Developed empirical refraction model for precise trigonometric levelling of the La Valette Landslide, France

Atmospheric refraction is the main source of deviations for laser-based sensors. Having a profound understanding of refraction, and the knowledge of the geometry of the line of sight, assists in identifying an accurate model to correct this error. The height of the point, similar to two other planar coordinates, is also impacted as a result of the refracted beam line. The height can be obtained through numerous geodetic measurement approaches such as spirit levelling or trigonometric levelling. An empirical refraction model was proposed in 1984 to better quantify the observations of trigonometric levelling. In this research, we propose a developed empirical model for the La Valette Landslide (Southeast French Alps) to determine the height of the target benchmarks in a landslide zone. The landslide is located in the Ubaye Valley, where the thrust fault of clay-shale sediments at the bottom and sandstone and limestone competent rocks at the top, control the occurrence of landsliding in this region. The deformation is attributed to the low resistance of the slope material and the increase in pore-fluid pressure resulting from the different hydraulic conductivities of the two geological units. The landslide has been monitored over many years, with several remote sensing techniques, and the task is undertaken as a part of the French Landslide Observation Service - OMIV. Since September 2019, an automated total station Long-Range Trimble S9 has been monitoring 54 reflectors’ positions every 1 to 3 hours with respect to three reference control points. The targets have been uniformly distributed over the landslides at distance from 350m to 2300m from the monitoring station, and at elevations varying from 1300m to 2100m. The research determined the point heights using the empirical trigonometric levelling model with the addition of an improved refraction model incorporating the development refraction correction for the observed angles of the control points.