Geodetic Datum and Geodetic Control Networks - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 90

in the form of closed loops, or close to leveling lines of a higher order in the form of
loops to curb the accumulation of systematic errors and facilitate densification of
the leveling lines of a lower order of accuracy.
The first-order leveling network is the backbone of the national vertical control
network and the primary basis for relevant scientific research. Hence, the first-order
leveling lines should be established along the traffic routes with gentle slopes in
geologically stable areas in order to satisfy the needs of high-precision leveling.
The leveling lines should be in the form of loops, and the loop circumferences in
plain and hilly areas are of 1,000-1,500 km and about 2,000 km in mountainous
areas. This density fits reasonably well for a country with as vast a territory as
China.
The second-order leveling network is the overall basis of the national vertical
control network. It should be established along railroads, highways, and rivers and
should form loops. The loop circumferences are generally specified to be 500-
750 km. In flat terrain, the loop circumferences can be decreased according to the
needs of construction and increased in mountainous regions or harsh areas, taking
into consideration the actual situations prevailing in each place.
Third- and fourth-order level networks provide the necessary vertical control
points directly for topographic mapping and engineering construction. The third-
order leveling lines are closed loops or annexed leveling lines densified within the
higher-order leveling network with a loop circumference designated to be no more
than 300 km. The fourth-order leveling lines generally form the annexed leveling
lines, which start from and finish on higher-level benchmarks. The lengths of the
annexed leveling lines are assigned to be no more than 80 km.
The precision in leveling of different orders is represented by M Δ , the random
mean square error for 1-km leveling, and M w , the total mean square error of altitude
difference. Their tolerances are as given in Table 3.1 .
By the end of 1984, the field operations of the national first-order leveling
network, which covered the whole mainland and Hainan Island of China, had
been completed. Figure 3.16 shows the establishment of this leveling network.
The China's national first-order leveling network has 100 leveling loops and
289 leveling lines in total. The total length of the leveling line is 93,360.8 km. The
overall adjustment was completed at the end of 1986. The “National Height Datum
1985” was adopted and the Qingdao leveling origin was defined as the reference
point for heights, belonging to the normal height system. Precision in actual
measurement are as detailed in Table 3.2 .
Overall, repeated leveling measurements of China's national leveling networks
were carried out over the years 1991 to 1997, concerned primarily with the original
first-order leveling network. Specific leveling routes and nodal points were
readjusted except for the spur lines. The total length of the re-leveled lines was
approximately 94,000 km with 99 loops and 273 leveling lines. The precision
targets after adjustment were that the random mean square error of altitude differ-
ence for 1 km was M Δ 0.45 mm and the total mean square error of altitude

difference for 1 km according to closing errors of 99 loops was M w 1.0 mm.