Key-Words: Surveying Engineering, Laser scanning airborne systems

Transcription

1 Laser Scanning Airborne Systems - a New Step in Engineering Surveying SAVU ADRIAN adisavu2002@yahoo.com DIDULESCU CAIUS caiusdidulescu@yahoo.com BADEA ANA CORNELIA badeacadastru@gmail.com BADEA GHEORGHE badeacadastru@yahoo.com Abstract: - The new laser scanning airborne technologies can be successfully used in engineering surveying and geodesy. They are mounted on light airplanes or on helicopters, have a fly speed comprised between 70 to 100 km/h; a height fly comprised between 50 to 100 m and can collect three-dimensional points with a density of 20 to 30 per meter. The advantages of these systems are: reduction of field work time, easy and safe access to objects (railway, roads), permit to create 3D homogeneous spatial data base, ensure the topographic and geodetic support for specific information systems. Key-Words: Surveying Engineering, Laser scanning airborne systems 1. Introduction Laser scanning airborne systems permit the measurement of points on the surface of the ground with the help of scanning equipment which can be mounted on the several types of aircrafts and helicopters. A visual flight of the zones of interests will collect GPS data, laser date and images. Laser scanning airborne system has a rate of collection of about points on second, flying to average altitudes of m and an average speed of 70 km/h, the density of the collected points being at least 10 points on square meter. This density is necessary to mark the difference between scanned objects like lines of transmission, conductors, distribution pillars, and electric lines, of railway, signals, switches and so forth, differentiation obtained through the recognition of spatial structure composed of the close points. 2. Main features ale of the laser scanner airborne systems The principle of the system is based on the energy reflected and fractionally absorbed by the ground or the objects on the ground. The reflected part is recorder by a sensor of the laser system and the difference of time among the transmission of the laser impulse and his reflection to the soil gives us the distance among a mark from helicopter and the targeted point placed on soil. The position, the orientation and the height of the helicopter are known with accuracy in the moment of transmitting the impulse so it can be calculated precisely the three coordinates of the terrain point. The speed of data acquisition and their accuracy permits the use of the system for surveying especially the communication ways. 2.1 Hardware parts of laser scanner airborne systems The laser scanner airborne system integrates many high-tech systems in complex equipment described above. ISSN: ISBN:

2 The platform of flight. The system can be installed on light aircrafts or helicopters adapted to the necessary conditions for precise measurements. These aircrafts are equipped with special platforms for mounting the laser sensor and the video system and also with special sensors for stability, orientation and direction of flight. The aircrafts have to be equipped with an Inertial Navigation System (INS) that includes sensors like roll; pitch and heading that control the navigation parameters. The determination of the coordinates and the speed of the vehicle through the remaking the information relating to this acceleration is so called inertial method of navigation. In principle, the inertial navigator consists in instruments that measure the components of the acceleration and has specialized computers that elaborate the navigational parameters of a certain system of reference. Typically for this method is the fact that, as opposed to all other navigation proceedings, the determination of parameters that define the evolution of the vehicle in space is apart from the exterior sources of information. These advantages made the INS to be used on large scale on air navigation The positioning system The laser scanning airborne system incorporates two GPS receivers (double frequency L1/L2) for the assurance of the position accuracy for the helicopter or the aircraft at one time. The receivers record both the carrying wave and differential correction from each satellite and in this way it can be obtained a high accuracy of the position of measured point after the final processing of measurements The aircraft orientation system. The aircraft orientation system consists in an inertial measurements system that determines instantaneous the angles of the aircraft: pitch, roll and heading (with a rate of 200 determinations on second), and also the acceleration of the aircraft. These measurements are used by INS for precise computation of the apparatus orientation. The accuracy of INS is of 0.05 degree for pitch and roll and of 0.08 degree for heading and in combination with the GPS receivers, assures the necessary data for the precise calculus of the three dimensional position of the aircraft The laser system. The laser system is apt to cover a band with the width between 15 and 175 m. Each scan delivers 200 measurements covering a sector of 60 degrees. Each scanning recording contains the time, the orientation of the laser, information about the verification and the correctness of the data and information about the intensity of the signal what forms infrared images for further visualizations. In operational mode the laser equipment scans with a frequency of 60 impulses on second which leads to obtain about points per second The video system The laser scanning airborne system is equipped with two digital video color cameras of high resolution having 750 horizontal lines and a CCD sensor. The images are recorded on S-VHS bands One camera is orientated downwards and it is coupled with the laser. The second camera is fixed in an oblique position and supplements the images obtained by the first camera. The time, calculated with high precision, is attached to every video frame, thus the image is correlated with laser data. All the information communication between the pilot and the operator of cameras are recorded on the audio band of the equipment and will be used later to the interpretation of the images The acquisition system. The laser scanning airborne system orientates and guides the pilot along the determined line of flight that appears on a screen with luminous spots which indicates the deviations on vertical (position insured by the feature AGL of the laser system) or on horizontal (insured by the navigation and positioning system). The pilot and the operator can see the images on forward and downwards direction on a color screen and the operator checks up and monitors the fly and the operations in process of development on an another screen with a special interface. The process is supervised by PC computers with Intel architecture, having the task to manage the data, to control the sensors and the navigation of the apparatus. The measurements made by the system - the carrying wave phase and pseudo range, the laser recording and the real-time differential and inertial solutions are stored on memory cards The integration system. Being projected in modular way, the system can be set in many different types of aircrafts and helicopters and it was built in two main pieces: the platform with sensors and the computing system. First component is mounted in a classic manner on the helicopter using the existing connections on this. It contains the laser, SMI and the video cameras. Outside is attached a support with two arms for the GPS antennas. The second component contains the GPS receivers, Omnistar, INS, video recorders, the computers and the equipments of storage data. It is fixed in the interior of the aircraft and it is connected to the first through cables Software parts of laser scanner airborne systems The processing of the measurements obtained after a laser scanner session supposes several stages: postprocessing at the end of each day of flight, processing the cloud of points obtained and finally to achieve the products that can be of several types. Each firm ISSN: ISBN:

