Kinematic 3D Laser Scanning for Road or Railway Construction Surveys

Transcription

1 1 st International Conference on Machine Control & Guidance Kinematic 3D Laser Scanning for Road or Railway Construction Surveys Gunnar GRÄFE* 3D Mapping Solutions GmbH, Germany Abstract Kinematic laser-scanning has been a key application for the Mobile Road Mapping System (MoSES) since a couple of years. The development of kinematic survey methods has reached a level, that allows the use of kinematic survey technology for high precision applications. Replacing static tachymetric surveys on the road itself, kinematic methods are more and more applied for construction projects. The technology is used for rapid airfield monitoring as well as an increasing number of highway construction projects or high-precision railway tunnel surveys. The resulting data is acquired and generated with static survey accuracy, but much faster and with much higher resolution - if needed. The results represent the basic survey information for planning tasks or high precision machine guidance. Within the last months, the next development steps have been taken with the aim to join static and kinematic scanner technology. High precision static 3D laser scanners, which are capable of performing profiling measurement modes, can now be used with the MoSES system. A special mounting and rapid-calibration procedure is required to enable the use of e.g. Zoller&Fröhlich or FARO 3D laser scanners for kinematic high precision applications. Pilot projects to demonstrate the capabilities of this new 3D laser scanner survey method have been high-speed surveys of automobile industry test sites or subway tunnel surveys. The new developments were completed by adding infrared photogrammetric camera technology to the MOSES system, which enables full night vision survey capacity, which is of high interest for tunnel or airfield monitoring applications. Keywords 3D Laser scanner, kinematic survey, digital road surface models, milling machine guidance 1 INTRODUCTION Road or rail construction tasks require reliable survey data. Due to highly increasing traffic load, combined with the age structure of the road or rail network, the responsible administrations are forced to put the main emphasis of maintenance activities into projects to ensure quality, safety and sufficient traffic capacity. The negative influences of maintenance tasks on the traffic flow, such as road blockings etc., have to be kept as short as possible. This is even more important for the necessary substantial engineering survey work, that is needed in advance to capture basic data for construction planning and preparation. For this task, kinematic survey methods offer new high-precision survey possibilities. Mobile technology minimizes the survey effort to capture the required data along existing roads and railways. Only a comparatively short driving time is needed for data collection, while the traffic flow is not significantly affected. This approach reduces both costs and safety risks. For these reasons, the capabilities of the Mobile-Road-Mapping-System (MoSES) have been expanded, so that high-precision engineering survey projects can be carried out with this kinematic survey system. 3D Mapping Solutions GmbH does service projects and develops and sells the necessary hardware and software to successfully operate mobile mapping systems. The latest developments enable the full kinematic usage of high performance 3D scanners, that are capable of profiling measurement modes. The kinematic survey with laser scanners plays a major role for new possibilities of construction machine control and guidance based on the kinematic survey results. This paper presents a new concept for road construction machine control based on digital road surface models.

2 Data Processing and Data Acquisition 2 2 KINEMATIC SURVEYING WITH STATIC SURVEY ACCURACY The prerequisites to reach static accuracy with a kinematic survey van are sophisticated calibration, data processing and quality management techniques [Graefe 2007b and Graefe 2007c]. These allow MoSES to carry out precise road and rail infrastructure surveys as a basis for planning and design. For roads, the main emphasis lies on the road surface. 2.1 The Mobile Road Mapping System (MoSES) MoSES has been designed as a multisensor survey van to capture all relevant road or railway data in a cost-effective way without obstruction of the traffic flow (Figure 1) [Gräfe 2007a]. MoSES combines sensors with complementary measurement characteristics. Photogrammetric stereo cameras and a number of laser scanners are mounted together on a calibrated measurement platform. The multisensor equipment covers a corridor along the route traveled with a total measured width to each side of up to 20 m at least. External orientation to georeference the collected data of all sensors is provided by a multisensor trajectory module combining IMU, DGPS and odometer measurements of high accuracy [Gräfe 2007b]. Reliability and stability of the trajectory module are the key to the MoSES s capability to serve high precision applications [Gräfe 2007c]. The system architecture is modular, so that it can be adjusted to different tasks by using various sensor configurations. Since the year 2000, MoSES has been successfully applied to a number of projects [Heister 2004]. Full sensor coverage of the road corridor is guaranteed by a variable number of digital cameras for documentation and photogrammetric measurement purposes, combined with laser scanners. Figure 1: The Mobile Road Mapping System (MoSES)

