Volume 6, Issue 3-4

Abstract. System identification is one main task in modern deformation analysis. If the physical structure of the monitoring object is unknown or not accessible the system identification is performed in a behavioural framework. Therein the relations between input and output signals are formulated on the basis of regression models. Artificial neural networks (ANN) are a very flexible tool for modelling especially non-linear relationships between the input and the output measures. The universal approximation theorem ensures that every continuous relation can be modelled with this approach. However, some structural aspects of the ANN-based models, like the number of hidden nodes or the number of data needed to obtain a good generalisation, remain unspecified in the theorem. Therefore, one faces a model selection problem. In this article the methodology of modelling the deformations of a lock occurring due to water level and temperature changes is described. We emphasise the aspect of model selection, by presenting and discussing the results of various approaches for the determination of the number of hidden nodes. The first one is cross-validation. The second one is a weight deletion technique based on the exact computation of the Hessian matrix. Finally, the third method has a rigorous theoretical background and is based on the capacity concept of a model structure.

Abstract. The adjustment of systems of highly non-linear, redundant equations , deriving from observations of certain geophysical processes and geodetic data cannot be based on conventional least-squares techniques, and is based on various numerical inversion techniques. Still these techniques lead to solutions trapped in local minima, to correlated estimates and to solution with poor error control. To overcome these problems, we propose an alternative numerical-topological approach inspired by lighthouse beacon navigation, usually used in 2-D, low-accuracy applications. In our approach, an m -dimensional grid G of points around the real solution (an m -dimensional vector) is at first specified. Then, for each equation an uncertainty is assigned to the corresponding measurement , and the sets of the grid points which satisfy the condition are detected. This process is repeated for all equations, and the common section A of the sets of grid points is defined. From this set of grid points, which define a space including the real solution, we compute its center of weight, which corresponds to an estimate of the solution, and its variance–covariance matrix. An optimal solution can be obtained through optimization of the uncertainty in each observation. The efficiency of the overall process was assessed in comparison with conventional least squares adjustment.

Abstract. Kalman filtering is a multiple-input, multiple-output filter that can optimally estimate the states of a system, and applicable for deformation analysis. The states are all the variables needed to completely describe the system behavior of the deformation process as a function of time (such as position, velocity etc.). The standard Kalman filter estimates the state vector where the measuring process is described by a linear system. In order to process a non-linear system an optimized aspect of Kalman filter is required. The main purpose of this research is to evaluate the optimized Kalman filter (OKF) as a non-robust method versus the iterative weighted similarity transformation (IWST) as a rigorous (also called robust) method. To satisfy this objective, first a detailed description on executing the optimized Kalman filter using the observation of angles and distances directly is provided. Then, 2-D total station data observations comprising distances and angles are used to demonstrate the OKF. For detecting the deformation, a real point-related test (single point test) is applied for every point as a local test. Consequently, the findings from OKF are compared and evaluated against the results from the IWST method. In general, the outcome of the Kalman filter algorithm is close to the preliminary results from the IWST method. The maximum and minimum differences in computed displacements are 0.2 and 2 millimeters respectively. Finally, Kalman filter approaches, having some properties, are recognized as suitable techniques for deformation analysis.

Abstract. Based on the model of coordinate S-transformation, a novel method of stability analysis of datum points in high-precision GPS deformation monitoring networks is proposed. The model of coordinate S-transformation is used to calculate seven transformation parameters in adjacent two measurement stages, in order to confirm the stability of stations by coordinate differences. To judge the stability of stations, in comparison to the traditional method by a fixed the same datum point, the “threshold approach” and “statistical test approach” have been developed and applied to evaluate the stability of datum points of a first-order GPS deformation monitoring network of a hydropower station located in the West Region of China.

Abstract. The optimal velocity estimation of the base stations relies on the optimal or at least realistic function model and stochastic model of the noises in the GPS monitoring position series. In this paper, we first analyse the theoretical effects of periodic signals and different noises on velocity estimates, and numerically analyse the impacts of common mode errors on the different noises. Then we numerically analyse the effects of the different types of noises on the velocity estimation using the GPS monitoring position series of base stations in China. The result shows the importance of the correct noise composition that is specified by the correct stochastic model. We therefore determine the optimal noise composition and its stochastic model by using variance component estimation and w -test theory on the daily monitoring position series with removed common mode errors. The result indicates that the noise of daily monitoring position series consists of both white noise and flicker noise rather than only the white noise in statistics. As a result, the realistic station velocities are computed with the associated noise stochastic model by using a 7-year GPS monitoring position series of 24 base stations in China, comparing with the results obtained using the pure white noise model. The results show that the root mean squared error of velocity derived from the realistic stochastic model is over 6 times larger than that derived from the pure white noise model, which indicates that the pure white model gets a too optimistic velocity estimate. Therefore, from the estimated velocities with the realistic noise stochastic model, the real-world tectonic motion trends can be recovered.

