Accurate Measurement of the Riverbed Model for Deformation Analysis using Laser Scanning Technology Fu tur e Iss ue Ju ly 3, 201 8 1 Accurate Measurement of the Riverbed Model2 for Deformation Analysis3 using Laser Scanning Technology4 Martin Štroner∗, Rudolf Urban, Tomáš Křemen and Bronislav Koska5 Department of the Special Geodesy, Faculty of Civil Engineering,6 Czech Technical University in Prague7 ∗Corresponding author: martin.stroner@fsv.cvut.cz8 Abstract. This paper presents an interesting application of the riverbed model shape and9 deformations monitoring using laser scanning technology and accurate local micro-network.10 The most interesting fact about this application is very high accuracy demand on defor-11 mation determination (maximum permissible error is only 2 mm) within quite large object12 (the size of the riverbed model is about 100 meters). Up to now, mechanical calipers in the13 selected profiles were used to detect the changes. This manual approach is very laborious14 and time-consuming and the gathered data resolution and accuracy is often not sufficient.15 The suggested solution contains two main parts. The first part deals with construction of16 highly accurate local micro-network for laser scanning needs (maximum required standard17 deviation in any coordinate is lower than 0.4 mm) and the second part deals with actual laser18 scanning and data processing. Design, measurement and processing of the experiment was19 conducted for the needs of the research project “Improvement of navigation conditions on the20 Elbe between Ústí nad Labem - state border CR / FRG - Navigation Step Děčín”. The main21 goal of this project is design and realization of the river regulation to improve the navigation22 conditions. The key benefit of using river model is the possibility to easily simulate various23 catastrophic scenarios (various degrees of the floods) and their impacts on riverbed changes.24 Keywords: engineering surveying; laser scanning.25 26 1. Introduction27 Engineering geodesy deals with measuring of all possible subjects for needs of construction,28 industry and related sectors. Coordinates or specific geometric parameters are usually main29 outputs of these measuring. Frequent and important task can be also determination of changes30 of the outputs. A common denominator of all these works is a mandatory requirement for31 accuracy of the collected data and evaluated results. Shifts and deformations are determined32 not only by various geodetic methods, but also by geotechnical and other methods.33 Frequently used geodetic methods for deformation measurements are trigonometric method34 as a part of geodetic network [18], geometric levelling [2], 3D scanning [11, 16] and an inter-35 esting application described in [12] and in [15], photogrammetry [13, 7], GNSS [17], and by36 combination of these methods and processing it altogether by adjustment.37 Besides the shifts and deformations of the buildings and constructions displacements of natural38 formations like hillsides (landslides) [1, 19] are also measured. There are often used methods39 based on terrestrial 3D scanning [10, 8, 5], airborne 3D scanning [14], combination of both40 [21], or in combination with other instrument’s measurement as ground penetrating radar [3].41 Measurement of deformations of the construction models can be classified as a very specific42 task. The size of the measured area is significantly smaller, on the other hand demanded43 Geoinformatics FCE CTU 17(2), 2018, doi:10.14311/gi.17.2.5 81 http://orcid.org/0000-0003-0070-7172 http://orcid.org/0000-0002-0996-1241 http://orcid.org/0000-0002-5043-8321 http://orcid.org/0000-0001-7590-3579 http://dx.doi.org/10.14311/gi.17.2.5 http://creativecommons.org/licenses/by/4.0/ Fu tur e Iss ue Ju ly 3, 201 8 M. Štroner et al.: Accurate Measurement of the Riverbed Model precision is significantly higher. The hydraulic models of the riverbed for simulation purposes1 of the various flows (including various degrees of the floods), assessing navigability and also for2 the purposes of the design of construction work are the main objectives of the research project.3 Description of the process including the numerical and physic modelling is available in [6].4 This paper describes the geodetic part of measurement of the deformations of the riverbed5 model due to the simulated flows. The deformations themselves were determined by the 3D6 laser scanning with the use of spherical ground control points on the concrete banks of the river7 model as a frame. The first task of the presented measurement was to determine coordinates8 of the spherical control points for subsequent 3D scanning with standard deviation in each9 coordinate better than 0.4 mm. The ground control point coordinates had to be determined10 with such a high accuracy, because the maximum permitted error of the changes evaluation11 by the 3D scanning was 2 mm, it was the second and main task.12 2. Characterisation of the hydraulic model13 A model for the water of the Elbe River with a total length of 7 km at a scale of 1:70 in outdoor14 areas of the Water Research Institute T.G.M. Prague (Czech