Large-scale and high-resolution 3-D cave mapping by terrestrial laser scanning: a case study of the Domica Cave, Slovakia

Mapping and modelling the complicated geometry of caves is a challenging task that has traditionally been undertaken by tacheometric surveying methods. These methods are excellent for capturing the general shape of a cave system but they are not suitable for high-speed, high-resolution mapping of complex surfaces found in this environment. Terrestrial laser scanning (TLS) technologies can acquire millions of points represented by 3-D coordinates, at very high spatial densities on complex multifaceted surfaces within minutes. In the last few years, advances in measurement speed, reduction in size / cost and increased portability of this technology has revolutionised the collection of 3-D data. This paper discusses the methodological framework and the advantages / disadvantages of adopting terrestrial laser scanning to rapidly map a complex cave system on the example of the Domica Cave in Slovakia. This cave originated in the largest karst region in the West Carpathians. The collected data set or 'point cloud' contains over 11.9 billion of measured points, captured in 5 days from 327 individual scanning positions. The dataset represents almost 1,600 m of the cave passages. Semi-automatic registration of these scans was carried out using reference spheres placed in each scene and this method archived an overall registration error of 2.24 mm (RMSE). Transformation of the final registered point cloud from its local coordinate system to the national cartographic system was achieved with total accuracy of 21 mm (RMSE). This very detailed data set was used to create a 3-D cave surface model needed for volumetric analyses. In the future, it will be used for spatial analyses or simulating the interaction of surface and subsurface processes contributing to the development of the cave system on the basis of a 3-D GIS platform.