Hans-Gerd Maas

Two algorithms for extracting building models from raw laser altimetry data

Two new techniques for the determination of building models from laser altimetry data are presented. Both techniques work on the original laser scanner data points without the requirement of an interpolation to a regular grid. Available ground plan information may be used, but is not required. Closed solutions for the determination of the parameters of a standard gable roof type building model based on invariant moments of 2 1r2-D point clouds are shown. In addition, the analysis of deviations between point cloud and model does allow for modelling asymmetries such as dorms on a gable roof. By intersecting planar faces nonparametric buildings with more complex roof types can also be modelled. The techniques were applied to a FLI-MAP laser scanner dataset covering an area of 500= 250 m 2 with a density of more than 5 pointsrm 2 . Within this region, all but one building could be modelled. An analysis of the variance of the parameters within a group of buildings indicates a precision in the range of 0.1-0.2 m. q 1999 Elsevier Science B.V. All rights reserved.

Particle tracking velocimetry in three-dimensional flows

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 todays 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 × 160 × 50 mm3.

Automatic forest inventory parameter determination from terrestrial laser scanner data

Terrestrial laser scanners find rapidly growing interest in photogrammetry as efficient tools for fast and reliable three‐dimensional (3D) point cloud data acquisition. They have opened a wide range of application fields within a short period of time. Beyond interactive measurement in 3D point clouds, techniques for the automatic detection of objects and the determination of geometric parameters form a high priority research issue. With the quality of 3D point clouds generated by laser scanners and the automation potential in data processing, terrestrial laser scanning is also becoming a useful tool for forest inventory. This paper presents a brief review of current laser scanner systems from a technological point of view and discusses different scanner technologies and system parameters regarding their suitability for forestry applications. Methods for the automatic detection of trees in terrestrial laser scanner data as well as the automatic determination of diameter at breast height (DBH), tree height and 3D stem profiles are outlined. Reliability and precision of the techniques are analysed on the basis of several pilot studies. In these pilot studies more than 97% of the trees could be detected correctly, and DBH could be determined with a precision of about 1.8 cm.

Photogrammetric Techniques in Civil Engineering Material Testing and Structure Monitoring

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.

Image sequence based automatic multi-camera system calibration techniques

An image sequence-based, fully automatic and rather flexible procedure for the calibration of stationary multi-camera systems for 3-D observation of dynamic events is presented and analysed. While conventional close-range camera calibration techniques are either based on a stable point-field with known reference coordinates or on a temporarily stationary point-field with only approximately known 3-D coordinates, which is imaged from different locations and under different orientations with one single camera, the presented technique is based on stationary cameras and moving targets, making use of the image sequence acquisition nature of most solid state cameras. In the simplest version, only one easily detectable marker has to be tracked through image sequences of multiple pre-calibrated cameras, thus avoiding the necessity of homologous feature identification for the establishment of multi-view correspondences; 3-D coordinates of the marker position are not required. This single-marker method does not allow for the determination of the interior orientation. In an extended version allowing for full camera orientation and calibration, a reference bar of known length is moved through object space, with the problem of feature identification and establishment of multi-view correspondences being reduced to the tracking of two targets. The method can only be used with multi-camera systems and is most useful for 3-D motion analysis applications, but may be adapted to a wide range of other applications. The advantages of the method over conventional self-calibration techniques are the trivial establishment of multi-view correspondences, the fact that no temporarily stable target field has to be constructed, and the fact that each camera has to be set up only once. After an explanation of the technique, its performance is examined in detail based on extensive computer simulations, and the practical effectiveness is shown in a pilot study on industrial robot calibration. Based on these studies, recommendations are given concerning the number of reference bar locations, preferable reference bar orientation schemes and the achievable accuracy potential.

Large-Eddy Simulation of Inhomogeneous Canopy Flows Using High Resolution Terrestrial Laser Scanning Data

The effect of sub-tree forest heterogeneity in the flow past a clearing is investigated by means of large-eddy simulation (LES). For this purpose, a detailed representation of the canopy has been acquired by terrestrial laser scanning for a patch of approximately 190 m length in the field site “Tharandter Wald”, near the city of Dresden, Germany. The scanning data are used to produce a high resolution plant area distribution (PAD) that is averaged over approximately one tree height (30 m) along the transverse direction, in order to simplify the LES study. Despite the smoothing involved with this procedure, the resulting two-dimensional PAD maintains a rich vertical and horizontal structure. For the LES study, the PAD is embedded in a larger domain covered with an idealized, horizontally homogeneous canopy. Simulations are performed for neutral conditions and compared to a LES with homogeneous PAD and recent field measurements. The results reveal a considerable influence of small-scale plant distribution on the mean velocity field as well as on turbulence data. Particularly near the edges of the clearing, where canopy structure is highly variable, usage of a realistic PAD appears to be crucial for capturing the local flow structure. Inside the forest, local variations in plant density induce a complex pattern of upward and downward motions, which remain visible in the mean flow and make it difficult to identify the “adjustment zone” behind the windward edge of the clearing.

Dynamic photogrammetric calibration of industrial robots

Todays developments in industrial robots focus on aims like gain of flexibility, improvement of the interaction between robots and reduction of down-times. A very important method to achieve these goals are off-line programming techniques. In contrast to conventional teach-in-robot programming techniques, where sequences of actions are defined step-by- step via remote control on the real object, off-line programming techniques design complete robot (inter-)action programs in a CAD/CAM environment. This poses high requirements to the geometric accuracy of a robot. While the repeatability of robot poses in the teach-in mode is often better than 0.1 mm, the absolute pose accuracy potential of industrial robots is usually much worse due to tolerances, eccentricities, elasticities, play, wear-out, load, temperature and insufficient knowledge of model parameters for the transformation from poses into robot axis angles. This fact necessitates robot calibration techniques, including the formulation of a robot model describing kinematics and dynamics of the robot, and a measurement technique to provide reference data. Digital photogrammetry as an accurate, economic technique with realtime potential offers itself for this purpose. The paper analyzes the requirements posed to a measurement technique by industrial robot calibration tasks. After an overview on measurement techniques used for robot calibration purposes in the past, a photogrammetric robot calibration system based on off-the- shelf lowcost hardware components will be shown and results of pilot studies will be discussed. Besides aspects of accuracy, reliability and self-calibration in a fully automatic dynamic photogrammetric system, realtime capabilities are discussed. In the pilot studies, standard deviations of 0.05 - 0.25 mm in the three coordinate directions could be achieved over a robot work range of 1.7 X 1.5 X 1.0 m3. The realtime capabilities of the technique allow to go beyond kinematic robot calibration and perform dynamic robot calibration as well as photogrammetric on-line control of a robot in action.

Large-Eddy Simulation Study of the Effects on Flow of a Heterogeneous Forest at Sub-Tree Resolution

The effect of three-dimensional plant heterogeneity on flow past a clearing is investigated by means of large-eddy simulation. A detailed representation of the canopy has been acquired by terrestrial laser scanning for a patch of approximately

From pixels to voxels: tracking volume elements in sequences of 3D digital images

328\,\mathrm {m}