Page - 776 - in Book of Full Papers - Symposium Hydro Engineering

2. VISUAL DOCUMENTATION OF DAM SURFACES WITH MODERN TOTAL STATIONS 2.1. DATA ACQUISITION For geodetic monitoring with total stations, the competent surveyor establishes stable points with known coordinates in the dam’s vicinity. These points ensure that observations of multiple epochs and/or setup points are referenced in a common coordinate frame. Regarding modern total stations, which enable scanning and imaging, this is a large benefit over TLS and SLR cameras, as point clouds are registered directly and image orientations are already known and stored when images are captured. Within a predefined polygonal area, modern TS instruments take images and acquire the scan data autonomously. The scanning resolution is not critical, as the shape of concrete (arch) dams is regular and thus well known. 2.2. DATA PROCESSING The process of combining geometry and texture content of an object’s surface is denoted as surface mapping. The principle workflow for processing the acquired scan and image data to derive a textured 3D photo model is illustrated in Fig. 2. The processing of the scan data (left box in Fig. 2) implies point cloud registration, filtering, outlier elimination and hole filling procedures. Based on the preprocessed point cloud, we compute a continuous digital representation of the surface with a combined approximation-interpolation method. We partition the data space into voxels, for which we compute characteristic points by averaging the respective points inside. This step provides a simple but effective approximation method and hence noise reduction. Fig 2: Workflow to compute 3D photo models from scan and image data of TS 776