The Use of Terrestrial Laser Scanning for Measurements in Shallow-Water: Correction of the 3d Coordinates of the Point Cloud

Abstract

Published as: Deruyter, G., Vanhaelst, M., Stal, C., Glas, H., De Wulf, A. (2015). The use of terrestrial laser scanning for measurements in shallow-water: correction of the 3D ABSTRACTAlthough acoustic measurements are a wide-spread technique in the field of bathymetry, most systems require a water depth of at least 2 m. Furthermore, mapping shallow-water depths with acoustic techniques is expensive and complicated. Over the last decades, the use of laser scanning for mapping riverbeds has increased. However, the level of accuracy and the point density which can be obtained by Airborne Laser Scanning (ALS), and Airborne Laser Bathymetry (ALB) in particular, are not as high as those of terrain measurements originating from ALS. Moreover, ALS and ALB are not yet suited for mapping shallow-water beds.Therefore, more recent research focuses on the use of Terrestrial Laser Scanning (TLS) from either a fixed or static position (STLS) or from a mobile platform (MTLS). An obvious advantage of using STLS and MTLS is that both the river beds and the river banks can be modelled by means of the same data acquisition system. This ensures a seamless integration of data sets describing both dry and wet surfaces, and thus of topography and bathymetry. However, although STLS and MTLS have the potential to produce high resolution point clouds of shallow-water riverbeds and -banks, the 15 th International SGEM GeoConference on…………… resulting point clouds have to be corrected for the systematic errors in depth and distance that are caused by the refraction of the laser beam at its transition through the boundary of air and water.In this research a procedure was implemented to adjust the coordinates of every point situated beneath the water surface, based on the refractive index. The refractive index depends on the wavelength of the laser beam and the properties of the media the beam travels through. The refractive index for a laser beam with a wavelength of 532 nm varies by less than 1% for a wide range of temperature and salinity conditions. Nevertheless, during the case studies, it became clear that it is important to use an estimate of the refractive index which approaches the actual value as closely as possible in order to obtain accuracies of less than 1 to 2 cm. Therefore, the refractive index was determined for each specific case by using water samples.

Remote sensing of river corridors: A review of current trends and future directions

“…The methods above focus on subaerial analysis, as despite the subsurface being equally important, it is considerably more challenging to measure. At fine scales, through‐water laser scanning shows potential in acquiring bed morphological data, requiring careful calibration for optimal results (Deruyter et al, ). However, there is evidence to suggest that increasing depths reduce accuracy due to laser attenuation (Smith et al, ).…”