Geodetic Datum and Geodetic Control Networks - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 70

principles and methods for determining the horizontal coordinates and heights are
fundamentally different, we cannot use the same datums in determination. Con-
ventionally, the establishment of geodetic control networks, as a consequence,
separates the horizontal from the vertical control network. The horizontal control
network determines the horizontal datum for a surface point whereas the vertical
control network determines the vertical datum for a surface point. Such control
networks established through two suites of systems are also called a “2 + 1”
network, which is still widely used in operations. In modern geodetic survey, the
horizontal control network is often established by GPS methods.
This chapter mainly discusses geodetic datums and the methods, principles, and
plans for establishing geodetic control networks.
3.1 The Horizontal Datum and Horizontal Control
Networks
The horizontal datum is realized by horizontal coordinates of a series of geodetic
control points, usually through establishing horizontal control networks. It serves as
the basis for determining the horizontal coordinates of points on the Earth's surface.
3.1.1 Geodetic Origin and the Horizontal Datum
Geodetic Origin
The geodetic origin is the initial or starting point for computing geodetic coordi-
nates in the national horizontal control network. It is chosen as an appropriate but
arbitrary point in the national geodetic network. Geodetic reference data are
obtained by determining with high precision the astronomic longitude and latitude
of the geodetic origin and the astronomical azimuth from there to another point;
and finding the geodetic longitude, latitude, and height of the geodetic origin and
the geodetic azimuth from there to another point according to the principle of
ellipsoid orientation. The reference ellipsoid and ellipsoid orientation will be
discussed in Chap. 7 .
The geodetic origin of China Beijing Geodetic Coordinate System 1954 is
located at the Pulkovo Astronomical Observatory of the former Soviet Union. In
the 1970s, China decided to build its own independent geodetic coordinate system.
After field survey and comprehensive analysis, the geodetic origin of P.R. China
was at last established in Shijisi Village, Yongle Township, Jingyang County,
Shaanxi Province, with the coordinate longitude of E 108 55 0 25.00 00 and latitude
N34 32 0 27.00 00 , and it was completed in 1978 (see Chen 2003). China's geodetic
origin consists of the survey mark at the center, the instrument platform, the main