Republic) was built for purposes15 of the research project “Improvement of navigation conditions on the Elbe River between Ústí16 nad Labem - state border CR / FRG - Navigation Step Děčín” in 2015. The main goal of the17 project is design and realization of the river regulation to improve the navigation conditions.18 The impact of adaptations will be tested on the specified physical model first.19 The model is made of concrete; the riverbed itself is modeled from sand with grain size20 corresponding to the real bottom cover (in scale 1:70). Model length is 100 m.21 The whole model is covered by the wooden roof from above and by a dense mesh fence from22 the sides to avoid a damage of the riverbed by the weather or small animals. The watercourse23 is considerably winding and measurement situation is therefore similar to measurements in a24 narrow tunnel (width of about 2 m). A situation is on Fig. 1, a photograph of the riverbed25 model in reality is on Fig. 2.26 Figure 1: Situation plan of river model 3. Creation of local micro-network27 The first part of the project was to design, create and determine a very accurate local area28 network (standard deviation in each coordinate is lower than 0.4 mm), which will include29 three pairs of spherical targets (at the beginning, middle and end model) as a ground control30 points for the 3D scanning measurement.31 As a method of the geodetic measurement was selected a trigonometric micro-network with32 measured slope distances, horizontal directions and zenith angles between standpoints, and33 Geoinformatics FCE CTU 17(2), 2018 82 Fu tur e Iss ue Ju ly 3, 201 8 M. Štroner et al.: Accurate Measurement of the Riverbed Model Figure 2: River model in reality (sand riverbed and concrete banks horizontal angles and zenith angles on the spherical targets. The spherical targets are used1 as a ground control points for the 3D scanner, and reference point is always a center of the2 sphere, calculated from the scanned points by fitting of the sphere with known diameter.3 The spherical targets cannot be targeted to the center, and hence the horizontal direction4 and zenith angle to the center of the sphere was always calculated from the values measured5 to the upper, lower, left and right visible edge.6 The best total station at the disposal of the department of the Special geodesy of the Faculty7 of Civil engineering of the Czech Technical University in Prague, the Trimble S8 (Fig. 3), with8 a nominal precision of the horizontal direction and zenith angle measurement 0.3 mgon and9 distance 0.8 mm + 1 ppm was used due to required high accuracy.10 Standpoints of the network were not permanently marked (due to the required precision),11 they were established only temporary in the central point of the prisms (Leica GMP 10112 Professional, Fig. 4) and total station. Only spherical ground control points and four witness13 points (see section 3.3) were permanently physically marked. Each temporary point was14 established by a tribrach on a tripod, where we placed either a prism or a total station with15 the same pivot point.16 As can be seen on the Fig. 1, the watercourse is considerably winding and measurement17 situation is therefore similar to the measurements in a narrow tunnel (width of about 2 m).18 The micro-trigonometric network was designed as two parallel polygonal traverses, where the19 measurement was done from each standpoint to the five directly neighboring points. Design of20 the measurement is further discussed in the next section, as it was a product of the accuracy21 planning process.22 Geoinformatics FCE CTU 17(2), 2018 83 Fu tur e Iss ue Ju ly 3, 201 8 M. Štroner et al.: Accurate Measurement of the Riverbed Model Figure 3: Trimble S8 Figure 4: Leica GMP 101 Professional prism 3.1. A priori precision planning1 The network was designed according to a local survey. To achieve the required precision, it was2 necessary to model the measurements and its precisions and prove that the required precision3 output will be met. Software PrecisPlanner 3D ([20]) was used for this purpose, with use4 of the approximate configuration of the determined points and standpoints. The procedure5 of the precision planning is described in detail in [9]. The configuration was designed at a6 guess and the optimization was made manually only by adding the measurement repetitions.7 The chosen points and planned measurements, generating a standard “braced quadrilateral”8 design, are shown on Fig. 5, where each line is a sightline described by a number of each type9 of the measurement performed (sd – slope distance, ze – zenith angle, di – direction).10 Distance between points was approx. 3 m in the transversal direction, 15 m in the longitudinal11 direction. The expected precision of the aiming was lower than nominal and therefore 1.012 mgon was used (due to the short distances) as a standard deviation of the horizontal direction13 and zenith angle measurement. Standard deviation of the distance measurement 0.5 mm was14 used. This value was calculated from differences in oppositely measured distances and from15 a posteriori analysis of similar micro-networks.16 Figure 5: Configuration for precision planning with use of the PrecisPlanner 3D software Geoinformatics FCE CTU 17(2), 2018 84 Fu tur e Iss ue Ju ly 3, 201 8 M. Štroner et al.: Accurate Measurement of the Riverbed Model The result of the modelling showed that in order to achieve the required precision it is nec-1 essary to execute the measurement in two sets. All the requirements were met, the worst2 standard deviation of the standpoint coordinate was 0.1 mm in the lateral direction, 0.3 mm3 in the longitudinal direction and 0.08 mm in the height direction.4 3.2. Measurement5 Measurement was done according to the planning. The only detected problem was higher6 dispersion of the angle measurement at the transverse neighboring point (short distance,7 approx. 3 m). That’s why standard deviation of angle measurement for these points in the8 adjustment was raised to 2.5 mgon.9 3.3. Processing and results10 The measurement was evaluated in the software EasyNet ver. 3.4.3 ([4]), including the field-11 book calculation, calculation of the approximate coordinates and reduction of the measured12 values for calculation of the coordinates. Adjustment was calculated completely in 3D, using13 also a robust estimation and other techniques for the outliers’ detection. Detected as outliers14 and removed were only 5 measurements from 348 totally.15 Achieved precision of spherical targets coordinates was sufficient, standard deviations after16 adjustment of each point coordinates (standard deviations in X, Y, Z coordinates σX, σY, σZ)17 are presented in Tab. 1. Error ellipses of the point positions can be seen at Fig. 6. Standpoints18 are numbered 4001 to 4010, spherical targets K1 to K6 and stabilized orientation points P119 to P4.20 Figure 6: Configuration and error ellipses of the real measurement 4. Laser scanning21 After determination of six spherical control points within the local micro-network the second22 part of the project, 3D laser scanning of the riverbed model was conducted successively in23 three separate stages, the first modelling the original model, the second and the third after24 the experiments simulating the influence of the water flow of a certain size (century flooding25 e.g.).26 Geoinformatics FCE CTU 17(2), 2018 85 Fu tur e Iss ue Ju ly 3, 201 8 M. Štroner et al.: Accurate Measurement of the Riverbed Model Table 1: A posteriori standard deviations in coordinates of the determined points P.no. σX [mm] σY [mm] σZ [mm] K1 0.12 0.35 0.10 K2 0.12 0.25 0.10 K3 0.11 0.20 0.07 K4 0.13 0.11 0.07 K5 0.13 0.13 0.09 K6 0.08 0.15 0.09 P1 0.05 0.16 0.08 P2 0.08 0.14 0.08 P3 0.09 0.16 0.08 P4 0.11 0.14 0.08 4001 0.07 0.11 0.07 4002 0.06 0.1 0.07 4003 0.09 0.08 0.06 4004 0.09 0.09 0.06 4005 0.11 0.07 0.05 4006 0.10 0.08 0.05 4007 0.06 0.08 0.06 4008 0.07 0.09 0.06 4009 0.06 0.11 0.07 4010 0.08 0.10 0.07 4.1. Measurement1 The Surphaser 25HSX scanning system in the second most accurate configuration IR_X was2 used for the measurement due to very high demands on outputs accuracy. The manufacturer3 states accuracy for this configuration lower than 0.5 mm for 5 m, noise/precision 0.1 mm for4 3 m. Other important parameters of these systems are: Scanning speed up to 1.2 million5 points per second, panoramic field of view, recommended measurement range 0.4-30 m.6 The measurement was carried out from twelve standpoints placed on both sides of riverbed7 model and 22 control points were permanently mounted and used in total. The average8 distance between standpoints was about 10 meters due to partially winding shape of the9 model see Fig. 7. Control points were stabilized by polystyrene spherical targets with a10 diameter of 150 mm.11 Scanning density was set 6 mm in orthogonal directions at the distance 10 m which leads to12 approximately 6 minutes measurement time on a standpoint and about 2 hours for the whole13 measurement of one stage.14 4.2. Processing15 Scanned data was exported in batch using console program ProcessC3d which is a part of16 used laser scanning system software SurphExpress Standard at first. It was necessary to use17 a format which keep ordered/structured format of the measured data to enable accurate and18 Geoinformatics FCE CTU 17(2), 2018 86 Fu tur e Iss ue Ju ly 3, 201 8 M. Štroner et al.: Accurate Measurement of the Riverbed Model Figure 7: Standpoints (Scan1-Scan12) and control points (1-22) configuration for 3D laser scanning. Control points 1, 2, 11, 12, 21, 22 are identical to K1 – K6 fast normal computation in the following steps. The most suitable format is “btx” which is a1 binary format for ordered points.2 The second used software was Geomagic Studio. It also supports batch processing and in3 addition to it macro scripting. It was used for point normal computation (exploiting struc-4 tured format of points), for sampling with minimal spatial distance 5 mm and saving in text5 format.6 The third step - registration took place in the Leica Cyclone software module Register. The7 control points were created by fitting spheres with the given diameter 150 mm. There were8 measured up to 8 control points on each standpoint from all together 22 control points. The9 actual registration