Mansoor Sabzali^1,, Lloyd Pilgrim¹

The atmosphere is an undeniable source of error for geodetic observations which cannot be underestimated or controlled by surveyors in the fieldwork measurements. Technicians using total stations and laser scanners require to have an accurate zenith angle measurement in order to accurately determine the 3D coordinates of points. Therefore, having a thorough knowledge of atmospheric influencing the zenith angle measurements is important. Terrestrial surveying instruments typically use a laser in the domain of visible light or near-infrared in the electromagnetic spectrum. The deviation is introduced by the different intersecting angles between the laser beam and the various atmospheric layers. The phenomenon is called the refractivity of waves. Snell’s law can be used to calculate the deviation of the laser as it moves through different layers of the atmosphere. This study aims to develop an understanding of the refractivity on zenith angle under the local refraction coefficient via considering vertical temperature gradient (VTG) as its major contributing factor into the assumed layers of atmosphere above the Earth’s surface, where the geodetic measurements might take place. The coefficient is also dependent on the other observable atmospheric elements such as temperature, pressure, and humidity. However, the accurate knowledge of VTG is not a straightforward task due to an inability to obtain direct measurements and its unexpected fluctuations. Consequently, the proper modelling of VTG through simulated and real datasets analyses with the aid of mathematical least square approach is pursued to achieve an updated atmospheric model for refracted zenith angle.

electromagnetic spectrum, least square, local refraction coefficient, refracted zenith angles, updated atmospheric modelling, vertical temperature gradient