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In this volume, a general view of Engineering Geologyis presented, its state of the art and up-to-date information about recent scientific tasks, aims and methods. But also the role of geo- desists in collaboration with civil and mechanical engineers, technical designers and architects is outlined. As a reference book, this volume will be useful for researchers, students and practitioners in Engineering Geodesy and neighbouring disciplines.
Passar bra ihop. Show all. Table of contents 38 chapters Table of contents 38 chapters Engineering networks in a threedimensional reference frame Pages Grafarend, Erik W. Robot vision based on an exact solution of the threedimensional resection-intersection Pages Grafarend, E. Show next xx. Read this book on SpringerLink. Recommended for you. Terrestrial laser scanning is one of the most efficient methods for providing the highly accurate and dense point clouds that can subsequently be used for measuring Earth surface processes [ 1 ], monitoring the deformations caused by natural hazards [ 2 , 3 ] or generating three-dimensional spatial models, e.
The point clouds collected by a terrestrial laser scanner require georeferencing [ 9 , 10 ], i. In terrestrial laser scanning, the point cloud is typically transformed into the geocentric coordinate frame using a 7-parameter Helmert transformation [ 6 ].
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The set of seven transformation parameters is determined using the least squares method on a basis of several georeferencing points whose coordinates are measured both by the laser scanner and a global navigation satellite system GNSS receiver. Alternatively, the optical head of a laser scanner may be equipped with several mounted GPS antennas [ 12 , 13 , 14 ]. Other methods of georeferencing of terrestrial scanner data comprise back-sighting, sensor-driven, and data-driven approaches [ 10 , 15 ] which also rely on control point targets. Therefore, additional terrestrial measurements may be needed in order to ensure the sufficient number of control points in such situations.
To avoid this possible additional effort, we propose a method for direct georeferencing of point clouds obtained using terrestrial laser scanning that allows for providing coordinates in the geocentric frame using a minimum number of GNSS measurements. To apply the proposed method, the scanner must be levelled. Furthermore, we show results from a field test that proves correctness of the results obtained using proposed method.
The paper is structured as follows: First, we introduce laser scanner geocentric orientation parameters, then an adjustment procedure for determination of the orientation parameters is described. In the following section, a field test of the proposed method is provided. Finally the method is discussed in an extended context and conclusions are drawn. The coordinates x , y , z of the point Q measured by a laser scanner can be obtained by Figure 1 :. The origin of the laser scanner coordinate system x , y , z is usually defined as a point of the electro-optical center of the scanner or a point of the intersection of the horizontal and vertical rotation axes of the scanner or a zero distance measurement point of the scanner [ 17 , 18 ].
The coordinates x , y , z of a particular point Q measured by a laser scanner are converted into the X , Y , Z external reference frame according to the formula e. Many authors, e.
Prior to the transformation, the orientation parameters can be adjusted and thus also improved by the least squares method which provides the expected values of estimated parameters as well as standard deviations of estimated parameters and adjusted observations. The nonlinear observational Equation 10 can easily be linearized using the expansion into the Taylor series and we receive the Gauss—Helmert model comp. The linear observational equation of the integrated laser scanner, GNSS, and EGM data 11 can be solved using the weighted least squares method:. The solution of the observational Equation 11 under the condition 20 is given by e.
The proposed method was validated using real GNSS and laser scanning measurements obtained in the framework of a field experiment.
The position of the laser scanner and the position of the GNSS point used in the experiment are shown in Figure 2. P X 0 , Y 0 , Z 0 denotes the laser scanner reference point position measured by a GNSS receiver; Q X , Y , Z is the georeferencing point measured both by the laser scanner and the GNSS receiver; EGM gravity denotes the direction of the plumb line from the global Earth gravity model EGM, assuming that the plumb line direction coincides with the vertical laser scanner rotation axis. Points 1, 2, 3, 4, 5 and 6 are remote testing points extracted from the point cloud and measured by the GNSS receiver, as well.
