Optical distance measurement (geodesy)

Geodesy methods are referred to as optical distance measurement, which allow the often required distance measurement by measuring the parallactic angle at the end points of a two-meter base staff . They were developed in the first half of the 20th century and were in use until the 1960s. Then they were replaced by electronic distance measurement (EDM) using laser and infrared light.

Up to the turn of the century (around 1900) theodolites were only suitable for measuring angles. As the first method of approximate distance measurement, Georg Friedrich von Reichenbach introduced the so-called distance threads in the thread network, between which a section was to be read on the measuring stick and multiplied by 100.

Then the company Wild Heerbrugg - driven by strong impulses from modern machine and construction technology - brought geodetic instruments to the market, which were equipped with additional devices for a more precise distance measurement. The parallactic angle was measured on a 2-meter-long, high-precision base rod and the distance was calculated trigonometrically.

Soon these measuring rods split up into those with a horizontal and vertical position: the former achieved higher accuracy and were used for national surveying , the latter in building surveying and engineering geodesy . Today these devices - although they still function perfectly after 50 years - have become pure collector's items or museum pieces, because they were able to compete against the electronic " distancers " that emerged in the 1960s (based on microwaves, infrared and soon also lasers ) don't argue for long.

The quick (but less precise) distance measurement with the 1: 100 spacer threads, however, is still sometimes used today when the construction company or the surveyor does not have a spacer available.