Investigating the Sensitivity of Terrestrial Laser Scanning Model to Control Coordinates in Deformation Monitoring

In deformation monitoring with Terrestrial laser scanning, volume comparison of the processed point cloud of an object (model) obtained at different periods is normally used to reveal if any deformation has occurred. Several environmental factors are known to cause deformation of structures ranging from stress to underground water as well as heat. The study investigated the sensitivity of Terrestrial Laser Scanner (TLS) model to target control points coordinates in deformation monitoring. The raw point cloud data of Nottingham Geospatial building (NGB) was processed with two different possible scenarios, firstly with the truth coordinates, and then with subtle changes in the coordinates (solutions), of the target points for the TLS. And to achieve this aim possible case scenarios were adopted during the data acquisition with the TLS. The first is if the target control points have shifted slightly, but the coordinates used are assumed to be the same as the original coordinates from an initial control survey. The second is if the coordinates of the target control points used are found to be slightly different from the original coordinates following a repeated control survey, but the target control points have not actually shifted. The methods adopted also include data processing with Cyclone software(data preparation, registration, filtering, and cloud unification) for different scenarios, firstly with the truth coordinates, and then carrying out coordinate extraction of points on the model for the purpose of coordinate comparisons/transformations of the different solutions. And thereafter modelling of different solutions of Nottingham Geospatial Building using LSS software to carry out volume comparison of the different surface model of NGB. Coordinate comparison and transformations of the different solutions (models) show systematic shifts from the original model in corresponding amount of the changes introduced to the truth coordinates. The results of the volume comparisons with LSS of the different solutions show a significant change in volume up to 27.809m3 from the original model. These results show that the deformation of the target control points used in deformation monitoring with TLS is transmitted into the model. The subtle changes in the target control points can significantly affect the accuracy of the model. This research has practically demonstrated the fact that TLS model is sensitive to the target control points in deformation monitoring. Errors in the target control point coordinates can cause deformation (volume changes) in the model, which is most often erroneously attributed to environmental factors alone. To mitigate this effect, controls for deformation monitoring should be established with high level geodetic instruments and the locations of controls should be confirmed to be in their right positions.