Geodetic Datum and Geodetic Control Networks - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 115

Table 3.4 Accuracy and density of GPS control networks

Order
AA Item A B C D E

Constant error a (mm) 3 5 8 10 10 10

Ratio error coefficient b (ppm) 0.01 0.1 1 5 10 20

Average distance between adjacent points (km) 1,000 300 70 15 ~ 10 10 ~ 5 5 ~ 0.2

dimensional coordinate system of WGS84 or ITRF. Practical engineering applica-
tions require national coordinate systems like Beijing Coordinate System 1954,
Xi'an Coordinate System 1980, China Geodetic Coordinate System 2000, or
another independent local coordinate system. Therefore, in the technical design,
the coordinate system and the initial data of the GPS network have to be specified,
which means making clear the datum adopted by the GPS network.
The GPS network datum consists of position datum, azimuth datum, and scale
datum. Position datum is usually determined by the coordinates of known initial
points. Azimuth datum can be determined by the value of the known starting
azimuth or the azimuth of the GPS baseline vector. Scale datum can be determined
by the side of the electromagnetic wave distance measurement on the Earth's
surface, by the distance between two initial points, or by the distance of the GPS
baseline vectors. So, the design of the GPS network datum is essentially the issue of
determining the position datum of the network.

Point Selection

Since GPS observation stations do not require intervisibility with each other, the
selection of points is much simpler than for conventional measurements. The choice
of GPS points has a significant influence on the smooth operation of GPS observa-
tions and the acquisition of reliable results. As a result, we should collect and fully
understand the geographical conditions of the survey areas and the distribution and
maintenance of existing control points based on the purpose of measurement and
the requirements of coverage, accuracy, and density of the survey areas so as to
properly choose the positions of the GPS points. The following principles should be
followed in the selection of GPS point positions:

1. It should be convenient to install antennae and GPS receivers around the point.
The point should be located where the view is not obstructed and the elevation
angle of the surrounding obstacles is less than 15 .
2. The point should be far away from high-power radio emission sources and high-
voltage wires to avoid interference from magnetic fields close to the signal.
3. In order to weaken multipath effects, there should be no objects that strongly
reflect or absorb electromagnetic waves around the point.
4. To improve operational efficiency, the point should be located where transpor-
tation is convenient.