Particle Tracking Velocimetry (PTV) is a well-known technique for the determination of velocity vectors within an observation volume. However, for a long time it has rarely been applied because of the intensive effort necessary to measure coordinates of a large number of flow marker particles in many images. With today's imaging hardware in combination with the methods of digital image processing and digital photogrammetry, however, new possibilities have arisen for the design of completely automatic PTV systems. A powerful 3D PTV has been developed in a cooperation of the Institute of Geodesy and Photogrammetry with the Institute of Hydromechanics and Water Resources Management at the Swiss Federal Institute of Technology. In this paper hardware components for 3D PTV systems wil be discussed, and a strict mathematical model of photogrammetric 3D coordinate determination, taking into account the different refractive indices in the optical path, will be presented. The system described is capable of determining coordinate sets of some 1000 particles in a flow field at a time resolution of 25 datasets per second and almost arbitrary sequence length completely automatically after an initialization by an operator. The strict mathematical modelling of the measurement geometry, together with a thorough calibration of the system provide for a coordinate accuracy of typically'0.06 mm in X, Y and 0.18 mm in Z (depth coordinate) in a volume of 200 x 160 • 50 mm 3.
Civil engineering material testing includes a wide range of applications requiring the determination of the threedimensional shape of an object and changes thereof. Large structure monitoring will often include the necessity of determining object deformations at a large number of points. Photogrammetric techniques offer a large potential for the solution of a wide range of measurement tasks in this field. A modular toolbox consisting of digital cameras, computer interfaces, illumination systems, calibration devices, combined with subpixel accuracy image measurement operators, multi-image matching techniques, and self-calibrating bundle adjustment in a suitable user interface, depicts a very powerful tool for tailoring custom-made solutions for material testing labs. Due to the wide range and the repetitive nature of measurements tasks in civil engineering, these applications could depict a significant future market for photogrammetry. This paper will briefly discuss the major hardware and software modules of a toolbox for civil engineering material testing and large structure monitoring. Based on several sample applications covering object dimensions from 10 cm to 500 meters, the potential of photogrammetric deformation measurement techniques will be shown. The major advantage of photogrammetric techniques can often be seen in the fact that they allow for highly automated measurements at a large number of points simultaneously. In many cases, object deformations can be determined at a precision in the order of 1:100,000 of the object dimension, based on off-theshelf hardware components.
[1] We performed three field campaigns in 2004, 2007, and 2010 at the southern margin of the Jakobshavn Isbrae, West Greenland, in order to infer flow velocities and their changes from photogrammetric time-lapse imagery with a temporal resolution of 20 min and a spatial spacing of about 30 m on the glacier surface. Area-wide analysis of more than 3000 three-dimensional trajectories at individual glacier positions allow for both the mapping of the grounding line and the detailed observation of flow variations during major calving events. From 2004 to 2010, the grounding line of Jakobshavn Isbrae retreated 3.5 AE 0.2 km. Considering previously published results, the grounding line retreat amounts to 6 km since 1985. The glacier has an ephemeral floating tongue that can establish during the readvance of the glacier front and break apart after large calving events. Observations of a major calving event show that an acceleration of flow velocities coincides with the onset of the break up during which flow velocities of up to 70 m/d can be reached. Moreover, large vertical displacements of the glacier front in the order of 15 m and lowering of 8 m at positions 500 m beyond the calving front were observed 2 days before the calving event.After the break up, the glacier slowly adjusts to the new boundary conditions within the next 4-5 days. Flow velocity variations caused by calving were detected up to 1 km upstream only which indicates that individual calving events have no immediate effect on the large-scale glacier dynamics.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.