Finally, a short evaluation of the indoor tests should be done to summarize their overall quality and efficiency already indicated in sections describing the experimental results. In order for the results of these tests to be free of errors produced by the realization process itself they had and were carried out with utmost care. It is firmly believed that the results, whatever they are, are not the outcome of errors introduced by the careless experimental set-up. Moreover, also the conceptual design of the indoor tests with its simplicity can be treated as reliable enough to provide the necessary high quality results.
Despite the credible execution of these tests, the measurement uncertainty at such a small error level would require the repetition of especially the first indoor test, during which the targets’ mechanical imperfections were analyzed by the manual observation process. Repeating the measurement pro-cess under various conditions would contribute to the reliability of results but the final conclusions of this test would probably not be altered. The fact is that the four targets used in the experiments are of slightly different mechanical quality and that the errors produced by the two degree rotation
are very difficult to model. Most of these errors seem to be confined to the ±1 mm boundary, within which the center deviation patterns should be verified regardless of the quality of the instrument used to carry out the observations. So-far, only the observed 1 mm vertical eccentricity error for target T4 was taken into account due to its apparent stability (see Figure 11 on page 32). The rest of the errors could not be modelled successfully, which means that probably some of their effect can only be minimized through the transformation process.
Compared to the level of errors from the first test, a much larger error level was uncovered during the second indoor test. Since both of the two tests were designed to analyze the influences of errors that eventually affected the quality of transformation parameters in the outdoor test 2, not modelling the range errors would have much more impact on the quality of the transformation. The extension of the maximum range from the scanner to the target came at a cost of scanning the retroreflective part in order to estimate the target distance. Selecting the points on the retroreflective part of the target surface can lead to distance errors of up to 1 cm, almost ten times higher than those from the first indoor test. In general, the correction functions from Table 3 can reduce the range errors below the millimeter level but the efficiency of modelling these errors can decrease if the error be-havior becomes unstable. The uncovered instability of the error bebe-havior is presumably caused by the retroreflective part of the target but could also arise from the scanner side. Therefore, further tests are needed to find out why these error fluctuations occur. It is possible that the inability of mo-delling such fluctuations with enough efficiency contributed to the exclusion of some of the station measurements in the outdoor test 2. Most likely the amplitude fluctuations could only be avoided by replacing the retroreflective tape on the target with a less “aggressive” one. Either way, the quest for finding a better target type, as well as the implementation of the modelling approach where the scanner and target contributions to the range errors are separated, remains open.
Concerning the indoor test where the scanner’s detectivity level was analyzed with respect to various surface conditions, the results of the test provide valuable information when deciding on scanner-object distance, incidence angle limitations and the point density. Based on the conclusions from the final indoor test, the scanning geometry and parameters in the outdoor test 2 were adjusted (maximum range of 30 m from the object, maximum incidence angle of 45◦ and the point density corresponding to a minimum of 400 point per patch). Furthermore, the a posteriori standard devi-ations, estimated for each material sample (ranging from 1.6 to 2.2 mm), can be used to upgrade the variance-covariance matrices of the scanner’s direct measurements (particularly for the distance component) when performing the error propagation process. On the basis of the estimated a poste-riori values, the modelling process in the outdoor test 2 was adjusted in order to exclude the patches with the planar noise higher than 3 mm. The results of the final indoor test further indicated the potentially negative effects caused by the scanner’s power supply. Therefore, the power shortages as well as the warm-up periods related to the instrument were avoided in the outdoor test 2 in order to minimize the impact on the stability of the scanner’s detectivity level. As already mentioned, the results of the final indoor test indicated that the 2 mm and 5 mm displacement level are well distinguishable, in turn supporting the findings from the deformation analysis.
5 CONCLUSIONS
The experimental results presented in the thesis must now be evaluated with respect to the initial hypothesis outlined in section 1.2. Moreover, the results offer a possibility to measure the quality of the methodological steps described in Chapter 2. Therefore, it is the purpose of this section to draw the final conclusions of the entire work described herein.
