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error of height of the gravity point, as regulated, is not to exceed
2 m, whereas that
of the coordinate is determined on a case-by-case basis. For the actually measured
coordinate, its mean square error should not exceed
5 m. For the gravity points in
the 1 0
1 0 and 5 0
5 0 grids, the mean square error is not to exceed
10 m, and not
to exceed
100 m for the gravity control points and gravity points in grids greater
than 5 0
5 0 .
If the gravity points coincide with geodetic control points and national bench-
marks of different orders, then their coordinates and heights can be used directly.
Otherwise, the coordinates and heights should be measured with GPS, traversing,
leveling, or trigonometric height traversing.
For further and more exhaustive information on the specific implementation
methods and process of gravity network establishment, please refer to the relevant
literature on the subject.
Review and Study Questions
1. What is geodetic datum and how many datums does it consist of?
2. How many types of geodetic control networks are there? Briefly describe the
methods for establishing each type of geodetic control network.
3. What are the principles of establishing the national horizontal control networks
and the national leveling networks?
4. Into how many orders can the national horizontal control networks be classified?
Outline the specifications (mean square error of angle observation, relative mean
square error of side length, and average side length) for establishing the national
horizontal control networks of different orders.
5. What are the specifications (constant error, ratio error) for establishing the
national GPS networks of different orders?
6. Explain the concepts of zero elevation surface and leveling origin.
7. Describe the overview of development of the international gravity datum.
8. Briefly describe the plans for establishing the national gravity network.
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