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УДК 528.72:629.7
ПРИМЕНЕНИЕ ЛАЗЕРНОГО СКАНИРОВАНИЯ С БЕСПИЛОТНЫХ ЛЕТАТЕЛЬНЫХ СИСТЕМ (БПЛА) ДЛЯ МОНИТОРИНГА, СЛОЖНЫХ И КОМПЛЕКСНЫХ ГЕОДЕЗИЧЕСКИХ ЗАДАЧ
Филипп Амон
RIEGL Laser Measurement Systems GmbH, RiedenburgstraBe 48, 3580 Хорн, Австрия, менеджер по международным продажам, тел. +43-2982-4211, факс: +43-2982-4210, e-mail: pamon@riegl.com
Урсула Ригль
RIEGL Laser Measurement Systems GmbH, RiedenburgstraBe 48, 3580 Хорн, Австрия, заместитель генерального директора, тел. +43-2982-4211, факс: +43-2982-4210, e-mail: uriegl@riegl.com
Петер Ригер
RIEGL Laser Measurement Systems GmbH, RiedenburgstraBe 48, 3580 Хорн, Австрия, менеджер по продукции и воздушному лазерному сканированию, тел. +43-2982-4211, факс: +43-2982-4210, e-mail: prieger@riegl.com
Мартин Пфеннигбауэр
RIEGL Laser Measurement Systems GmbH, RiedenburgstraBe 48, 3580 Хорн, Австрия, директор по научной работе и интеллектуальной собственности, тел. +43-2982-4211, факс: +43-2982-4210, e-mail: mpfennigbauer@riegl.com
Описывается технология лазерного сканирования с БЛА/БЛПА, ее сравнение с технологией воздушного и наземного лазерного сканирования, и потенциальные возможности объединения данных. Приводятся примеры с использованием коридорного картографирования для мониторинга линий электропередач и трубопроводов, контроля за состоянием объектов производственной инфраструктуры и объектов жизнеобеспечения населения.
Ключевые слова: БЛА/БЛПА, лазерное сканирование, коридорное картографирование, мониторинг линий электропередач и трубопроводов, контроль за состоянием объектов производственной инфраструктуры и объектов жизнеобеспечения населения, воздушное лазерное сканирование, радиометрическая калибровка.
UAV BASED LASER SCANNING FOR MONITORING APPLICATIONS AND CHALLENGING, COMPLEX SURVEYING TASKS
Philipp Amon
RIEGL Laser Measurement Systems GmbH, Riedenburgstrasse 48, 3580 Horn, Austria, Manager, International Sales, tel. +43-2982-4211, fax. +43-2982-4210, e-mail: pamon@riegl.com
Ursula Riegl
RIEGL Laser Measurement Systems GmbH, Riedenburgstrasse 48, 3580 Horn, Austria, Assistant to the CEO, tel. +43 2982 4211, fax. +43-2982-4210, e-mail: uriegl@riegl.com
Peter Rieger
RIEGL Laser Measurement Systems GmbH, Riedenburgstrasse 48, 3580 Horn, Austria, Product Manager, Airborne Laser Scanning, tel. +43-2982-4211, fax. +43-2982-4210, e-mail: prieger@riegl.com
Martin Pfennigbauer
RIEGL Laser Measurement Systems GmbH, Riedenburgstrasse 48, 3580 Horn, Austria, Director, Research & Intellectual Property, tel. +43-2982-4211, fax. +43-2982-4210, e-mail: mpfennigbauer@riegl.com
We present the workflow of ULS (unmanned-aircraft-based laser scanning) in comparison to the well-established high-altitude airborne laser scanning or stationary terrestrial laser scanning and discuss the potential of data fusion. Example applications include corridor mapping for power line and pipeline monitoring, and inspection of industrial and public infrastructure.
Key words: UAS, laser scanning, corridor mapping, power line and pipeline monitoring, inspection of industrial and public infrastructure, data fusion, airborne laser scanning, radiometric calibration.
INTRODUCTION
Airborne laser scanning (ALS, often also called airborne LIDAR) is an active remote sensing technique that samples the landscape in a sequential manner by laser pulses that are deflected across the flight path (Vosselman and Maas, 2010).
