Sec. 89. Types of Geodetic Network

A geodetic network is a net of fixed points on the earth's surface, called geodetic or control points, whose position in the general frame­ work of geodetic coordinates is defined. The net having coordinates valid for the entire area of the USSR and is the initial one used for the construction of other geodetic nets, is termed the na­tional geodetic network.

There exist geodetic nets for horizontal and vertical control. The horizontal geodetic networks include geodetic points with known horizontal coordinates X and Y. Vertical control is provided by bench marks with known freights H.

The national geodetic network may be filled by a geodetic densification net between the geodetic points of the national network which can then be associated with a survey geodetic net. A survey geodetic not is a densification net constructed for topographic surveys or other geodetic and engineering operations.

Sec. 90. Horizontal Geodetic Networks

Horizontal geodetic [networks are built by triangulation, trilateration or polygonometry.

Triangulation, as the name suggests, relies on the construction of triangles. All their angles and some of their sides are measured (Fig. 124). If only the sides of the triangles in a geodetic network are measured, the method is termed trilateration (Fig. 125). Polygonometry is used to construct geodetic nets by running traverses between geodetic points. All distances and angles are measured in these traverses (Fig. 126). When astronomic coordinates and azimuths are determined for some of the points of a geodetic network, it is called an astronomical geodetic network.

The USSR national geodetic network is subdivided into first, sec­ond, third and fourth-order (or primary, secondary and tertiary) nets.

The first-order network is constructed by building rows of triangulation along the meridians and parallels. The rows intersect to form

a system of polygons. The section of a row where it intersects

with the rows running perpendicular to it is called a link (section). Links are 200 to 250 km long. The 2-3 exit sides are located at the ends of each link in the diagram. The length of a poly­gon's perimeter is 800 to 1000 km.

The areas inside the first-order polygons are filled with a continuous net of second-order triangles. These are then filled with the third and fourth-order nets.

In the absence of a first or second-order geodetic network it is customary to construct independent lower-order nets. In doing this it is necessary to measure out at least two bases or exit sides followed by a tie-in of this net to a higher-order triangulation.

The points of the national geodetic network are chosen so that they are uniformly distributed over the area to be surveyed and that at least three points are visible from each station. The points chosen are carefully and reliably fixed by geodetic signals, struc­tures that mark the position of the geodetic points on the ground (Fig. 129). Each signal has a centre that carries the coordinates of the particular geodetic point.

By comparing the reference data of a geodetic point with the measured results a surveyor can determine discrepancies, which are called errors of closure. It is possible to judge the adequacy and quality of the work he has done from the values of the errors of closure.