UAV and SfM in Detailed Geomorphological Mapping of Granite Tors: An Example of Starościńskie Skały (Sudetes, SW Poland)

Pure Appl. Geophys. Ó 2017 The Author(s) This article is an open access publication https://doi.org/10.1007/s00024-017-1730-8 Pure and Applied Geophysics UAV and SfM in Detailed Geomorphological Mapping of Granite Tors: An Example of Starościńskie Skały (Sudetes, SW Poland) MAREK KASPRZAK,1 KACPER JANCEWICZ,1 and ALEKSANDRA MICHNIEWICZ1 Abstract—The paper presents an example of using photographs taken by unmanned aerial vehicles (UAV) and processed using the structure from motion (SfM) procedure in a geomorphological study of rock relief. Subject to analysis is a small rock city in the West Sudetes (SW Poland), known as Starościńskie Skały and developed in coarse granite bedrock. The aims of this paper were, first, to compare UAV/SfM-derived data with the cartographical image based on the traditional geomorphological field-mapping methods and the digital elevation model derived from airborne laser scanning (ALS). Second, to test if the proposed combination of UAV and SfM methods may be helpful in recognizing the detailed structure of granite tors. As a result of conducted UAV flights and digital image post-processing in AgiSoft software, it was possible to obtain datasets (dense point cloud, texture model, orthophotomap, bare-ground-type digital terrain model—DTM) which allowed to visualize in detail the surface of the study area. In consequence, it was possible to distinguish even the very small forms of rock surface microrelief: joints, aplite veins, rills and karren, weathering pits, etc., otherwise difficult to map and measure. The study includes also valorization of particular datasets concerning microtopography and allows to discuss indisputable advantages of using the UAV/SfM-based DTM in geomorphic studies of tors and rock cities, even those located within forest as in the presented case study. Key words: UAV, SfM, geomorphological mapping, granite tors, Sudetes. 1. Introduction Among characteristic elements of granite landscapes are isolated solid rock residuals known as tors. They may also form clusters or occasionally combine into more complicated landform assemblages called rock cities (Migoń et al. 2017). Their existence is connected with spatial variability of bedrock 1 Institute of Geography and Regional Development, University of Wrocław, pl. Uniwersytecki 1, 50-137 Wrocław, Poland. E-mail: geological features, mainly jointing patterns, whereas joints also control the shape, evolution and patterns of degradation of individual tors (Linton 1955; Jahn 1974; Dumanowski 1968; Migoń 1996, 2006; Twidale and Vidal Romani 2005). These structural conditions have long been surveyed through traditional geological and geomorphological mapping, occasionally aided by interpretation of aerial photographs where the absence of vegetation allowed one to do this. Fieldwork is usually based on ground-level observations, including geological compass measurements. More recently, this type of studies may involve an analysis of high-resolution (1 9 1 m or more) digital terrain models (DTMs). These datasets are usually derived from airborne laser scanning (ALS)—a method which, in recent years, allowed for a dynamic progress in representing and modeling of Earth surface (e.g. Höfle and Rutzinger 2011; Bishop 2013; Migoń et al. 2013), especially within inaccessible areas or those of complex relief. In certain situations the ALS data can be substituted by photogrammetric methods using unmanned aerial vehicles (UAV) and digital image post-processing technique known as structure from motion (SfM). This combination appears to be cheap and efficient and thus, the use of these methods is currently becoming increasingly popular, mainly in case studies focused on small objects or areas (e.g. James and Varley 2012; Hugenholtz et al. 2013; Mancini et al. 2013; Lucieer et al. 2014; Ryan et al. 2015; Clapuyt et al. 2016; Cook 2017; Da˛bski et al. 2017; Marteau et al. 2017; Miziński and Niedzielski 2017). This paper presents a case study which aims to determine to what extent the use of UAV and SfM methods can help to recognize the geomorphological structure of granite tors and supplement landform M. Kasprzak et al. inventory attempted through traditional methods. The reference object in this study is the Starościńskie Skały rock city in Rudawy Janowickie Mts. (Sudetes, SW Poland, Fig. 1), recently subject to detailed mapping using the combination of traditional field-based geomorphological mapping (Michniewicz et al. 2016). The latter was based on high-resolution, 1 9 1 m LiDAR-based DTM, which was produced in years 2010–2014 for most part of the area of Poland (We˛z_ yk 2014). Aside from the intention to gather new information about the morphology of the particular group of tors, the main objective of this study was to compare the potentially available methods and datasets concerning granite tors in terms of the degree of detail returned. Contrary to some high altitude, treeless areas where tors occur, this analysis concerns objects located within a forest. Consequently, both field mapping and measurements as well as spatial data post-processing were far more difficult and extra problems had to be solved. Pure Appl. Geophys. 2. Methods The main dataset used in this study is the digital model of the tor group, prepared on the basis of aerial photography. The digital images of 4000 9 3000 px resolution were made by DJI Phantom 3 Professional quadrocopter. It is equipped with the digital camera of 1/2.300 CMOS sensor, 94° field of view, 20 mm (35 mm format equivalent) focal length and f/2.8 focus (Phantom 3 Professional 2015). Flights during which the photos were taken were conducted on 7th March 2017. The time of flights was set before the start of vegetation season and was carried out in the presence of stable wind conditions, sufficient visibility and lack of snow cover. In order to obtain appropriate image coverage of the study area as well as the high level of detail, orthogonal photos were taken during three consecutive flights at the altitude of 80, 50 and 30 m above the starting point (ca. 65, 35 and 15 m above the top of the highest tor). Additionally, oblique photos were taken during the flight around the group of tors in order to provide information about areas invisible directly from above. Figure 1 Granite rock city of the Starościńskie Skały tors in the Rudawy Janowickie in the Sudetes, SW Poland (photo M. Kasprzak) UAV and SfM in Detailed Geomorphological Mapping of Granite Tors The aforementioned fieldwork yielded a set of 239 digital images, which were used as an input data in the process of digital terrain model creation with the use of SfM method. The essence of SfM lies in using the rules of stereoscopy—a series of overlapped images enables creation of a three-dimensional model. Contrary to traditional photogrammetry, information about camera position and angle is unnecessary (however, the camera position was registered by the UAV built-in GPS (...truncated)