3 1 st International Conference on Machine Control & Guidance Kinematic Surveying with high Performance Laser Scanners The laser scanner module combines 2-dimensional scanners with the trajectory module. Various types of scanners are supported by the 3D Mapping technology. Prerequisite for kinematic laser scanning applications is, that the scanner is capable of operating in profiling mode. The orientation of the scanners is flexible, so that 360 field of view for tunnel survey applications etc. is possible. Very rigid outdoor capable scanning systems may be used as well as high performance 3D scanners in profiling mode, e.g. like Zoller&Fröhlich or FARO products. The scanners are mounted at right angles to the driving direction and reliably calibrated. This means for high performance scanners, that the scanner operation concept has been designed following a dual-use philosophy. The scanners may be used stationary for 3D laser scanning tasks. Additionally they can directly be mounted to the specially designed roof-rack of MoSES and can be calibrated in the survey area within short time, so that kinematic survey tasks can be fulfilled. After completion, the scanner may be used in stationary 3D mode again. The main feature of modern laser scanning systems is the enormous point density combined with a measurement resolution below 1 mm. The frequency of profiles in driving direction varies with driving speed. At a speed of about 10 meters per second along a highway with average width, up to points per meter of road surface may be obtained. The measurement accuracy of high performance laser scanners for kinematic applications is currently being evaluated. The first results show, that the standard deviation for a single point can be expected in a range of 2 mm. Former investigations on laser scanner accuracy for much simpler scanners led to the specification of a distance measurement accuracy better than 5 mm [Gräfe 2007a]. The scanner point cloud is georeferenced by the trajectory module, which forms the core component for this kind of high-end mapping system application. MoSES can operate various position and orientation systems for trajectory determination, that contain different types of Inertial Measurement Units (IMU) for different tasks, such as highway surveys or tunnel survey applications. The following examples show the capabilities of kinematic high end laser scanning survey. Figure 2 and 3 contain images of the raw 3D laser scanner point cloud resulting from a demonstration survey in downtown Munich. The survey was carried out at posted driving speed. The same visual resolution would be obtained along roads outside city limits, like highways. Modern software packages enable the operators to work efficiently within these raw data results, which may contain point numbers in the range of 10 8 to The examples show, that objects parallel to vehicle movement may easily be extracted automatically [Gräfe 2007a] or digitized manually from the scanner data, e.g. road markings, edges, curbstones etc. Figure 2: Example for a kinematic raw scanner point cloud out of one single mission showing the Briennerstraße approaching the Königsplatz in downtown Munich.

4 Data Processing and Data Acquisition 4 Figure 3: Example for a kinematic raw scanner point cloud out of one single mission showing the Stiglmaierplatz in downtown Munich. Figure 4: A kinematic raw scanner point cloud showing the survey result along a subway tunnel. Especially figure 3, which contains a section of the Stiglmaierplatz in Munich, gives a impression, how efficient even complicated lane structures or the road markings themselves may be obtained from kinematic scanner surveys without the necessity to personally enter the road area or to block the road. The figures shown, have a average point density around 1 cm between the scattered scanner profile points. Figure 4 shows the result of a railway tunnel survey. In this case, a subway network has been captured kinematically with MoSES.

