Patrick Westfeld

Recent Developments in 3D-PTV and Tomo-PIV

The article gives an overview on recent developments to 3D flow measurement techniques, focusing on photogrammetric aspects of 3D geometric modelling and multi-camera system configuration. A geometric model for a virtual four-camera system, which is realized by a single camera and a four-fold mirror system, is presented. The 3D-PTV model is extended by a ray tracing technique to strictly handle the spatial intersection of manifold broken optical beams when determining coordinates of particles in a liquid from images from multiple viewpoints taken by cameras observing the flow through multiple curved interfaces. Transferring these geometric models to PIV, we present some algorithmic improvements to tomographic PIV, which have the potential to achieve a significant reduction of the computational effort.

An Approach to Determining Turbidity and Correcting for Signal Attenuation in Airborne Lidar Bathymetry

Airborne lidar bathymetry is an efficient technique for measuring the bottom of shallow water bodies. A characteristical feature of lidar bathymetry beam propagation is given by scattering and absorption effects in the water column, both leading to a loss of received signal intensity. This loss of signal intensity depends on the turbidity of the water body. Inversely, an analysis of the decay of the recorded waveform signal allows for deriving statements on the local degree of turbidity in the water. The paper shows a first approach on the determination of one turbidity measure per laser pulse by analysing the recorded waveform and fitting an exponential function, wherein the decay coefficient depicts an integral measure describing turbidity. The technique was applied to a shallow inland water, and the results were validated by conventional point-wise turbidity measurement techniques. An obvious consequence of attenuation and loss of signal intensity in lidar bathymetry is the fact that the bottom returns become rather weak. In many cases, conventional ground pulse echo detection techniques fail in detecting water bottom points, leading to a reduced number of water body bottom points and thus limiting the application range of the technique. To partly compensate for this effect, a differential backscatter cross section determination based signal attenuation correction method has been developed, which allows for a signal-derived re-amplification of the ground signal. Although the technique also amplifies noise, it could be shown that it is capable of delivering a higher number of additional ground points and thus extending the applicability of the technique.

Statistical Modeling of Interpersonal Distance with Range Imaging Data

The presented work combines automated highly resolved spatio-temporal photogrammetric data acquisition and analysis with statistical approaches for the determination of the interpersonal distance between interacting persons. This topic forms an interesting bridge between engineering and educational research, delivering a new efficient measurement technique to educational research and opening new application fields to photogrammetry.

Triangulation methods for height profile measurements on instationary water surfaces

Abstract The precise measurement of water surface models or profiles plays an important role in experimental hydromechanics. Conventional gauge-based techniques often come with a large instrumental effort and a limited spatial resolution. The paper shows an efficient non-contact photogrammetric technique for the measurement of water-surface profiles, which is based on an extension of the well-known laser light sheet projection technique. While the original laser lightsheet triangulation technique is limited to surfaces with diffuse reflection properties, the developed technique is capable of measuring on reflecting instationary surfaces. This article presents the basic principle, potential and limitations of the method. Several evolution steps of the system with different applicability and different complexity are shown. A double projection plane system capable of simultaneously measuring water surface height and tilt profiles marks the ceiling of the development. Besides the geometrical models of different levels of complexity, system calibration procedures are described. The applicability of the techniques and their accuracy potential are shown in several practical tests.