Geodetic Datum and Geodetic Control Networks - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 80

The establishment principle of traversing is fundamentally the same as that of
triangulation and can also be classified into four orders. The accuracies required for
the measured distances and angles of various-order traverses should generally
accord with the accuracies obtained from computations by the triangulation chain
of the corresponding order.
The first-order traverse usually runs along the main traffic routes and criss-
crosses into larger traverse loops; several traverse loops constitute the traverse
network. Figure 3.9 shows a schematic diagram of the traverse layout. The first-
order traverse network should be properly connected with its adjacent triangulation
chain to form an integral geodetic control network. The circumference of the first-
order traverse loop is usually around 1,000-2,000 km. The second-order traverse is
set within the first-order traverse loop (triangulation chain) with its two ends closed
at the first-order traverse points (or triangulation chain), in the form of a connecting
line. Simultaneously, mutually intersecting traverse loops are also formed between
the second-order traverses, which constitute a network. The circumference of the
second-order traverse loop is usually 500-1,000 km or so. The lengths of the first-
and second-order traverse legs vary from 10 km to 30 km. To control the azimuth
errors of the traverse legs and reduce the lateral errors of traverses, the astronomical
longitude, latitude, and azimuth at the two ending points of each traverse leg should
be measured in order to determine the initial azimuth of this traverse leg at a
distance interval of 100-150 km from the first- and second-order traverses, where
the first- and second-order traverses meet the first- and second-order triangulation
chains and where the first- and second-order traverses intersect, so that the initial
azimuth of this line in question can be obtained. Traverse is not as durable as a
triangulation networks and the error propagation in calculating azimuths accumu-
lates fast; the intervals of the initial azimuth should therefore be small. The traverse
controlled by azimuths at the two ends is called the traverse section, which should
be laid straight. Traverse section that generally runs along the same direction in the
traverse network between two intersections is called a traverse segment. The
propagation of errors in azimuth increases with the increasing of number of sides.
Therefore, the legs of each traverse section in the first- and second-order traverses
should be no more than seven.
The third- and fourth-order traverses are densified based on the first- and second-
order traverse networks (triangulation chains), which are closed on a second known
point, called the connecting traverse. The total length of such a single connecting
traverse should be less than 200 km and 150 km for the third- and fourth-order
networks, respectively. When several such connecting traverses are established, it
would be preferable that these traverses constitute a network to reinforce the
traverse's structure. The lengths of the third- and fourth-order traverse legs can
be determined according to the performance of the side and angle measuring
instruments and the density required for geodetic points. Generally, the lengths of
the third-order traverse legs range from 7 to 20 km, while those of the fourth-order
range from 4 to 15 km. A traverse leg with greater length is preferred in fieldwork.