ABSTRACT
Refraction is a physical phenomenon that a surveyor encounters relatively often during practical field measurements. Atmospheric refraction, either vertical or horizontal, which is caused by the diffraction of optical properties of the environment by the temperature difference of air layers, is one of the most frequent types of refraction. In nontraditional cases, we can encounter the refraction even when measuring objects under water. In that case, the bending of light when passing through the air/water interface is more substantial. While the distortion caused by atmospheric refraction becomes evident slowly and its effect is noticeable and significant (from a geodetic point of view) up to distances in order of several 10 meters, the effect of distortion in refraction when measuring through water-level is in the order of several mm and detectable up to distances in order of several 10 centimetres. In technical practice, there can sometimes be the case that it is necessary to survey the course of the terrain that continues below the water level. The selection of measurement methodology and used instrumentation depends on the specific conditions and accuracy requirements. When measuring the relief of the bottom by the spatial polar method using total station through the air/water interface, we encounter errors caused not only by refraction but also by the change of the velocity of electromagnetic wavelength propagation of the laser distance meter in a water environment. Therefore, we do not measure the correct position and height of the given point (object), whose image we see above the water level. When determining the horizontal position of points, there are length differences between their real and virtual position. This is in particular related to the change in the velocity of electromagnetic waves passing through the water layer, so the position of measured points is always located further away from the survey station than their actual position. Height differences between the measured and real position can have positive or negative values depending on the size of the measured vertical angle.