3 implied in development of laser scanner airborne systems supplies processing software that has in essence same characteristics; small differences can appear depending on principal surveying works for which were created The processing of raw data At the end of each working day, the platform flight is pre-processed, verified and transposed in a specific file that contains information about the orientation of aircraft. During the pre-process, the software executes the check of data quality to eliminate the discontinuities of data and to verify the specifications of the project. In this stage will be defined the horizontal projection, the model parameters of and the measure units The post-processing of data The processing software will have to determine the coordinates XYZ from data provided by GPS systems, the orientation of the aerial platform and the laser systems. The laser scanner system has extended CAD facilities and correlated with the software that process video images and laser data obtain a spectacular multimedia presentation of the flight zone. Each software has the facility to visualize and ortho-rectify the video images that enriched the gathered information. Fig. 1. Images from the laser scanning airborne system 3. Interpretation of different processed data results. The hardware part of the system consists in two performance computers, two video recorders S-VHS, monitors and video image cards. The software part is usually an intuitive user interface that respects the Windows standards. The software has a module of transformation data and includes different types of projection systems. The vertical component of data can be converted from ellipsoidal heights in orthometric heights using different geoids models and afferent information of measured zone. There are about points per kilometer and these points provide many possibilities of interpretation: - Color after height : the laser points are disposed in a scheme of color that permits the visualization of forms with relief. - Color after intensity : the objects and forms that have no relief can be distinguished after the intensity of the reflection. - Transversal and longitudinal profiles. The software permits extraction of 3D information about the position of any object or form that can be visualized. The user extracts the information in the same manner as he should draw across the background presented on-screen. He can define an unlimited number of drawing objects and layers that can be colored as needed. All the extracted features maintain the original available information. The software has an efficient although sophisticated scheme of attached attributes. Each layer contains drawing objects that includes points, polylines or groups of points and polylines. Each object class has owns sets of attributes defined by user, and also these attributes have an unlimited number of predefined values.. When the user extracts a point from a certain layer where previously was defined a flag, an introduction window is displayed with fields having predefined values for selection. The products obtained with the laser scanning airborne system are: - Sets of data (general data X,Y, Z); - Transversal and longitudinal profiles; - Contour lines and the Digital Model Terrain; - Digital video images incorporated with the lasers 3D points; - GIS Databases; - Object inventory; - Schemes of intensity for the classification of the grounds; - Transfer between another CAD/GIS packages. ISSN: ISBN:

4 4. Technical data of laser scanner airborne systems MODEL DA42 MPP, HK36 MPP, AS350B Engine Top speed. Speed of cruise. Consumption Autonomy of flight. Length Proceedings of the 11th WSEAS International Conference on Sustainability in Science Engineering Width. The span of the wings 2*THIELERT Centurion 1.7, TTC115 ROTAX 914 F3 (115HP), 1TURBOMECA ARRIEL 2B 359km/h 255 km/h 33.3 l/h km 8.56 m 2.49 m m Number of places 2+2 Weight 1260 kg Maximum capacity 1785 kg Capacity reservoir 300 l Table 1. Fig 2. Components of laser scanner airborne system Producer Furgo, IGI, Leica Geosystems, Optech, Riegl LMS, TopEye, TopoSys GmbH FLIP-MAP 400, LiteMapper 2400/5600, Name of laser scanner airborne systems ALS50-II, ALTM Gemini, LMS-Q240i- 60/LMS-Q240-80/LMS-Q560, TopEye Mk II, Falcon III, Harrier 24/ Harrier 56 Date of appearance/last update 2002/2007 Dimensions -weight[kg]/dimensions [cm] of laser system 30kg, 50*30*30cm -weight[kg]/dimensions [cm] of total system aproximative100kg -power supply 24V/30W ISSN: ISBN:

5 The features of the laser pulse Wavelength [nm] 1,500nm The pulse of the laser wave [ns] 4ns Diverging the laser wave [mrad] 0.45mrad (radial) Type/Class of laser Fiber/Class I M Resolution 0.3m The method of recording Method of scanning Rotation of mirrors The speed of rotation mirror 150Hz Frequency of pulse [Hz] 250,000 Hz Maximum angle of scanning [deg] 60 deg Maximum echoes number of received pulse 4 The method of detect of received signal At returning The intensity of the signal [bits] 11 bits Positioning system GPS system 2*Trimble DB950 L1/L2, 10Hz Planimetric / altimetric relative accuracy of GPS σ 2 [cm] 2/10cm INS Inertial Navigation System Applanix PosAV 410, 200Hz INS accuracy (roll/pith/heading) [deg] 0.008/0.008/0.015 deg Processing software GPS/INS GrafNav/PosProc Accuracies and resolutions Accuracy of distance determination [cm] 2-3cm Accuracy of different of height determination/1km [cm] Depends of quality of network Total planimetric accuracy [cm] Depends of quality of network Maximum number of points/m points, depends of altitude and fly speed The aisle among points along the route [m] 0.27m la 150km/h The aisle among points perpendicular to route [m] 0.46m la 150km/h Another components of the system 2 cameras, one with taking over in front and Camera one with taking over in lateral, 11Mpix, inclusive video mode Facility of data storage Removable hard disk, 80 GB Another features The platform of flight Helicopter Height of flight (min/max)[m] m Maximum duration of flight [hours] 3-6 ore Air temperature [C grade] Celsius grade Table 2 5. Conclusion The laser scanner airborne system has the following advantages: - From the point of view of collection spatial data, this system can take over spatial information without interact with objects. The large rate of gathering data shortens the period of terrain works, what leads to the decrease of the costs. - From the point of view of processing the spatial data: - The transfer of information from the scanner system to the software and inversely is achieved quickly through cable or wireless. All information can be stored in a database and selected on different categories. Thus, it can be performed an easy organization of regarding information of video files, the position of gathered data, the name of the project, the zone of flight. - The software has an implemented database that contains maps in the shape of images (bmp, tiff, jpg) and digital maps (dxf, ESRI, shape). The necessary parameters for georeferrencing the images can be calculated and this information can be used for planning the flights if the zone is in the database. - The final products are interchangeable in ASCII and DXF formats that are recognized by the most CAD software s. - The cloud of points obtained after scanning enable explicative and complex analyses. ISSN: ISBN:

6 - The software generate easily the Digital Model of Terrain that permit realization of longitudinal and transversal profiles, the calculus of the area of different sections and the calculus of volumes. - The results obtained after such measurements are homogeneous and can be correlate with data taken over from navigation and positioning systems on real time. The laser scanning airborne systems will represent the future in collecting and processing 3D data in many fields where the classic surveying of large areas implies a huge amount of human efforts, logistics and time. Fig. 3 Longitudinal profile References [1] Deumlich F., Instrumentenkunde der Vermessungstechnik, VFB Verlag, Berlin, 1995 [2] Grobmann W., Vermessungskunde III - Walter de Gruyter, Berlin, 1999 [3] Heunecke F., Ingenieur Geodäsie, VEB Varlag für Bauwesen, Berlin, 1986 [4] Heunecke F., Handbuch Ingenieurvermessung Band 1, Karlsruhe, 1993 [5] Witte B., Vermessungskunde für Bauingenieure, Stuttgart, 1991 ISSN: ISBN: