Abstract
Abstract The concept of obtaining uniform and high-precision 3D models of large and complex engineering structures based on terrestrial laser scanning surveys highly depends on the accuracy of registering a series of adjacent scans. If it is not possible to place the required number of reference points in the observation scene, and there are no identifiable structural elements that could serve as such reference points, the issue can be solved by using the virtual reference points. The virtual reference points are not marked, but their position can be re-created in the post-processing of the scans on the basis of the geometrical features of the object. In the case of an industrial chimney, these can be the structural axis points determined at arbitrarily selected altitudes. In the solution presented herein, the coordinates of the reference points were independently determined twice: in the geodetic coordinate system using traditional method, in the local system of the scanner using the bisector of the chord of the circle, and by approximation of the circular arc shape of the chimney shell. Both procedures are preceded by the filtration of outliers. The concept of virtual reference points was verified by the measurement of the verticality of the axis and the structural geometry of a steel industrial chimney, which was accessible only on the foundation level.
Highlights
The evolution of scanning technology (TLS), we have seen in recent years, has made this method more and more eagerly used in broad-based engineering, but in many applications has given it a leading role [1]
One should definitely point out the ease and speed of use; precision with a high point density, noninvasive, and remote character of the measurement; and the possibility of remote operation. All these contribute to the fact that the TLS technique is applied in as built surveys of buildings and structures for the purpose of documentation of their existing condition and checking compliance with the project [1,2,3,4]; creating the precise 3D models of structures of various types for design purposes [5]; documenting historical objects [6,7]; or destruction, deformation, and monitoring of engineering constructions and excavations [3,6,8,9,10,11,12]; surveying of tunnels and rail tracks [13]; calculating volumes; applying building information modeling (BIM) [1,2]; monitoring the stability of the earth’s structures [14]; and geotechnical and geological mapping in mining
From the engineering surveying perspective, one of the most interesting features of this technology is the possibility of obtaining a full model of the surveyed structure [3,9,10]. This contributes to the ability to perform advanced analyses [3,12] as well as to obtain the desired information on the properties or state of an object at any point, not just at selected spots [2,3,9]
Summary
The evolution of scanning technology (TLS), we have seen in recent years, has made this method more and more eagerly used in broad-based engineering, but in many applications has given it a leading role [1]. The verticality of the industrial chimney axis is usually analysed by measuring the directions of the tangents to the circular sections of the structure This method is accurate and, depending on the number of stations, makes it possible to assess the deformation of the structure [19]. Solving the task requires determining the coordinates of the chimney axis in the local point cloud system and in the geodetic coordinate system The former is performed using the scan point approximation method and the latter by geodetic method used for analysing slender structures. In the processing of scanned images, the curvature is determined with the assumption that the analysed sections of the chimney shell are circular sectors In this context, the outcome of alignment of conventional observations of the control network points is an important input. The determined mean errors and the error ellipse of the circle centre coordinates indicate the importance of the irregularity of the circular shape on specific levels
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