5 1 st International Conference on Machine Control & Guidance Kinematic Surveying using Multi-Camera-Systems The laser scanner point cloud examples of figure 2 to 4 reveal, that much information can be derived from scanner data. Road surfaces especially are digitized perfectly. On the other hand, scanner point clouds contain lots of digital information, that is not needed. Additionally, due to geometric configuration, single, point-concentrated objects may get lost between the profiles. For this reason, MoSES applies sensors with complementary measurement characteristics. The camera module consists of up to 8 cameras covering the whole measurement corridor. The geometric camera configuration is variable, so that e.g. a configuration with 4 cameras looking down on the road surface is possible as well as 360 corridor coverage. The cameras provide precise visual documentation information about the driven corridor. For tunnel or bridge inspection surveys infrared cameras are used. All cameras are precisely georeferenced and calibrated. For precise object measurements, stereophotogrammetric measurement techniques are used. For some geometric configurations, especially for cameras looking downward onto the road surface, orthophoto transformations may automatically be applied to the images. Figure 5: Example for a road documentation image; The MoSES can carry up to 8 cameras in grayscale, infrared or color mode. 2.4 Trajectory Quality Management The trajectory quality management does not only contain the processing control of vehicle position and orientation. The trajectory quality management contains two main steps, that are prerequisite for the practical use of kinematic survey data for high precision applications, like road construction: 1.) Homogenization of independent missions 2.) Adjustment to ground control points The background of the homogenization of trajectories is, that the positioning accuracy for a land vehicle is highly affected by DGPS positioning quality, even if high precision inertial measurement systems are used. In any case, the absolute accuracy of positioning is decreased, compared with the relative accuracy of the trajectory and the sensor measurements. Described in a simplified way, this means, that a surveyed road is measured relatively in a very precise way, but may in absolute coordinates be significantly shifted as a whole, depending on DGPS absolute accuracy. The examples

6 Data Processing and Data Acquisition 6 in figure 3 and 4 only contain one mission. It is obvious, that measurements to other moving vehicles, parking cars etc., leave gaps within the point cloud. For this reason most major roads are regularly driven once in each direction to guarantee as much sensor data coverage as possible. Naturally, the results of those missions show absolute differences between each other related to trajectory accuracy. These systematic differences can be completely removed by automatic feature extraction, e.g. road markings, and difference adjustment [Gräfe 2007a]. The result is a homogenization of all missions along the same road section, which leads to one homogenous result, e.g. one unbiased scanner point cloud out of all independent survey missions. If static survey accuracy is needed, the uncertainty of absolute trajectory determination requires the transformation of all results to ground control points using a special, time dependent transformation. Ground control points may be determined within the camera data using stereo-photogrammetric measurement techniques or they may be determined within the scanner data points, which requires ground control point signalization. The detected control points are used as a means of transforming the kinematic results to the local coordinate system. Depending on the driving speed, approximately every 250 m a ground control point is required. The transformation to the local coordinate system can be determined with an accuracy better than 3 cm in horizontal position and 3 mm in height. 3 KINEMATIC SURVEY RESULTS FOR ROAD OR RAIL CONSTRUCTION 3.1 Survey Results Precise survey data is needed e.g. for road planning and for road pavement maintenance tasks. Kinematic methods, compared with traditional static survey, offer comparable accuracy at higher speed, lower costs, much higher result data resolution and most important with much lower safety risks. During practical projects, MoSES is used to survey the road itself and to collect as much additional required object information as possible. An example contains figure Digital Road Models Digital 3D road surface models may be generated from laser scanner data. The surface model determination can be regarded as a filtering process, that transforms the laser scanner point clouds to road survey results, that match civil engineering data representation standards. The generation of 3D digital surface models based on laser scanner measurements with MoSES requires robust model determination and sophisticated quality control. The grid for the road model is orientated along the road axis. The grid is based on cross-sections at regular distances, with constant spacing between the points within each profile (Figure 8). This method of generating the grid was chosen, because the results can easily be integrated into standard road planning software packages, which organize their data in the same way. The width of each profile is limited by the edges of the road. The grid density is flexible. Surface models of a length between 0.5 to 40 km with a point spacing between m and 0.50 m have been processed so far. Normally the point clouds of one or two scanners out of at least two independent missions have to be combined to one homogenous result. The basic observations are the laser scanner profiles of each scanner (Figure 7). For each grid coordinate, a best-fit, multidimensional surface function is determined using robust estimation techniques. The estimation is based on the surrounding scanner measurements. The resulting function is used to calculate the smoothed height for the grid point. The effects of this approach contain smoothing and quality gain for the grid point height as well as large data reduction effects compared with the amount of raw data. The results are digital road models of high quality (Figure 9). The height precision of the digital road surface models reaches 3 4 mm using the standard MoSES scanners.