Abstract. Building structures are extremely sensitive to the influence of outdoor conditions. The most frequent types of outdoor conditions are the influence of wind, sunshine, changes in the temperature of a building's surroundings and, lastly, the effect of a buildings own loading or the improper loading by another source. According to the resonance of the structure with its surroundings, vibrations and oscillations at relatively high frequency intervals (0.1–100.0 Hz) also occur. These phenomena significantly affect the static and dynamic characteristics of structures, as well as their safety and functionality. The paper provides an example of the monitoring of these phenomena using geodetic methods at two different types of structures. The first example is an industrial structure with a cylindrical shape, the monitoring of which was made by a total station with a measuring frequency of approximately 2 Hz. The second example is the Apollo Bridge on the Danube in Bratislava (Slovakia), the steel structure of which was measured by acceleration sensors with a frequency of up to 10 Hz. The central aim of the paper is an analysis of the dynamic behavior of both structures using spectral analysis methods. The use of the Fast Fourier Transform (FFT) and the Lomb–Scargle periodogram is described; the structure's own frequencies and the amplitudes of the structure's oscillations are calculated.

Abstract. This paper examines the potential of microwave radar interferometry for monitoring the dynamic behaviour of large civil engineering works. It provides an overview of the method, its principles of operation with particular emphasis given on the IBIS-S system. Two areas of application are considered and the results of the analyses are presented and discussed. The first experimental study involves the monitoring of the dynamic response of a tall power plant chimney due to wind load. The second example examines the dynamic behaviour of a long cable-stayed bridge. In this case, the focus is placed on the effects that individual traffic events impose on the vibration response of the main span of the bridge deck and the bridge pylons. Analysis of the results provides detailed displacement time-histories and the dominant frequencies observed at the top of the chimney and along the bridge deck and the top of the towers. Also, cross-comparisons and discussions with the results obtained at the same structures using different sensor configurations are provided.

Abstract. The paper presents the mobile multi-sensor system Orthos Plus for the monitoring and mapping of tunnel walls, a scan data processing method for the evaluation of 3-d tunnel wall displacements from subsequent wall scans and, finally, a virtual reality tool supporting the interpretation of data. The measuring system consists of a 3-d laser scanner, a motorised total station and a digital camera that are integrated on a light metal frame that is installed on a mobile platform. It has been designed to perform tunnel measurements most efficiently and to meet the special requirements of tunnels under construction. The evaluation of 3-d displacements is based on a 3-d matching algorithm that takes advantage of the particular conditions of tunnel (shotcrete) surfaces. The virtual reality tool allows viewing of data in a 3-d virtual reality tunnel model and their animation in time and space in order supports understanding in an optimal way. The measuring system Orthos Plus has been developed in the course of a national research project, the 3-d matching method in the frame of the Austrian Christian Doppler Laboratory Spatial Data from Laser Scanning and Remote Sensing and the VR tool in the Austrian COMET K1 Competence Center VRVis Center (www.vrvis.at).

Abstract. Quality descriptions are parts of the key tasks of geodetic data processing. Systematic errors should be detected and avoided in order to insure the high quality standards required by structural monitoring. In this study, the iterative closest point (ICP) method was invested to detect systematic errors in two overlapping data sets. There are three steps to process the systematic errors: firstly, one of the data sets was transformed to a reference system by the introduction of the Gauss–Helmert (GH) model. Secondly, quadratic form estimation and segmentation methods are proposed to guarantee the overlapping data sets. Thirdly, the ICP method was employed for a finer registration and detecting the systematic errors. A case study was casted in which a dam surface in Germany was scanned by terrestrial laser scanning (TLS) technology. The results indicated that with the conjugation of ICP algorithm the accuracy of the data sets was improved approximately by 1.6 mm.

