Side refraction

In geodesy and meteorology , lateral refraction is an irregular, lateral deflection of the light beam. It can falsify measurement results only slightly, but systematically.

Side refraction has to be expected in tunnels and mines, for example , but it also plays a role in precision measurements under strong sunlight or in thermally stressed machine shops.

While the vertical refraction of rays is a normal phenomenon - see terrestrial refraction - and can be easily determined using meteorological formulas, the side refraction represents an anomaly that can only be calculated with greater effort. It is created by a horizontal gradient in the air temperature , which mostly results from heated components that are close to the sighting beam .

In astronomy , the effect is known as hall refraction , it is caused by unequal temperature in the dome of an observatory . Lateral refraction can also occur in open air if the layers of the same air density are not horizontal. Over mountain slopes at different temperatures , this effect as "zenith refraction" can reach a maximum of 0.5 ", but also larger amounts at flat elevation angles.

In order to reduce the effect of possible side refractions, precision measurements in astrogeodesy and astrometry are usually repeated several times , if possible under different weather conditions . In tunnels or in buildings that are heated on one side, however, the corresponding precautions are more complex. In some cases, systematic errors can also be identified through detailed analysis of the measurement results.