Geodetic Datum and Geodetic Control Networks - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 71

Fig. 3.1 Outside
appearance of China's
geodetic origin.
Source: SBSM
building, and the projection platform. The instrument platform was set in the
observation room on the top floor of the seven-storey building which is 26 m
above the ground. The top of the building is a semicircular dome made of fiber-
reinforced plastic (FRP) and automatically controlled by electric power for celestial
observation (cf. Fig. 3.1 ). The survey mark at the center of the geodetic origin is set
in the basement center of the main building, which is made of red agate with a
diameter of 10 cm, and is delicate and durable (Fig. 3.2 ).
The Horizontal Datum
The horizontal datum provides the basis for establishing the national geodetic
coordinate system and for calculating the geodetic coordinates of each point in
the national horizontal control network. It includes a set of initial data, i.e., the
geodetic longitude and latitude of the initial point and the geodetic azimuth from
the initial point to its adjacent point in the national geodetic control network. (The
initial point is the geodetic origin in classical geodetic survey.)
The extension of the horizontal datum is realized by the horizontal control
network formed by a series of control points. Coordinates of the control points
are computed from the geodetic origin and obtained by classical geodetic methods
such as traversing, triangulation, and so on. In modern geodetic survey, the hori-
zontal datum is usually realized by 3-D datum obtained from the GPS method
(cf. Sect. 3.3 ).