subtense bar

The base bar is used in geodesy for high-precision measurement of distances up to 20 m. The mathematical basis of the distance determination are the trigonometric functions in the right-angled triangle. The base bar forms with its two outer targets the baseline of an equilateral triangle, at the third vertex of a theodolite is set up. With this, the angle between the two legs is measured by accurate targeting of the targets.

The distance is obtained by mathematical resolution of the triangle, the length of the baseline ( = length of the base bar ) and the measured angle are known. The height of the triangle corresponds to the desired distance.

Base slats are about 2 m long usually. The two outer targets are exactly 1m away from the central target. The distance between the targets must be extremely accurate, so that the distance determination achieved a correspondingly high accuracy. In particularly precise devices, this distance is more accurate than / - 0.1 mm. For this, the targets of these devices are protected by mechanical devices from temperature- induced changes in distance. For the measurement of the base bar is horizontally positioned and oriented at right angles to the measuring path. It is equipped with a bubble level and an optical aiming device.

Due to the advancements in the field of electronic distance measurement, the measurement has lost with the base bar in importance since the 1980s, however, continues to apply in the vicinity (eg when measuring machines, industrial surveying ) as one of the most accurate methods of measuring length. Up to a distance of 10 m, an accuracy of less than 0.5 mm and in the range of 10 to 20 m can be achieved accuracies of less than 1 mm. At 100 meters, the accuracy is only 25 mm.

Principle of measurement

At the finish point B is the base bar is situated leveled and aligned so that they to the vertical plane through the distance s is vertical. The end marks L and R of the base bar be adequately with horizontiertem theodolite and found the horizontal angle gamma.

• Geodetic instrument