Chinese English

A collection of DATA in a common location, relating to a given set of subjects. Usually comprised of several DATA BASES considered as an entity.

An organised collection of DATA stored so as to be capable of use by relevant applications with the DATA being accessed by different logical paths. It should be application independent.

The line coinciding approximately with the 180th MERIDIAN, at which each calendar day first begins, the BOUNDARY between the -12 and +12 TIME ZONES. The date on each side of this line differs by one DAY, but the TIME is the same in these two ZONES. Sometimes called calendar line or international date line.

Any numerical or geometrical quantity or set of such quantities which may serve as a reference or base for other quantities. In geodesy a datum is defined by a set of parameters specifying the reference surface or the reference coordinate system. Therefore two types of datums are required: a horizontal datum which forms the basis for computations of horizontal control surveys in which the curva­ture of the earth is considered, and a vertical datum to which elevations are referred.

A permanently established surface from which SOUNDINGS or tide heights are referenced, usually LOW WATER. Also called DATUM, datum level, DATUM PLANE, hydrographic datum, reference level, reference plane. See DATUM: TIDAL.

A set of parameters specifying the reference surface or the reference coordinate system used for geodetic control in the calculation of coordinates of points on the earth. Commonly datums are defined as horizontal and vertical datums separately. For a local geodetic datum the reference surface is defined by five parameters: the LATITUDE and LONGITUDE of an INITIAL POINT, the AZIMUTH of a line from this point, and the parameters of the REFERENCE SPHEROID. Absolute datums specify the INITIAL POINT of the REFERENCE ELLIPSOID to be (ideally) located at the earth's centre of mass. For modern reference systems using datum information given by satellites additional parameters are defined, e.g. gravity models. See also HORIZONTAL CONTROL DATUM.

描述在地球表面点位坐标计算中用作大地测量控制的参考表面或者参考坐标系统的一系列参数。通常大地基准分为水平基准和垂直基准。对于地方大地基准，参考面由5个参数定义：起始点的经度和纬度，自起始点出发的某一直线的方位和参考椭球体的两个参数。绝对基准的参考椭球体的起点位于地球质心。在现代参考系统中，利用卫星观测提供的基准信息定义了一些其他的参数，例如重力模型。亦可参见horizontal control datum（水平控制基准）。

Any LEVEL SURFACE (e.g., MEAN SEA LEVEL) taken as a surface of reference from which to reckon ELEVATIONS. Also called datum level, reference level, reference plane, levelling datum, datum for heights. See also DATUM PLANE.