Methods For Feature Extraction of an Outcrop Using Terrestrial LiDAR

Abstract

The following report brings to light how LiDAR data of an outcrop can be acquired and subsequently processed in order to extract the geometry of the outcrop and its pertaining parameters. It makes the use of a sandstone outcrop in the Trooz quarry, Belgium, near Liege. The outcrop was scanned using a terrestrial laser scanner, after which the point cloud data recorded information, pertaining to position and intensity was then processed and subject to interpretation. The report aims to systematically and chronologically take the reader through the entire workflow used by the author, its drawbacks and advantages. Whilst providing a specific analysis of the outcrop in question, creating a general guideline is also an aim. Prior to the processing the significance of the geologic history of the area is touched upon along with the practical acquisition of that data, concerning equipment and procedure. The physical principles of LiDAR and other possible options of acquiring data such as photogrammetry are explained, each with their benefits and shortcomings. The practicality of LiDAR is especially analysed followed by explanation and usage of processing methods such as registration, geo-referencing and computation of geometrical parameters, specifically with the help of software’s such as CloudCompare and MATLAB. Once the results of such parameters are derived, their integrity is discussed by comparing MATLAB oriented methods to those of CloudCompare. These quantified results include values of dip angle and direction over the entire outcrop and the corresponding normal vectors. Varying roughness over the surface of the outcrop and analysis of the intensity distribution present. Whilst also validating these results by comparing them with manual recorded results of dip angle and direction taken at the site. The significance of these parameters is discussed and their application in the characterization of layering. Finally, suggestions regarding the methodology both during acquisition and processing such as during geo-referencing are made by displaying obstacles encountered and flaws realized in the author’s own work.