Implementation of virtual reference points in registering scanning images of tall structures

The concept of obtaining uniform and highprecision 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.