The backscattered echo information is typically used to determine the range to the objects within the laser beam. By merging the range information and the deflection angle of the laser beam with synchronized position and orientation information the geo-location of the backscattering surface elements can be determined. The 3D point cloud of the surveyed area results from a multitude of single measurements. Next to spatial information ALS and ULS sensors typically also provide information about the intensity of the backscattered signal. However, for the practical usage of this information and eventually for target classification, a radiometric calibration of the acquired signal strength, taking into account atmospheric attenuation and angle of incidence of the laser beam is essential.
Recent publications showed that a practical radiometric calibration workflow is feasible (Hofle and Pfeifer, 2007) (Briese, Hofle, Lehner, Wagner, Pfennigbauer, Ullrich, 2008) (Wagner 2010) (Briese. Pfennigbauer, Lehner, Ullrich, Wagner, Pfeifer, 2012) (Briese, Pfennigbauer, Ullrich, Doneus, 2013). Furthermore, the paper "Radiometric Information from Airborne Laser Scanning for Archaeological Prospection" (Briese, Pfennigbauer, Ullrich, Doneus, 2014) demonstrates a first practical application of the calibrated radiometric information for archaeological prospection.
ALS and ULS instruments typically operate with one single laser wavelength, but due to different application requirements instruments utilizing different wavelengths are available and the parallel or sequential use of different sensors allows even estimating a multi-wavelength radiometric representation of the area of interest (Briese, Pfennigbauer, Lehner, Ullrich, Wagner, Pfeifer, 2012) (Briese, Pfennigbauer, Ullrich, Doneus, 2014).
This publication focuses on the radiometric calibration of single-wavelength close-range ALS and ULS instruments. This radiometric calibration can be used to facilitate the calibration of passive multispectral imagery concurrently acquired over the same area. The presented workflow is demonstrated on the basis of an ALS data set from 2013 and an ULS data set from a later flight mission in 2014. The RIEGL VUX-1 as extremely lightweight ALS and especially well adapted UAS sensor is presented and its performance capacity is demonstrated by high-resolution data sets.
RADIOMETRIC CALIBRATION OF ALS DATA
This section summarizes the basic theory and practical workflow for the radiometric calibration of ALS data which is presented in more detail in the publications (Briese, Höfle, Lehner, Wagner, Pfennigbauer, Ullrich, 2008) (Briese, Pfennigbauer, Lehner, Ullrich, Wagner, Pfeifer, 2012) (Ullrich, Briese, 2014) (Briese, Pfennigbauer, Ullrich, Doneus, 2014). The process of ALS and ULS data acquisition can be described by the LIDAR equation that describes how the power of the laser pulse (PT at emission) is altered along its path from the sensor emission to the target and back so the detector where the received power PR is observed (Wagner 2010) (Briese, Pfennigbauer, Lehner, Ullrich, Wagner, Pfeifer, 2012):
4 1 nD2
P' RÖÜ a' 4nR '^ATM 'T ^SYS=Pr
with the atmospheric attenuation ^ATM, the intensity reduction due to range R, the opening angle 0T, the receiver optics of diameter D, the target backscattering properties a (backscatter cross-section), and some further system loss ^SYS. By the assumption of a single echo per laser shot on an extended target (bigger than the laser footprint) the equation can be simplified to:
P D
2
' WâTM ' VSYS '/ = PR
16 R1
Furthermore, the above equation introduces the backscatter coefficient y=&/ Ay, with Ay representing the laser footprint area. Moreover, by assuming that the targets hit by the laser pulse behave like a Lambertian reflector the so-called diffuse reflectance pd can be estimated with the angle of incidence # by:
Yd
Pd =
4 cos#
Finally, it can be assumed that ?]sys, Pt, and D are constant for a certain ALS sensor or a certain flight mission. Therefore these factors can be summarized in a so-called calibration constant CCAL that can be estimated with the help of targets with known reflectance at the laser's wavelength (Briese, Hofle, Lehner, Wagner, Pfennigbauer, Ullrich, 2008).
RIEGL V-line instruments provide a value for the calibrated relative reflectance. This value is the ratio of the actually observed echo amplitude and that resulting from a fictive, large Lambertian reflector of 100% reflectance at the same distance as the target object (Pfennigbauer, Ullrich, 2010). This is a valuable indication, yet the calculation disregards the effect caused by the angle of incidence. Therefore, for the presently demonstrated procedure, we use the calibrated amplitude provided by the instruments.
