High target triangulation

from Wikipedia, the free encyclopedia

Earlier methods of land surveying are known as high-target triangulation , where the majority of the target points are not on the surface of the earth, but instead aiming at balloons , artificial missiles, stars or inaccessible high mountain peaks . With the exception of the latter, their coordinates are variable, so that they have to be eliminated in the calculation of the viewpoints.

The simplest way to do this is to aim at these high targets from at least two terrestrial positions at the same time . Mathematical cutting processes (cutting planes) can then be set up in a formula that the coordinates of the high targets fall out of the calculation process (see also Stellar triangulation ).

The high-target triangulation offers several advantages, which have now taken a back seat compared to the satellite methods:

  1. Formation of extensive survey networks
  2. and thus more favorable error propagation
  3. Less influence of atmospheric light refraction ( terrestrial refraction ), avoidance of side refraction
  4. no need for complex stabilization ( marking , measuring pillars , etc.)
  5. completely independent control of the terrestrial land survey .

The methodology probably dates back to the 19th century, but was further developed in several ways in the 1950s and 1960s:

The purely terrestrial triangulation with inaccessible distant or high points was u. a. used in expeditions - for example the Austro-Hungarian North Pole Expedition from 1872 to 1874 by Julius Payer and Carl Weyprecht with the icebreaker "Tegetthoff". While driving past Novaya Zemlya , the only imprecise coastal maps could be improved by repeatedly targeting capes and distinctive double peaks, and Payer succeeded in measuring some islands and peaks in Franz Josef Land , which was discovered from afar , even though the ship was already stuck in the ice .

The Indian Great Trigonometric Survey took a similar approach to measuring the high Himalayan peaks, the highest of which was recognized as Mount Everest , named after the chief geodesist .

The cosmic version of the high-target triangulation, however, is based on the use of the starry sky as a background. It is recorded along with the missiles by ballistic cameras and thus provides a reference surface in the star's inertial system. The missiles (from 1959 also satellites) are photographed simultaneously by two ground stations by chopping up their tracks in the sky using time stamps and displaying them against the star background. Later, special satellite cameras were used for this .

The photographic plates are measured on photogrammetric comparators with an accuracy of a few µm . The "observation vectors " recorded at the same time for each track point then each define a plane which contains the connecting line of the two viewpoints and which is oriented directly in the fundamental system of the stars - that is, absolutely . The planes are intersected with each other, which gives the exact connection vector between the cameras.

From the early 1960s, the satellite triangulation methodology was further developed. With improved, partially automated satellite cameras, several intercontinental satellite triangulations were carried out around the world in the 1970s , which was previously technically impossible due to the curvature of the earth . For the first time, the astronomers and geodesists of the SAO were able to determine the mutual position of four continents directly , with measuring distances of over 5000 km having to be bridged. The balloon satellites Echo 1 and 2 with their 1000–1500 km high orbits served as high targets . The meter accuracy achieved exceeded the previous data by 10 to 20 times. Even Western Europe was covered with a dense network of between about 20 university departments, the name WEST received and precise definition of the European network European date in 1979 contributed.

The climax of this development is the " World Network of Satellite Triangulation ", completed in 1974 , with which 46 stations around the world were geodetically linked. By evaluating a few thousand photo plates (ballistic cameras of the type BC-4 ), the geodetic date of all participating countries could be determined internationally to an accuracy of 3 to 5 m, which was 10 to 30 times more accurate than before, depending on the vertical deviation . Only the Soviet Union and China stayed away from this hitherto unique global cooperation for military reasons (confidentiality of coordinates). About half of the plates were recorded correctly (with stars and the precalculated satellite), but their counterparts on the 1–2 opposite stations failed (mostly due to surprising cloud cover or too strong wind ). These calculated failures were compensated for by the large over-determination in the network balancing .

The only disadvantage of satellite or stellar triangulation is that balloons, missiles or satellites must be visible on at least two (better 3) distant ground stations or observatories . This is made easier by reliable weather forecasts . The fact that this has been successful around 1000 times in the “world network” has also contributed to international understanding and the end of the Cold War . Today's earth measurement , on the other hand, has become free of these restrictions through the transition from light to microwaves ( GPS , GLONASS and Galileo systems).

literature

  • Karl Ledersteger : Astronomical and Physical Geodesy (Earth Measurement) , JEK Volume V (870 p., Especially Ch. 2, 5 and 13), JB Metzler-Verlag, Stuttgart 1968.
  • A.Berroth, Walter Hofmann : Kosmische Geodäsie (356 p., Especially Chapters 1, 5, 13-15), Verlag G.Braun, Karlsruhe 1960
  • Hellmut Schmid : The world network of satellite triangulation . In: Wiss. Communications from ETH Zurich and Journal of Geophysical Research , 1974.

See also