The remaining points are located on the ground. In order to ensure unambiguous identification of the testing points in the point cloud, the identification targets were placed on both the roof points and on the ground points while scanning. The heights of the laser scanner i and reflector j are also the measured quantities see Table 1. The standard deviations, provided in Table 1 , are the a priori values used for parameter estimation.
The GNSS coordinates of the testing points 1, 2, 3, 4, 5, 6 are not included in the least squares adjustment of the laser scanner position, as they are only used for the assessment of the laser scanner geocentric georeferencing accuracy. Each point was measured three times in 2 min sessions.
The values provided in Table 2 are the mean values. For the practical applications in the lowlands, the components of the vertical deflections can be easily interpolated from the global vertical deflection GVD model provided on the internet e.
Thus, such an approximation may be applied only to the lowland areas, where the differences between vertical deflections on the geoid and Earth surface are negligible. For mountainous areas, Equations 28 and 29 should be used for deriving correct values of the vertical deflections. According to producers of laser scanners, the direction of the vertical axis is consistent with the plumb line direction to the order of 1 arcsec. The EGM gravity has been selected as one of the most highly accurate global gravity field models, which is based on a combination of satellite laser ranging data for the longest wavelengths of the gravity field, the Gravity Recovery and Climate Experiment GRACE data for the long and mid wavelengths of the gravity field, as well as from the altimetry and terrestrial measurements for the local gravity.
Pavlis et al. The accuracy of this transformation is tested by comparing the transformed geocentric coordinates of the test points 1, 2,…, 6 with the corresponding coordinates obtained by the GNSS receiver. Table 3 shows that all the differences of the coordinates measured by GNSS and from the transformation using the vertical deflection are below or at the level of 1 cm for the all test points. Differences between geocentric converted and measured coordinates of the test points of the point cloud.
Assuming that the accuracy of the X , Y , Z coordinate determination by the laser scanner depends on only GNSS positioning which is not true, because additional errors come from scanning technology , the accuracy of coordinate differences can easily be calculated.
This is the most optimistic evaluation. In fact, this error value could be significantly greater. Thus the coordinate differences summarized in Table 3 are in the range of their errors. As opposed to the classical georeferencing methods of point clouds, the proposed solution introduces the a priori information on two orientation angles of the measurement reference frame.
Therefore, the geometry of the observations is directly linked to the local gravity field which gives the possibility of transforming the coordinates to the global geocentric reference frame. In the classical solution, there is no a priori information on the gravitational field, and thus, the orientation of all three axes must be provided by geocentric coordinates, which is why at least three georeferencing GNSS points are needed.
The proposed method allows for reducing the number of required GNSS georeferencing points to two through a direct link between the geometry provided by GNSS in the global reference frame, and terrestrial scanning measurements in the local reference frame, and the gravity information from the global Earth gravity model EGM, which provides the connection between the local and the global reference frames.
In particular, the proposed method can be useful when the number of georeferencing GNSS points is limited or the accuracy of GNSS reference point positions have deteriorated due to poor satellite visibility, e.
It is typical that the GPS satellite visibility may be poor in city corridors. However, it is possible to find single gaps with a proper satellite configuration that can allow for georeferencing. Our method needs only two GNSS points with a proper determined position. The advantage of the proposed method is more evident in case of terrestrial laser scanning performed indoor and outdoor of a building, e.
In such a case, the indoor point cloud can be joined with the outdoor point cloud via one georeferencing point located outside the building. The scanner position inside the building can be determined when applying the method proposed in [ 29 ]. The integration of the vertical deflection with the classical method for georeferencing can also be used for the validation of the GNSS coordinates when any of the GNSS points are suspected to contain outliers or substantial systematic effects due to poor satellite observability or multipath.
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Even if the number of GNSS referencing points is small, the outliers or points affected by systematic errors can be successfully detected by a constraint derived from the orientation of the vertical axis in the local gravity field using the subsequent formulae. The incorporation of the geometrical observations and the global gravitational field constitutes an important issue in the framework of the integration of precise surveying techniques.