Based on the above described theory the following workflow for the radiometric calibration of ALS data can be applied to the ALS data (Briese, Pfennigbauer, Ullrich, Doneus, 2014):
1. Selection of the in-situ reference targets based on the ALS flight plan
2. Determination of the incidence angle dependent diffuse reflectance pd of the reference surfaces utilizing a spectrometer or reflectometer (Briese, Hofle, Lehner, Wagner, Pfennigbauer, Ullrich, 2008) that operates at the same wavelength as the laser scanner
3. Recording of meteorological data (aerosol type, visibility, water vapor, etc. for the estimation of an atmospheric model, or the visibility at visible wavelengths) during the flight mission in order to estimate the atmospheric transmission factor
5. Direct georeferencing of the ALS echoes and maybe strip adjustment in order to get an advanced relative and absolute georeferencing of the ALS data
6. Estimation of the local surface normal in order to consider the local incidence angle 6
7. Estimation of CCAL based on the ALS echoes within the in-situ reference targets (e.g. defined by a polygon area)
8. Radiometric calibration of all echoes based on the determined value of CCAL and the angle of incidence 6
STUDY SITE AND RESULTS
Study Site
For the application of the mentioned workflow for radiometric calibration the archaeological study site Carnuntum in Austria was selected. Carnuntum is located in the south-east of Vienna and is one of the case-study areas of the Ludwig Boltzmann Institute for Archaeological Prospection and Virtual Archaeology (LBI-ArchPro). As a consequence, a lot of different reference data sets (ALS, photogrammetry, terrestrial measurements, etc.) are available. Further details about the study site can be found in the paper "Radiometric Information from Airborne Laser Scanning for Archaeological Prospection" (Briese, Pfennigbauer, Ullrich, Doneus, 2014).
Radiometric Calibration Results
This subsection summarizes the radiometric calibration results for the study area that were published in „Radiometric Calibration of Multi-Wavelength Airborne Laser Scanning Data" (Briese, Pfennigbauer, Lehner, Ullrich, Wagner, Pfeifer, 2012) and "Multi-Wavelength airborne laser scanning for archaeological prospection" (Briese, Pfennigbauer, Ullrich, Doneus, 2013). Figure 1 presents the radiometric information from one ALS strip acquired by the ALS sensor RIEGL VQ-480i (laser source with 1550 nm) before (upper part of the figure) and after (lower part of the figure) the radiometric calibration workflow.
It can be clearly seen in Figure 1 that the darkening at the strip's borders (across the flight direction) mainly caused by an increasing range R can be eliminated by the radiometric calibration procedure. Furthermore, it is visible that larger field systems are represented in the calibrated reflectance image with the same gray value after the application of the presented workflow.
Figure 1: Upper image: Amplitude image of one ALS strip (1550 nm; rotated counterclockwise by 25°); Lower image: Calibrated reflectance image of the same strip (Briese, Pfennigbauer, Ullrich, Doneus 2013)
Figure 2: Upper image: Calibrated reflectance image (1550 nm) of the complete study area Carnuntum (Briese, Pfennigbauer, Ullrich, Doneus, 2013); Lower Image: Detail of the reflectance image; the length of the red line in the upper right part of the image represents 200 m. In both visualizations the reflectance images (255 gray values) are linear scaled from 0 (black) to 0.5 (white)
After the application of the radiometric calibration workflow for all available ALS strips a calibrated reflectance value for every single ALS echo is available. Based on this 3D point cloud with the assigned reflectance attribute a reflectance image can be estimated by an interpolation method. In the examples presented here the software OPALS with its grid interpolation method moving planes (selected grid width 0.25m) was utilized (OPALS, 2014). The resulting true orthophoto that contains the reflectance values for the laser wavelength of 1550 nm can be inspected in Figure 2.
NEW SENSOR AND AIRBORNE CARRYING PLATFORM FOR ULS
In 2014, RIEGL introduced the new ULS sensor, RIEGL VUX-1 (see fig 3, fig 4 and (RIEGL VUX-1, 2014). This new, compact and lightweight sensor was developed especially for use on UAS, gyrocopters, helicopters and ultralight aircraft. The effective measurement rate of the instrument is 500,000 measurements per second (with 200 scan lines per second) and offers a field of view of up to 330°. The maximum scan mission flight altitude is 350 m AGL. The instrument offers multiple target capability and a ranging accuracy of 10 mm.