7 1 st International Conference on Machine Control & Guidance Figure 6: Highway intersection, that has been surveyed kinematically as part of a larger project. All features have been determined from kinematic survey data, as there are: road axes, edges, curbstones, markings, crash barriers etc. Figure 7: Example for a laser scanner cross profile along a narrow road of doubtable quality.

8 Data Processing and Data Acquisition 8 Figure 8: Road axis at a test site with transversal profiles plotted along the axis. The digital road surface model uses such cross profiles as grid structure. Figure 9: Detail out of a road surface model for a rural road displayed as color coded height model. The digital surface model results are normally handed over to road planning and design software packages in the form of cross profiles.

9 1 st International Conference on Machine Control & Guidance CONCEPT FOR ROAD CONSTRUCTION MACHINE GUIDANCE BY PRECISE 3D ROAD SURFACE MODELS The digital road surface model determination combines the following advantages: - The road surface is digitized completely without gaps our outages. Usually the measurement point density is much higher than the resulting grid resolution, so that any pothole or rut is precisely captured. - The road surface data is used as a basis for planning and design of the new road surface. For road sections with simple design, the new road surface may even be generated automatically out of the existing road surface model using best fit surface adjustment techniques. This means, that after completion of planning and design, the height differences between the existing road surface and the new, planned road surface are known precisely, so that for example the surface cutting depth for a milling machine is exactly known for each point of the road surface grid. Figure 10 illustrates the concept, that results from this new technical possibility, using the milling machine as a example for machine guidance. Figure 10: Concept for the possible machine guidance application using digital road surface models for steering the cutting depth of a milling machine. In practical terms, the concept illustrated in figure 10 would mean, that a MoSES-like van could survey building sites, e.g. on highways, in advance. Due to the systems mobility, several construction sites can be handled in short time. The data processing results are used for surface planning and design. The resulting cutting depth for each grid coordinate may be used to guide a milling machine. After completion of the milling process, the survey van can be used again to control quality and volume of the extracted pavement material. The same control method could be applied for each part of the working process to build the new road surface. Such a kinematic survey system can also easily be used for final road surface quality control.

10 Data Processing and Data Acquisition 10 5 CONCLUSIONS Kinematic surveys with high precision laser scanners open a number of new survey application areas for road or rail construction tasks. In this paper some road construction examples have been presented. The result quality has been proved under practical survey conditions. A standard deviation for the scanner points of m in height is obtained even with comparably simple scanners. High performance scanners are expected to reach m height accuracy. The huge point resolution and precision of the scanner data on the road surface leads to ideas for new concepts for machine control and guidance, that have been illustrated using a milling machine as an example. Future developments will concentrate on this field. As far as scanner technology is concerned, calibration and scanner technology evaluation will also require some development effort in future. REFERENCES Books: [Gräfe 2007a] GRÄFE, G.: Kinematische Anwendungen von Laserscannern im Straßenraum, PhD thesis, Universität der Bundeswehr München, Institut für Geodäsie, Neubiberg, Journal articles: [Gräfe 2007b] GRÄFE, G.: High precision kinematic surveying with laser scanners, Journal of Applied Geodesy, Editors in Chief: H.Kahmen / C. Rizos, Walter de Gruyter Verlag, 4/2007, p , [Gräfe 2007c] GRÄFE, G.: Quality Management in Kinematic Laser Scanning Applications, Proceedings 5th International Symposium on Mobile Mapping Technology (on CD), Padua, [Heister 2004] HEISTER, H., GRÄFE, G.: Projekterfahrungen beim Einsatz der kinematischen Messsysteme KiSS und MoSES, 58. DVW-Seminar: kinematische Messmethoden Vermessung in Bewegung, Volume 45 of DVW-Schriftenreihe, , Stuttgart, Wissner Verlag, * Dr.-Ing. Gunnar Gräfe, 3D Mapping Solutions GmbH, Talanger 4a, Oberhaching, Germany.