LiDAR and 2D Electrical Resistivity Tomography as a Supplement of Geomorphological Investigations in Urban Areas: a Case Study from the City of Wrocław (SW Poland)

MAREK KASPRZAK 0 ANDRZEJ TRACZYK 0 0 Institute of Geography and Regional Development, University of Wrocaw , Plac Uniwersytecki 1, 50-137 Wrocaw, Poland In urbanized areas, particularly in lowland terrains and floors of large river valleys, the natural land configuration is often hard to recognize due to a long history of human activity. Accordingly, archaeological works in cities, which supply knowledge on settlement conditions, are usually accompanied by geological and geomophological research. Lately, data from light detection and ranging (LiDAR) have become a valuable source of information on urban land configuration. Geophysical methods are also becoming increasingly popular in background studies. The paper presents a method of using and linking these sources of spatial information about landforms in such areas. The main aim is to identify to what extent these complementary sources of data and the proposed method can be used in such a specific environment to reconstruct natural, buried terrain morphology. The city of Wrocaw in Central Europe serves as an example. To this end geomorphometric studies were conducted with the use of digital elevation models (DEMs) based on LiDAR scanning and derivated land-surface parametersSAGA Wetness Index, Channel Network Base Level and Altitude above Channel Network. The study also involved determining morphological edges and measurements of the meanders of the Odra, as well as expanding information on the spatial distribution of alluvia and the structure of slope breaks. To this end, geophysical measurements were conducted using the Two-Dimensional Electrical Resistivity Tomography method. Additionally, five typical sequences of man-made ground present within the perimeter of the city were distinguished. As a result, a map of the main landforms of Wrocaw is presented. Finally, we argue that although high resolution DEM and derivate land-surface parameters are very useful in terrain analysis, places with thick man-made ground or strongly levelled areas must be recognized by geoarchaeological excavations or geological bore holes. The geophysical survey is useful to identify buried morphological edges and older relief elements in open areas. 1. Introduction Geomorphological studies in urban areas are difficult to carry out due to dense development and artificial transformations of natural surfaces involving, among others, the formation of man-made ground and modifications of the drainage system (ROSENBAUM et al. 2003; SZABO 2010). As a result, the natural land configuration, especially in lowland areas, is obscured and hard to identify. This often prevents an unequivocal identification of initial landforms and the correct analysis of the local palaeogeographic development. Such difficulties accompany detailed geomorphological studies conducted during archaeological works (KVAMME 2006). These studies are greatly supported by digital elevation models (DEMs) and geophysical methods of ground penetration (KAMPKE 1999; CHALLIS 2006; PAPADOPOULOS et al. 2006; TSOKAS et al. 2011). Unfortunately, DEM analysis for flat, urbanized areas causes many errors associated with a models accuracy, impact of land cover, existence of buildings and artificial transformations of ground surface (SITHOLE and VOSSELMAN 2004; BONK 2007; LIU 2008; REUTER et al. 2009). In addition, the analysis of terrain morphology in urban areas based on DEMs are rarely supported by results of other investigations. This article presents a method to use and link multiple sources of spatial information for determining boundaries of natural landform units in relatively flat (low relief gradients) urban areas, heavily altered by human activities. The investigation is based on GIS landform parameterization based on high resolution DEM, geophysical surveys and geological or geoarchaeological documentation. The main aim of the study is to identify to what extent these complementary sources of data and the proposed method can be used in such a specific environment for the reconstruction of natural terrain morphology. The land-surface parameterization using the submeter DEM is the first approach of this type in the study areathe city of Wrocaw. The DEM analysis draws on the existing knowledge about changes in the drainage system and information about natural and man-made sediments collected from various sources (archaeological excavations, geomorphological and archaeological literature). Due to the scarcity of information concerning the spatial variation of alluvial deposits outside of the city center and the structure of valley escarpments, geophysical profiling was conducted using the Two-Dimensional Electrical Resistivity Tomography method (2D ERT). The obtained results allowed for the reinterpretation of previous geomorphological and geological views and data. Among others, five typical sequences of made grounds and the range of the main landform units within the perimeter of the city were identified. The proposed method is suitable for medium and large-sized cities of Central Europe, located in valley floors. Wrocaw, selected as a test area, has a history closely related to the development tendencies of other cities of Central Europe (PIEKALSKI 1999) e.g., Cracow, Leipzig, Dresden or Frankfurt; hence, it serves as an example as to how these new data sources add to the current state of knowledge on the natural configuration of urban land. 2. Methods In this paper the combination of geomorphological research methods was used. Data obtained in the traditional way (field mapping, analysis of archaeological outcrops and geological bore holes) were linked with an analysis of high resolution DEM and results of geophysical surveys. This comprehensive aggregation of research methods, rarely seen in the literature, was attempted for an urban area, where many factors interfere with the recognition of natural landforms. The flow chart in Fig. 1 illustrates the course of investigation. The basis for this study was the spatial analysis of the area of Wrocaw, based on DEM provided by the Municipal Office of Wrocaw. The original resolution of DEM is 1 9 1 m and it was created based on light detection and ranging (LiDAR). The airborne scanning was conducted in May 2006. The scanning point density was 2 points per sq metre, and the vertical measurement accuracy amounted to 15 cm. LiDAR data underwent pre-processing (YOU et al. 2003) by the supplier: the land surface (bare Earth model) was extracted from the point cloudinformation about the natural land cover, buildings and man-made elements that obstruct the view of waters (bridge fragments) were removed. Light detection and ranging (LiDAR) data are one of the most accurate sources about land configuration, surpassing in precision global DEMs obtained from satellites, using radar system (SRTM) or stereo-pair images (Aster GDEM) and existed models from digitization of topographic maps contours (DTED) (GAMBA et al. 2002; HIRANO et al. (...truncated)