# UNCLASSIFIED

Glossary

Circular Error (CE)

Confidence Interval

Coordinate

Datum

Elevation

Ellipsoid

Equator

Equatorial Aspect

Equipotential Surface

Geocentric Coordinates

Geodetic Coordinates (Geodetic Position)

Geodetic Height (Ellipsoidal Height, h)

Geodetic Latitude (f)

Geodetic Longitude (l)

Geoid

Geoid Separation (N)

Grid Reference System

Horizontal Datum

Linear Error (LE)

Map Projection

Map Scale

Mean Sea Level (MSL)

Meridian

MSL Height

Oblique Aspect

Orthometric Height

Parallel

Polar Aspect

Prime Meridian

Reference Ellipsoid

Scale Factor (Projection)

Spherical Error (SE)

Vertical Datum

Circular Error (CE)

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).

Confidence Interval

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.

Coordinate

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.

Datum

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.

Elevation

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.

Ellipsoid

The surface generated by an ellipse rotating about one of its axes. Also called ellipsoid of revolution.

Equator

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.

Equatorial Aspect

A variation of a particular map projection where the center of the projection is located on the equator.

Equipotential Surface

A surface with the same potential, usually gravitational potential, at every point; a level surface.

Geocentric Coordinates

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.

Geodetic Latitude (f)

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.

Geodetic Longitude (l)

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.

Geoid

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.

Geoid Separation (N)

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.

Grid Reference System

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.

Horizontal Datum

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.

Linear Error (LE)

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).

Map Projection

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.

Map Scale

The ratio between a distance on a map and the corresponding actual distance on the earth's surface.

Mean Sea Level (MSL)

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. Mean Sea Level and the surface of the geoid are often assumed to coincide though in reality they are approximations to one another and can be offset by meters in some locations. GEOTRANS computes orthometric heights. However, because the MSL surface and the geoid surface are good approximations to each other in many locations, the more commonly used term MSL height is used to refer to these heights.

Meridian

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.

MSL Height

An elevation or height referenced to mean sea level.

Oblique Aspect

A variation of a particular map projection where the center of the projection is located neither at a pole, nor on the equator.

Orthometric Height

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.

Parallel

A line on the earth, or a representation thereof, which represents the same latitude at every point.

Polar Aspect

A variation of a particular map projection where the center of the projection is located at a pole.

Prime Meridian

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 Greenwich, England. For new refined coordinate systems, the location of the prime meridian is defined by the International Earth Rotation Service, Paris, France.

Reference Ellipsoid

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.

Scale Factor (Projection)

A multiplier for reducing a distance in a map projection to the actual distance on the chosen reference ellipsoid.

Spherical Error (SE)

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).

Vertical Datum

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.

# UNCLASSIFIED