all stages of the tide, used as a reference for elevations. Also called Sea Level Datum.
A north-south reference line, particularly a great circle through the geographical poles of the earth, from which longitudes and azimuths are determined; or the intersection of a plane forming a great circle that contains both geographic poles of the earth, and the ellipsoid.
An elevation or height referenced to mean sea level.
A variation of a particular map projection where the center of the projection is located neither at a pole, nor on the equator.
The distance of a point from the geoid measured along the direction of gravity at that point, with heights of points outside the geoid being treated as positive.
A line on the earth, or a representation thereof, which represents the same latitude at every point.
A variation of a particular map projection where the center of the projection is located at a pole.
A meridian from which the longitudes of all other
meridians are reckoned. This meridian, of longitude 0°, was traditionally
chosen to pass through the Greenwich Observatory in
An ellipsoid, usually a bi-axial ellipsoid of revolution, whose dimensions closely approach the dimensions of the geoid; the exact dimensions are determined by various considerations of the section of the earth's surface concerned.
A multiplier for reducing a distance in a map projection to the actual distance on the chosen reference ellipsoid.
The radius of a sphere within which there is a specified probability of locating a point. Spherical error is the three-dimensional analog of circular error. Commonly used are Spherical Error Probable (50 percent), and Spherical Error (90 percent).
A vertical datum is the surface to which elevations are referenced. A local vertical datum is a continuous surface, usually mean sea level, at which elevations are assumed to be zero throughout the area of interest.