Hardly Accessible Morphological Structures – Geological Mapping and Accuracy Analysis of SfM and TLS Surveying Technologies

Abstract

The aim of geological field mapping is to collect and interpret data on the relief of the Earth's surface. From thus created geological maps, we can obtain information about mineral units and their structure – rock and mineral types, their thickness, lithological deposits, faults, folds, fractures, and thus interpret information as they originated over time. However, the accessibility of such structures is affected by various morphological elements – terrain notches, watercourses, but also by vegetation. Simultaneous geodetic and geological mapping could be a solution for surveying hardly accessible morphological structures. Non-contact surveying technologies – terrestrial laser scanning (TLS) and close-range photogrammetry (terrestrial and remotely piloted aircraft system (RPAS) photogrammetry) provide reliable, high-quality and accurate data on the topographic surface with a high temporal resolution, as the spatial accuracy of the measured point can be mXYZ ≤ 10 mm at an imaging distance of about 20 – 30 m. From the measured data, it is possible to generate point clouds, digital terrain models, and orthophoto maps based on automated data processing. However, the disadvantage of photogrammetric imaging is a proportional decrease in accuracy with increasing imaging distance. The accuracy of TLS is not significantly affected by increasing distance. The paper presents a case study of the use and comparison of non-contact surveying technologies and their application for in-situ mapping of hardly accessible geological structures in the area of Spišská Magura (Slovak-Polish border). The results are given for two localities on two outcrops - Jurgów (PL) and Bachledova valley (SK), while analyzing the usability of TLS and RPAS photogrammetry, with and without the use of artificial ground control points (GCP). The paper presents a mutual comparison of all obtained graphical outputs in terms of 1D and 2D quality depending on the type of GCPs used, depending on the terrain and accessibility. The results show that by using photogrammetry when creating map data, in comparison with TLS, we are able to get sufficient accuracy of outputs for in-situ geological mapping.