Basic measurement

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Basic device for measuring base slats (a wooden ruler in the picture) made by the Brunner brothers in Paris, years of construction 1876 to 1878

In geodesy, the basic measurement is the terrestrial measurement of very precise distances between high-ranking survey points. A second meaning of the word is the precise optical distance measurement with a 2-meter base staff .

In the classic national surveying of the 19th and early 20th centuries, the most important method for measuring geodetic networks was triangulation ( angle measurement with precision theodolites ). The distance measurement was too complex and had to be limited to a few “ baselines ” of a few kilometers in length.

Measuring chains and Invar wires

These lines were established in the lowlands and measured initially with measuring chains and later with precisely calibrated Invar wires. In the basic apparatus according to Jäderin , the wires, the alloy of which has only a minimal thermal expansion coefficient , were each 24 m long. They were laid out horizontally by specially trained measuring teams, stretched with calibrated tension weights and read off to the nearest tenths of a millimeter on measuring stakes . The accuracy of such basic measurements reached a few millionths (about cm over a distance of 5 km), but took weeks to months per line.

The measured route was extrapolated to the nearest triangulation route of the first-order network using a basic expansion network made up of ever-increasing triangles , thus giving the network scale of the geodetic triangular network .

Basic measurement from 1800 to 1950

Around 1800 Carl Friedrich Gauß , Joseph Liesganig and other scientists developed the theory of earth measurement and triangulation networks were spanned over wide stretches of land with triangular lengths of 20 to 50 km. The nets were initially stored in regional fundamentals, later in cross-federal projects ( Potsdam in Germany , the Hermannskogel near Vienna in Austria ). The network scale was determined by baselines at intervals of about 200 to 300 km. In Austria-Hungary they were z. B. on Neunkirchner Allee (today B 17) near Wiener Neustadt ( called “Wienerneustädter Basis” since Maria Theresa ) in Lower Austria, near Josefstadt in Bohemia and near Hall in Tyrol .

Based on the triangulation points of the 1st order network (TP) created in this way, local “network densifications” were carried out later. For this purpose, the “geometer” or “engineer topographer” established a geodetic network in the local area (e.g. a municipality), which was measured and calculated using the first-order TP that had already been calculated. This resulted in further triangulation points of subordinate hierarchy (network of 2nd to 4th / 5th order), which already formed a narrow grid of fixed points every 1 to 3 km. From the 1950s , the increasing demand was met by so-called switch - on points, which were closer than 1 km in the country, in cities even down to 200 to 300 meters. In some cases, the aerial photogrammetry method was used for this .

Optical and electronic distance measurement

For shorter distances, on the other hand, optical distance measurement was (and is in some cases still used today). Here, a is with special theodolite bar targeted and between two fine graduations visible bar section multiplied by a factor of 100. Measurements with the so-called base staff (a horizontal, precisely manufactured 2 m measuring staff) were also used, but were limited to distances below about 100 m.

In the 1960s , the technical development of longer distance measurements using radio, infrared and light waves succeeded. As electronic distance measurement, it has been the standard method of land surveying and engineering geodesy since around 1980 and is now mostly easier than traditional angle measurement . GPS measurements are also being used more and more often. Therefore, today, geodetic networks are usually created as combined networks.

See also

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