was carried out twice. Only scanned data were registered in first step to10 verify absence of gross error and check inner data consistency. In the second step scanned11 data was registered together with control points determined within the local micro-network,12 which were placed in separated leveled “ScanWorld” (Leica Cyclone name for standpoint).13 This ScanWorld was set up as a “Home ScanWorld” (target coordinate system). Used control14 points were 1 (K1), 2 (K2), 12 (K4), 21 (K5) a 22 (K6) see Fig. 7 and 6. Control point 1115 (K3) was damaged and couldn’t be used.16 This second registration was necessary to level scanned data (used scanning system is not17 equipped with inclinometer) and to check and compensate systematic influences caused by18 line character of measured object and by use of laser scanning system with relatively (to19 object size) low range. Systematic errors present in the measurement would sum and result20 in significant overall length difference between the scanned data and geodetic measurement.21 The resulting “Mean Absolute Errors” of registration (accuracy property used in Cyclone22 software for registration) were for laser scanning data only 1.4 mm, 1.2 mm and 1.2 mm for23 three realized stages and 1.4 mm, 1.3 mm and 1.2 mm for second registration with control24 points from local micro-network after elimination of systematic influences (see section 4.325 below). The visualization of registered point cloud part is on Fig. 8.26 The last processing step was transformation from local coordinate system to the national co-27 ordinate reference system (S-JTSK) and national vertical reference system (Balt po vyrovnání28 / Baltic Vertical Datum - After Adjustment). The client requested a transformation to place29 the measured deformations to the real world in the scale 1:1.30 This part was quite difficult because only few suitable common known points in both coor-31 dinate systems exist on the barrage (a dam placed in a watercourse to increase the depth32 of water or to divert it into a channel for navigation or irrigation) at the beginning of the33 Geoinformatics FCE CTU 17(2), 2018 87 Fu tur e Iss ue Ju ly 3, 201 8 M. Štroner et al.: Accurate Measurement of the Riverbed Model Figure 8: Screenshot of registered point cloud riverbed model. The barrage itself exists only in measured Elbe river model (local coordinate1 system of measurement) and CAD design project (national coordinate reference system), but2 not in reality. The barrage fills only small part of the riverbed model (maximum barrage size3 is about 8 meters and model size 100 meters) and that’s why it was not possible to use it for4 bearing determination (orientation/rotation) of scanned data and only one common known5 point on barrage was used. The rotations from local coordinate system to the national co-6 ordinate reference system (S-JTSK) was calculated as a difference between bearings in both7 systems from selected point on the barrage to the point in the middle of riverbed at the8 opposite side of the river model in exact distance (on point cloud in local coordinate system9 in distance 85.714 m and in CAD design project in the national coordinate reference system10 S-JTSK in distance 6 km). On the base of calculated (orientation, shift) or known (scale)11 values the parameters of similarity spatial transformation were computed and simple console12 program for large (about 9 million) point cloud transformation was created. The type of final13 transformation is “point and rotation” in horizontal space in addition with shift in vertical14 component and a priori known scale of the model.15 4.3. Accuracy checks and improvements16 The first accuracy check were the resulting “Mean Absolute Errors” of registration in Leica17 Cyclone for laser scanning data only. On the base of scanning system specification and lots18 of previous experience we had expected Mean Absolute Error about one millimeter, but the19 first result was nearly 3 millimeters which point to some unexpected error.20 The second check, mostly for systematic errors, was comparison of shape and size between the21 laser scanning and “geodetic” outputs. The comparison was conducted using identity (rigid22 body) and similarity spatial transformation on five “identical” points in both coordinate23 systems. The result of the first application of identity transformation showed very high24 Geoinformatics FCE CTU 17(2), 2018 88 Fu tur e Iss ue Ju ly 3, 201 8 M. Štroner et al.: Accurate Measurement of the Riverbed Model spatial position standard deviation 20 mm, which indicated important systematic influence.1 Moreover the significantly highest deviations were in longitudinal direction of riverbed model2 (and vertical deviations were order of magnitude lower). By the analysis of these results it3 was found out that the laser of 3D scanning system rangefinder penetrates the surface of used4 polystyrene spheres. Subsequently made experiments showed that the average value of this5 penetration is very close to 3 millimeters. The experiment was based on the same principle6 as determination of a prism – total station system constant (distance measured