Figure 3: the new lightweight airborne „ULS" scanner RIEGL VUX-1,
dimensions and weight
10 20 30 40 50 60 70 80 90 100 110
Speed [kn]
Example: VUX 1 at 550.000 pulses/second
range to target = 80 m, speed = 40 kn
Resulting Point Density - 56 pt/m'
Figure 4: ULS scanner RIEGL VUX-1, performance characteristics
Figure 5: ULS data point cloud in degrees of calibrated reflectance
Figure 5 shows the ULS data point cloud, again in degrees of calibrated reflectance. The flight and scan parameters for the data acquisition were: Speed 48 kts (25 m/s)
Altitude approx. 150 m AGL
Scan rate 380 kHz
Point density 18 pts/m2
With regards to versatility and cost-efficiency, UAS as remotely piloted sensor carrying platforms offer a promising complementary method to terrestrial and airborne surveying, especially for smaller-scale and/or repetitive data acquisition. Figure 6 shows as an example the RiCOPTER, developed as turn-key solution, a completely integrated UAS LiDAR system.
With the extremely wide scan angle together with the low flying altitude and the consequently small angles of incidence occurring in this setup, radiometric calibration taking into account the angle of incidence becomes even more important than for ALS.
Figure 6: RIEGL RiCOPTER with RIEGL VUX-SYS
38
SUMMARY AND DISCUSSION
This paper highlights the ability of ALS and ULS to deliver, next to geometric information of the sensed surface, radiometric data of the illuminated target surfaces. Based on the theory of the LIDAR equation a practical workflow for the radiometric calibration of ALS/ULS data sets was presented. This process was demonstrated with the help of already published results (Briese, Pfennigbauer, Ullrich, Doneus 2014) and a later acquired ULS data set. As a result radiometric information is added to the acquired 3D point cloud. Based on this point cloud information a true orthophoto displaying the ALS reflectance information can be estimated. Due to the active laser illumination this orthophoto is not affected by the sunlight. The resulting highresolution radiometric quantities might be a valuable reference data set for the radiometric calibration of passive airborne image spectroscopy data.
ACKNOWLEDGEMENTS
The Ludwig Boltzmann Institute for Archaeological Prospection and Virtual Archaeology (archpro.lbg.ac.at) is based on an international cooperation of the Ludwig Boltzmann Gesellschaft (A), the University of Vienna (A), the Vienna University of Technology (A), the Austrian Central Institute for Meteorology and Geodynamic (A), the office of the provincial government of Lower Austria (A), Airborne Technologies GmbH (A), RGZM-Roman- Germanic Central Museum Mainz (D), RAA-Swedish National Heritage Board (S), IBM VISTA-University of Birmingham (GB) and NIKU-Norwegian Institute for Cultural Heritage Research (N).
REFERENCES
Briese, C., Hofle, B., Lehner, H., Wagner, W., Pfennigbauer, M., Ullrich, A., (2008): Calibration of full-waveform airborne laser scanning data for object classification, SPIE: Laser Radar Technology and Applications XIII, Orlando, 6950/2008, S. 8.
Briese, C., Pfennigbauer, M., Lehner, H., Ullrich, A., Wagner, W., Pfeifer, N., (2012): Radiometric Calibration of Multi-Wavelength Airborne Laser Scanning Data, XXII ISPRS Congress, Melbourne, Australia, in: "ISPRS Annals of the Photogrammetry, Remote Sensing and Spatial Information Sciences (ISPRS Annals)", 37/2012, ISSN: 1682-1750; S. 335 - 340.
Briese, C., Pfennigbauer, M., Ullrich, A., Doneus, M., (2013): Multi-Wavelength airborne laser scanning for archaeological prospection, International Symposium of CIPA, Strasbourg, France, in: "International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences", Volume XL-5/W2.
Briese, C., Pfennigbauer, M., Ullrich, A., Doneus, M., (2014): Radiometric Information from Airborne Laser Scanning for Archaeological Prospection. International Journal of Heritage in the Digital Era, volume 3, number 1/2014, in press.
Hofle, B., Pfeifer, N., (2007): Correction of laser scanning intensity data: Data and model-driven approaches. ISPRS Journal of Photogrammetry and Remote Sensing 62(6).
OPALS, Software OPALS (Orientation and Processing of Airborne Laser Scanning data), http://geo.tuwien.ac.at/, accessed at 31.3.2014.
