In point-wise geodetic surveying it is before the acquisition process that the choice on where to place the representative points has to be made in order to control the position and integrity of the structure in question. From the TLS point of view, it is only after the point-to-surface transformation that the decision has to be made on how the evaluation should be performed. In general, when analyzing multi-temporal TLS models, the extensiveness of geometric description of variations in the object’s condition is very much limited by the amount of surface detail. Hence, it is not always possible to approach the displacements and deformation inspection in the same way as in the case of point-wise geodetic monitoring with signalized points inherently enabling the determination of movements in all three coordinate directions. In an extreme imaginary case, for example if scanning a flat wall, any changes within this plane will be undetected by the device. Though rare, in such cases, the only option is to fix some control points onto the surface. More likely, when lacking surface features, it may be possible to extract some information on the magnitude and direction of displacements of individual surface segments. Provided enough features exit, the surface model can ultimately be reduced to some single specific (representative) points on the object surface and their position tracked with respect to time. The latter two cases, i.e., deformation inspection models, are discussed in more detail in the rest of this section.
2.8.1 Model 1: Truncated direction
The word truncated refers to the inability of this inspection model to describe the direction of dis-placements of individual surface segments in all coordinate system directions regardless of the type of system being chosen (cartesian or object dependent, such as cylindrical which is widely used
when modelling the surface of tunnels from the TLS data). This disadvantage is a direct conse-quence of the absence of any distinct object features preventing the surface models to be reduced to some identical points in all measurement campaigns. Consequently, the tracking of model segments is only possible along the predetermined normal vector directions (tracking directions), as shown in Figure 9. In Figure 9, the magnitude of displacements of individual surface segments is presented as
Figure 9: Segment-wise displacement inspection. The direction of movements is determined by the initial segment normal vectors.
the distance between the initial segment position (black dots) and the points where lines composed by the initial normal vector directions intersect with segments coming from any successive epoch.
Such an approach also provides the ground for studying how the normals of successive segments diverge from the initial one and indicates how the deformation is progressing.
To apply this inspection model, the first requirement is that the point clouds are already split and modelled by segments as describes in section 2.7.1 in order to have normal vectors available. The more surface segments of different orientation the structure contains, the larger the number of dis-placement tracking directions. The overall description of the object status (position and integrity) and its changes should be composed by analyzing the vector fields consisting of all tracking direc-tions available, especially since the effects of forces acting onto the structure may be transmitted to various number of tracking directions with different magnitude depending on their nature. In
certain cases the direction of forces may coincide with the tracking directions to a high degree, such as when monitoring water pressure effects behind hydro plant dams or bridges being exposed to the force of gravity or heavy loads. If so, despite its limitations this inspection model can be success-full in answering how the structure reacted at the presence of forces whether internal or external in origin.
Another advantage of this model worth noticing is the high number of model segments compared to the number of representative points which can typically be extracted from the surface models of anthropogenic structures and objects. Hence, the truncated direction model should be seen as a surface-wise inspection model enabling a near continuous study of displacements and deformations of the surface in question. Therefore, even when representative points are at disposal, it is advisable to evaluate the spatio-temporal changes of surface models according to this approach in order to confirm the consistency of results of both inspection models.
2.8.2 Model 2: Representative points
Compared to the truncated direction model, the reduction of surface models to single specific points leads to a point-wise displacement analysis which can provide full (3D) geometric information on the object’s condition and its changes without the need to install any control points or other sensor compositions onto the object’s surface. The determination of identical representative points in all measurement campaigns is of great importance in order to treat their displacements correctly (i.e., referring to the same point at all times). Again the definition of these points is problem dependent;
still some guidelines can be drawn as to where and how they should be extracted.
Regarding the location of their extraction, if the object’s shape has been deformed, the representative points must be determined on the surface itself with the extraction process resulting in exact point solutions. In such deformable cases when the number of points is low, the results of this inspection model may have to be combined with those coming from model 1. Contrary to that, if the object’s shape has remained unchanged and it has only changed its position, the object may be presented by some specific points on its surface whereby their number depends on the size of the object. In special cases when monitoring objects with well-defined geometry, which have not changed their form, we may choose the representative points which do not necessarily lie on the surface of the object (e.g., object axis); an example that will be described in section 3.3.
During the process of extraction of representative points, these can be obtained by intersecting different kinds of surface model descriptors, such as planes or lines which have been modelled be-forehand and have their parameters estimated on the basis of redundant observations. Referring to Figure 8 on page 24 for demonstration, the representative points can be determined in the inter-section of three adjacent planes. Furthermore, if identical representative points are to be extracted on a line (axis), this cannot be done straightforward because of the line’s direction vector having a slightly different position in each measurement epoch depending on point configuration. The only
way to overcome the problem is by intersecting the line by another model descriptor (plane, line, etc.) or by projecting a point with known position onto the line. In the case of outdoor test 1 the pro-jection was only possible by using one of the network’s control points since no other single point could be extracted from the surface model. In addition, with models containing structured lines (breaklines), these can be also included in the evaluation process following the same steps as pre-sented above. Finally, in the presence of certain surface features, e.g., spheres or hemispheres, the models can directly be reduced to single points without the need of any adjacent model descriptors.
After obtaining the points, one common and simple approach for the determination of displace-ments is to use the rule of thumb (Savšek-Safi´c, 2006). In this case, the point has moved, if the displacement vector is bigger than the positional standard deviation of end points increased by a factor of 3 or 5. Three sigmas are sometimes taken as the limit value to what can be regarded as the random error of the determined position. Therefore, any larger deviation from the estimated po-sition is usually considered a blunder or an actual displacement. Regardless of the model selected for the displacement and deformation analysis, careful error propagation steps have to be performed alongside in order to make the final conclusions reliable enough.
3 EXPERIMENTAL RESULTS
According to the methodology presented in Chapter 2, the proposed workflow (section 2.1) was tested in the two outdoor experiments described herein (see section 3.3 and 3.4). The characteristics of the two outdoor experiments are in many ways quite different with respect to:
• the terrain features and ground stability;
• the size of the object under inspection;
• the object’s accessibility;
• the limitations of the geodetic network formation;
• the complexity of displacement and deformation evaluation process.
Comparing test 1 and 2 based on these factors, test 2 most certainly appears to be more challenging for TLS when deciding whether to accept or reject the working hypothesis with enough confidence.
During the two outdoor tests two different scanners were used, namely the Leica scanner in test 1 and the Riegl scanner in test 2 both belonging to a similar instrument quality class. The fact that the first scanner was used with the targets coming from the same manufacturer has resulted in a significant reduction of systematic range errors due to the possibility of scanning the target surface with a specially designed acquisition mode. By using this mode the target centers are estimated with the amplitude weighted mean algorithm built into the scanner’s software for data processing. On the other hand, the range error behavior had to be tested and modelled prior to test 2 in order to reveal how the second scanner will comply to the same targets (see subsection 3.1.2). Consequently, the scanner-target ”compatibility” poses an additional factor to the ones listed above that needed to be analyzed for the purpose of test 2 alone. Finally in test 2, objects of different material composition and surface roughness were involved which meant tests on the scanner’s detectivity level had to be performed to determine its sensitivity to small scale displacements and deformations (see subsection 3.2).