Fig. 2. Flowchart of the applied schematic methodology.Figure optionsDownload full-size imageDownload as PowerPoint slide
3.1. Data acquisition
For image acquisition, two different perspectives have been considered — ground-based as well as airborne. The airborne images were conducted with an UAV (AscTec Falcon 8) which was applied at a flying altitude of approx. 15 m. To ensure sufficient area coverage, flight planning accounted for an overlap in flight direction of 80% and an overlap of flight strips of 60%. The UAV was equipped with an active stabilizing camera mount that RVX-208 compensates for system vibrations and flight disturbances due to wind, so the camera is nearly always nadir oriented. A non-metric digital compact system camera (Sony NEX-5 N), with a fixed lens and a focal length of 20 mm, was utilised for image acquisition. Sensor size is 23.5 × 15.6 mm2 with a corresponding pixel resolution of 4.8 μm.
For the ground based image acquisition two different hand-held single-lens reflex cameras with fixed lenses were used — Nikon D100 in October 2013 and Canon 600D in February 2014. For the terrestrial case, the use of a second camera was necessary due to a technical defect of the camera from the first campaign. For realizing an approximately identical geometric resolution, a configuration of camera und related objective was used to achieve similar ground resolution during both campaigns. The lenses of the cameras have focal lengths of 28 mm and 20 mm respectively. Sensor sizes are about the same size as the aerial utilised camera (23.7 × 15.6 mm2 and 22.3 × 14.9 mm2). The Canon 600D has also a similar pixel size of 4.3 μm. However, pixel size of the Nikon D700 is significantly larger (7.8 μm), resulting in a higher signal to noise ratio.