Pfennigbauer, M., Ullrich, A. (2010): "Improving quality of laser scanning data acquisition through calibrated amplitude and pulse deviation measurement", Proc. SPIE 7684, 7684-53.
RIEGL VUX-1: new lightweight ALS sensor RIEGL VUX-1, http://www.riegl.com/products/uasuav-scanning/, accessed at 31.3.2014.
Ullrich, A., Briese, C., (2014): Radiometric Calibration of LIDAR instruments and LIDAR data. Presentation, EuroCOW, Barcelona.
Vosselman and Maas, (2010): Airborne and Terrestrial Laser Scanning, Whittles Publish-ing, ISBN: 978-1904445876, S. 336 .
Wagner, W., (2010): Radiometric calibration of small footprint full-waveform airborne laser scanner measurements: Basic physical concepts. ISPRS Journal of Photogrammetry and Remote Sensing 65 (6 (ISPRS Centenary Celebration Issue)), S. 505-513. International Archives of Photogrammetry, Remote Sensing and Spatial Information Sciences 38, Part 7B, S. 360-365.
Biographical Notes
Philipp Amon is working with RIEGL Laser Measurement Systems GmbH located in Horn, Austria since 2010, currently as Manager International Sales.
He graduated from a higher-level secondary college for industrial engineering and is currently working on his BSc in Industrial Engineering from the HFH Hamburg.
His publications are releated to terrestrial and mobile laser scanning, applications of laser scanning and photogrammetry, as well as UAS/UAV applications of laser scanning.
Ursula Riegl is assistant to the CEO at RIEGL Laser Measurement Systems GmbH located in Horn, Austria. She holds a Magister degree in comparative literature and roman languages from Vienna University, Faculty of Humanities.
Peter Rieger is responsible manager for airborne laser scanning products at RIEGL Laser Measurement Systems GmbH located in Horn, Austria.
He received a Dipl.-Ing. degree in telecommunications engineering from the Vienna University of Technology in 2002. His research interests cover ranging techniques in scanning LiDAR, with emphasis on methods for resolving range ambiguities, full waveform analysis, and inertial navigation/GNSS.
Martin Pfennigbauer holds a Dipl.-Ing. Degree and a PhD from Vienna University of Technology. From 2000 to 2005 he was working at the Institute of Communications and Radio-Frequency Engineering, focusing on free-space optical intersatellite communication and quantum communication. Since 2005 he is with RIEGL Laser Measurement Systems, presently as Director, Research & Intellectual Property. He manages research projects funded by the European space agency, the European Union and national funds.
Dr. Pfennigbauer's special interest is the design and development of lidar instruments for surveying applications with focus on rangefinder design, waveform processing, and point cloud analysis.
Contacts Philipp Amon
RIEGL Laser Measurement Systems GmbH, Riedenburgstraße 48, 3580 Horn, Austria, Manager, International Sales, tel. +43 2982 4211, fax +43 2982 4210, e-mail: pamon@riegl.com
Ursula Riegl
RIEGL Laser Measurement Systems GmbH, Riedenburgstraße 48, 3580 Horn, Austria, Assistant to the CEO, tel. +43 2982 4211, fax +43 2982 4210, e-mail: uriegl@riegl.com
Peter Rieger
RIEGL Laser Measurement Systems GmbH, Riedenburgstraße 48, 3580 Horn, Austria, Product Manager, Airborne Laser Scanning, tel. +43 2982 4211, fax +43 2982 4210, e-mail: prieger@riegl.com
Martin Pfennigbauer
RIEGL Laser Measurement Systems GmbH, Riedenburgstraße 48, 3580 Horn, Austria, Director, Research & Intellectual Property, tel. +43 2982 4211, fax +43 2982 4210, e-mail: mpfennigbauer@riegl.com
Christian Briese
Department of Geodesy and Geoinformation, Vienna University of Technology, Austria christian.briese@geo.tuwien.ac.at
Michael Doneus
LBI for Archaeological Prospection and Virtual Archaeology, Vienna, Austria Department of Prehistoric and Historical Archaeology, University of Vienna, Franz-Klein Gasse 1, 1190 Vienna, Austria
VIAS - Vienna Institute for Archaeological Science, University of Vienna, Franz-KleinGasse 1, 1190 Vienna, Austria
mi chael.doneus@univie.ac.at
© Philipp Amon, Ursula Riegl, Peter Rieger, Martin Pfennigbauer, 2015