from middle7 and outside of a line).8 After the application of corrected sphere diameters spatial position standard deviation of9 checking identity (rigid body) spatial transformation decreased from 20 to 3 millimeters and10 average registration Mean Absolute Error decreased near the expected one millimeter value.11 The last check was spatial comparison of final point clouds of any two stages. Determined12 deformations should be lower than required 2 mm besides deformations on some parts of13 riverbed model bottom made from sand. There should be especially no deformation on14 concrete elevated sides of the model. Comparison was made using tool “3D Compare” in the15 software Geomagic Studio. Required accuracy was met on the vast majority of model surface16 see Fig. 9, where is one of the comparisons presented.17 Figure 9: Hypsometric visualization of two stages distances (changes) [meters] 5. Conclusion18 Experience with realization of riverbed model measuring for changes determination is pre-19 sented in the paper. Due high demands on accuracy and precision and also on measurement20 resolution two different technologies must be combined. In the first step ground control points21 have to be determined. The technology of micro-network measured with total station was22 used for this task. These ground control points were used for referencing of individual stand-23 points for the second technology - 3D laser scanning. Laser scanning can achieve very high24 resolution of measurement but in the case of the most accurate devices it can be usually used25 only for short distances. A common denominator of both technologies was requirement on26 the highest possible quality of its outputs.27 The high accuracy of geodetic micro-network points was made possible by using the most28 accurate total station, high quality tools and also a priori accuracy planning. Very important29 is choice of devices and tools with small eccentricities and also high quality tripods and30 Geoinformatics FCE CTU 17(2), 2018 89 Fu tur e Iss ue Ju ly 3, 201 8 M. Štroner et al.: Accurate Measurement of the Riverbed Model tribrach, where repeated clamping of different devices (total stations, carriers with prism)1 causes very small movements. The eccentricities may encounter up to a 0.3 mm with the use2 of the conventional tools, but on the project were used tribrach with a maximum standard3 deviation of repeated clamping 0.05 mm. Furthermore, it is very important to carefully aim4 or, in the case of automatic aiming, accurately turn prisms toward total station.5 Due to the short distances in this case, automatic aiming cannot be recommended. The6 advantage is motorized/robotized total station which greatly speeds up measurements, and7 which is not such demanding on the stamina and the concentration of the operator.8 In the case of laser scanning part, the required accuracy was achieved using accurate scanning9 system (the most accurate 3D laser scanning system on the market at the time of release),10 suitable chosen scanning configuration and processing and especially using high accuracy11 local micro-network. This network was necessary to level scanned data and to check and12 compensate of systematic influences. Because of this accuracy checks the significant system-13 atic error caused by rangefinder laser penetration through the surface of used spheres was14 discovered. After determination and correction of this error, the expected check results were15 finally achieved. The final result was a detailed 3D model of the deformations caused by16 the test water flow with demanded precision.17 Researchers from T. G. Masaryk Water Research Institute were greatly satisfied with delivered18 outputs and recommend implementation of this technology for all similar research projects in19 their institution.20 Acknowledgements21 This work was supported by the Grant Agency of the Czech Technical University in Prague,22 grant No. SGS18/067/OHK1/1T/11„Optimization of acquisition and processing of 3D data23 for purpose of engineering surveying and laser scanning“.24 References25 [1] Jaroslav Braun and Pavel Hánek. “Geodetic monitoring methods of landslide-prone26 regions – application to Rabenov”. 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International Archives of18 the Photogrammetry Remote Sensing and Spatial Information Sciences. Bahnhofsalle:19 Copernicus Gesellschaft MBH, 2016, pp. 917–924. doi: 10.5194/isprsarchives-XLI-20 B5-917-2016.21 Geoinformatics FCE CTU 17(2), 2018 92 https://doi.org/10.14311/gi.14.1.2 https://doi.org/10.1007/s10346-010-0241-9 http://k154.fsv.cvut.cz/~stroner/PPlanner/ http://k154.fsv.cvut.cz/~stroner/PPlanner/ http://k154.fsv.cvut.cz/~stroner/PPlanner/ https://doi.org/10.5194/isprsarchives-XLI-B5-917-2016 https://doi.org/10.5194/isprsarchives-XLI-B5-917-2016 https://doi.org/10.5194/isprsarchives-XLI-B5-917-2016 M. Štroner et al.: Accurate Measurement of the Riverbed Model Introduction Characterisation of the hydraulic model Creation of local micro-network A priori precision planning Measurement Processing and results Laser scanning Measurement Processing Accuracy checks and improvements Conclusion