Railway precision survey work: determination of the effect of atmospheric refraction on altimetry measurements

The use of a tacheometer (total station) during precision surveying makes it possible to combine the altimetry and planimetric components for the rapid execution of the work. Tacheometers offer the advantage of defining with enough precision the position of the points in the plane. But for a comparison of the altimetry component to the results of a direct leveling it is desirable to take into account certain factors in order to improve the accuracy of the measurements. The evolution of technology gives the possibility not only to carry out certain works quickly but also with the necessary precision. The tacheometer makes it possible to make a tacheometric survey (detail survey) simultaneously in planimetry and altimetry. This avoids performing this task first with the theodolite for the planimetric component and then with the level for the altimeter component. The section of the railway adjaha crossroads "Commune of Cotonou" – Pahou crossroads "Commune of Ouidah" was chosen in order to be able to compare the results of the measurements on different ranges. The fieldwork was divided into two stages. The first is based on the use of theodolite for planimetric measurements and the level for altimetry. For the second phase the tacheometer was used for a combination of the two measurements (altimetry and planimetry). For ranges greater than 150m it is recommended to evaluate the joint influence of refraction and curvature of the earth in order to improve the altimetry component of the measurement.). The study focuses on the methodology and content of geodetic engineering work on railways (Carrefour Adjaha – Carrefour Pahou). It gives an analysis of existing methods and describes the characteristics of the work using electronic tacheometers.