Target mark
In metrology and instrument making , a target mark is a precisely scored mark on glass or metal.
Target marks either serve as a measuring device themselves (for example in the measuring microscope of a stereo comparator or in the image plane of a telescope ), or they are aimed at by the crosshairs of an optical system, for example the fiducial marks on a photo plate , or a mire attached at a greater distance .
In geodesy target boards are often used, which can be set up on a tripod just like a theodolite . They have two or four high-contrast, wedge-shaped target marks to enable precise targeting even from a great distance. By alternately using and exchanging the target plate and theodolite - the so-called forced centering - the measurement accuracy can be increased to around 0.1 mm, for example in a polygon , while it is in the range of mm to cm when aiming at a measuring stick .
Until the development of electronic distance measurement, the 2-meter basic staff was often used for precise distance measurements at 10–30 m . It also has a target mark at both ends, with which the parallactic angle is measured.
In astronomy , astrogeodesy and medullary science , target marks are used which are provided with devices for lighting .