Network propagation
In classical geodesy, network expansion is the step-by-step construction of a nationwide surveying network, starting from an astronomically measured fundamental point located approximately in the center of the country . The individual network or measurement points are determined by angle measurement of long triangles on the surface of the earth, today also by electromagnetic distance measurement . Since it used to be difficult to measure long distances, the network scale was determined by only one base line , but several kilometers long, until around 1960 .
The reference area of such networks is a reference ellipsoid suitable for the country (regional approximation to the mean earth ellipsoid ), the calculation method is spherical trigonometry . While smaller triangles can be calculated on a sphere with the mean earth radius , for larger distances the main geodetic task has to be solved on the ellipsoid.
The first phase of a network expansion on the reference ellipsoid is the first-order network , which consists of triangles (mostly between mountain peaks ) with sides of about 20 to 50 km. The starting value is the astronomical latitude of the fundamental point and an astronomical azimuth to a neighboring point. The geographical coordinates of the other network points result from the coordinate differences to the fundamental point ("geodetic transmission").
Then a second-order network with a mesh size of around 10 km is (or was) inserted into this basic network . The further consolidation to 4th to 5th order networks (approx. 1–2 km) took place through simpler methods of local point activation and is therefore no longer referred to as network expansion.
From around 1950 onwards, several Laplace azimuths were added to the networks in order to minimize small disturbances caused by the earth's gravity field . Methods of satellite geodesy were added later, for example in the European network WEST in the 1970s. If, on the other hand, surveying networks are being revised in some developing countries today, an international geocentric reference system such as the ITRF is used instead of the regional network expansion .
See also
literature
- Karl Ledersteger : Astronomical and physical geodesy . Handbook of Surveying Volume 5, 10th edition. Metzler, Stuttgart 1969
- Bernhard Heck, calculation methods and evaluation models for national surveying . ISBN 3-87907-173-X , Wichmann-Verlag, Karlsruhe 1987.