Abstract. Terrestrial laser scanners are high-accuracy 3D imaging instruments that are capable of measuring deformations with sub-millimetre level accuracy in most close-range applications. Traditionally, deformation monitoring via laser scanning is performed by measuring distinct signalised targets. In this case, the centroid of these targets must be determined with great accuracy for optimum detectability. To achieve this, a least-squares target centroid extraction algorithm suitable for planar checkerboard-type targets is proposed for irregularly organised laser scanner data. These target centroids are then used in a free-station network adjustment for performing deformation analysis with no a priori assumptions about the deformation pattern. To ensure the optimum measurement accuracy, all systematic errors inherent to the instrument at the time of data acquisition needs to be removed. One of the methods for reducing these systematic errors is by performing self-calibration of terrestrial laser scanners. In this paper, this was performed on-site to model the systematic errors of the scanner. It is demonstrated that the accuracy of the recovered translational movements were improved by an order of magnitude from the millimetre level to the sub-millimetre level using this approach. Despite the success of using laser scanners with signalised targets in deformation analysis, the main benefit of active sensors like terrestrial laser scanning systems is their ability to capture 3D information of the entire scene without installing markers. A new markerless deformation analysis technique that utilises intersection points derived from planar-features is proposed and tested in this paper. The extraction and intersection of planes in each point cloud can be performed semi-automatically or automatically. This new method is based on free-stationing and does not require a priori knowledge about stable control points or movement patterns. It can detect and measure both translational and rotational movements of the planes with minimum human interaction. The effectiveness of the proposed methodology is studied through simulations and real datasets captured with a phase-based Leica HDS6100 scanner.

Abstract. In September 2007 a geodetic network based on GPS measurements, precise levelling and continuous borehole tilt measurements was established for surface displacement monitoring on the high bank of the River Danube at Dunaszekcső where a serious landslide began to develop at that time. The landslide took place on February 12, 2008. After rapid, enormous movements, which resulted in significant subsidence (8–10 m) and lateral displacements (4–5 m) on the high bank at Dunaszekcső and the emergence of a peninsula consisting of clastic material in the Danube's bed, the deformation rates significantly dropped. This observation was explained by the secondary metastable equilibrium of the moving blocks at that time. However, our subsequent geodetic data clearly showed that slow post-event movements (mainly subsidence, 0.5–1.0 cm/month) on both the northern and southern sliding blocks have been occurring for the last two years, which means that the sliding blocks did not reach an equilibrium in the mentioned period. At the same time, measurements on our extended geodetic network in the second half of 2010 referred to a possible southward spread of sliding (southern part of the Vár Hill) in the near future. Later on (spring 2011), field observations were also indicative of the initiation of movements on the southern part of the Vár Hill as predicted by the geodetic measurements. The areal extent of the block becoming unstable closely corresponds with the danger zone assigned by GPS observations.

Abstract. Shanghai maglev is a very fast traffic tool, so it is very strict with the stability of the roadbed. However, the ground subsidence is a problem in Shanghai because of the poor geological condition and human-induced factors. So it is necessary to monitor ground subsidence in the area along the Shanghai maglev precisely and frequently. Traditionally, a precise levelling method is used to survey along the track. It is expensive and time consuming, and can only get the ground subsidence information on sparse benchmarks. Recently, the small baseline differential SAR technique plays a valuable part in monitoring ground subsidence, which can extract ground subsidence information with high spatial resolution in a wide area. In this paper, L-band ALOS PALSAR data and C-band Envisat ASAR data are used to extract ground subsidence information using the SBAS method in the Shanghai maglev area. The results show that the general pattern of ground subsidence from InSAR processing of two differential bands of SAR images is similar. Both results show that there is no significant ground subsidence on the maglev line. Near the railway line, there are a few places with subsidence rates at about -20 mm/y or even more, such as Chuansha town, the junction of the maglev and Waihuan road.

Abstract. The geographical scope of the experiment in this paper is Dangxiong in Tibet. The rift basin of Yangbajing-Dangxiong-Gulu locates at the east side of the Nyainqentanglha. The crustal activities in Dangxiong basin are frequent and a great deal of geothermal energy converged under the surface. The geothermic resources were used through the exploitation of underground water. The excessive extraction caused the surface deformation in surrounding areas of the pump wells. In this paper, we investigate the surface deformation caused by the geothermal extraction in Dangxiong with 23 Envisat ASAR images from 2006 to 2010 based on the time series analysis. The results show that the center of the basin was relatively stable. The deformation rate was between -1 mm/yr and 2 mm/yr. The surface subsidence rate of the region with the pump wells was about 8 mm/yr.

Abstract. This article briefly describes the structure and activities of the International Association of Geodesy (IAG), Commission 4 – Positioning and Applications. Working closely with other international organizations and professional institutions the focus of this commission is to leverage current and emerging positioning techniques and technologies to deliver solutions for a range of engineering, scientific and mapping applications.