Based on the results of the thesis, clear evidence exists for significant confidence in accepting the working hypothesis. Following the proposed methodology can lead to a high precision deformation determination in the long-term for objects and not only for signalized (i.e., marked) points. TLS has proved to be capable of providing high precision data and therefore could be considered a complementary surveying technique which cannot only be combined with other well established high precision surveying technologies but can also contribute to a more complete understanding of deformations. However, the analysis shows that working in the millimeter domain with TLS comes with a certain price since the field work as well as the processing of the measurements has to be done with a lot of care. Furthermore, the level of detectable deformations in the millimeter domain can be influenced by many factors, such as:
• the selection of the surveying equipment (a scanner, targets);
• the conditions in the field (surface properties, distance to the object, incidence angle, geodetic network geometry);
• the efficiency of the modelling of systematic error (calibration parameters);
• the proper error propagation schemes incorporating all the subsequent data processing steps.
The estimation of displacements and deformations below the scanner’s nominal capabilities can be achieved in general. This fact has been proved in the outdoor test 1 where the TLS has been used to determine the pillar axes. In this test the TLS results did match the ones from the precise classical terrestrial method to a very high degree. The analysis of the data from test 1, which have been first described in Vezoˇcnik et al (2009), was taken a step further in the thesis but the final results pub-lished in the paper remain unchanged. On the other hand, the role of TLS in the outdoor test 2 was even bigger and despite some problems with the instrument as well as with the absolute orientation, the final results are promising in terms of the ability of TLS to be used for the high precision moni-toring tasks. In both outdoor experiments, the establishment of the same surveying conditions in the field and following the same data processing algorithms was considered an important aspect of the general workflow in order to avoid the accumulation of any additional errors. For the monitoring to be effective in the long run, these errors have to be minimized as much as possible.
The modelling of systematic errors and obtaining the calibration parameters for all the measuring equipment is also a necessity. Without this step implemented in the workflow, the capabilities of deformation inspection in the millimeter domain may be severely reduced. Not only should the
modelling of systematic errors be considered in the process, but the calibration parameters (e.g., range error functions for the TLS targets, scanner systematic errors) would have to be determined by frequent tests in order to be estimated from highly redundant observations (and possibly different ambient conditions) and particularly to see how their temporal stability is behaving. Moreover, tests are definitely needed also in terms of the surface material response to evaluate the stability of the instrument’s detection capabilities.
The extraction of the significant displacements and deformations cannot be approached without the proper error propagation process. During the modelling stage it is also important to consider the introduction of proper stochastic models as well as the estimation of realistic precision parameters for all the input quantities of both deformations models so as to not be deceived by too optimistic standard deviations, which are in many cases simply the outcome of highly redundant TLS measure-ments. According to the results presented in Chapter 4 such error propagation schemes revealed that TLS can in certain cases reach below the 5 mm level in the displacement and deformation detection, but to go as far as 1 mm would not be possible after applying the threefold point precision. With respect to the magnitude of detectable deformations, a 5 mm level is more realistic when dealing with objects larger in size and non-ideal surveying conditions. The detectivity level may be slightly extended by scanning the object with multiple scans and averaging the results. Scan multiples can also contribute to the stability of the deformation detection as shown during the surface material response test.
The surveying conditions can eventually decide whether the data can be used for the change in-spection at this small scale level. In the outdoor test 2, the non-ideal conditions with respect to the number of TLS targets and the network geometry did contribute to the exclusion of some of the station measurements. To overcome this obstacle, using more targets per station is advisable in or-der to increase the quality and stability of the estimated transformation parameters. Not to increase the overall time spent in the field to perform the classical terrestrial measurements, some of these targets do not necessarily have to be included into the geodetic network but can be used for the relative orientation purposes only. The quality of the relative orientation can further be increased by the proper integration of targets and the ICP algorithm (Haring, 2007). Once the relative orien-tation parameters between the adjacent scanner sorien-tation would be estimated, the whole block (i.e., all the TLS station measurements) could be transformed into the reference frame simultaneously.
In order to be able to analyze the quality of the transformation also at the object side, the size of the overlapping areas has to be taken into consideration. With today’s scanner providing very fast scanning rates, the size of the overlapping areas can be increased beyond 50 % with no significant time delays. The more such areas and the bigger they are, the better control could be established over the quality of transformation.
Finally, to strengthen the confidence in the results of the deformation analysis it may be sometimes advisable to install some control points onto the object of inspection and estimate their positions by means of an alternative surveying technology. This multi-sensor monitoring approach is most certainly one of the nowadays trends and will be important also in the near future. The involvement
of control points and complementary geodetic techniques does not decrease the quality of the pro-posed methodological workflow but can ultimately contribute to the level of trust in the outcome of such intricate geodetic tasks regardless of the type of technology employed.