uav-based laser scanning to meet special challenges in ......uav-based laser scanning to meet...

Geomatics Indaba Proceedings 2015 – Stream 2

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UAV-based laser scanning to meet special challenges in lidar surveying

by Philipp Amon, Ursula Riegl, Peter Rieger and Martin Pfennigbauer, Riegl Laser Measurement Systems

Abstract

In UAS-based mapping most of the demands have so far been met by the use of imaging sensors. However, in many applications photogrammetry finds its limitations. We highlight some of the advantages of lidar technology - such as multiple target capability or independence of environmental light conditions – proving that laser scanning is perfectly suited for demanding surveying applications.

After more than a year of experience with the first survey-grade laser scanner, the Riegl VUX-1UAV - especially developed and designed for UAV integration - we assess the potential of unmanned aircraft based laser scanning by means of several practical examples. Capable for very low level (VLL) flight, the new class of small UAVs offers as yet unknown possibilities and perspectives to close the gap between high-altitude airborne and ground-based laser scanning.

We differentiate and analyse typical applications for which ULS is proposed and assess the respective challenges. As examples we discuss corridor mapping and route monitoring in the energy sector, applications in agriculture, forestry, mining, and archeology. These applications request highly automated solutions, whereas for the survey of otherwise inaccessible areas or complex structures, the agility and maneuverability of multi-copter systems come into play.

We present the Riegl VUX-1 UAV sensor system integrated into the high performance Riegl RiCopter and discuss acquired data with respect to coverage and level of detail in comparison with high-altitude airborne and ground-based laser scanning.

Keywords

UAV-based laser scanning, surveying, UAS, lidar, echo digitisation, precision farming, corridor mapping, forestry

Introduction

Until now, remote sensing operated from commercial civil unmanned aircraft systems (UAS, also referred to as RPAS, remotely piloted aircraft systems) has mainly relied on photogrammetric techniques [2][4], making use of small and lightweight consumer-grade digital cameras to account for payload limitations. Significant development on the unmanned aerial vehicle (UAV) sector over the last years resulted in longer endurance and higher payload capacity. At the same time, extremely compact laser scanners became commercially available. On the basis of these developments it became possible to operate lidar instruments integrated into UAS [9].

UAV-based laser scanning (ULS) provides extremely flexible and cost effective operation possibilities, delivering comprehensive and accurate data in combination with ease of operation and low cost [12][13][14]. This enables on-demand or repetitive accurate surveying.

After the debut of the extremely lightweight and compact airborne laser scanner VUX-1 – especially developed for integration into UAS – in February 2014 [15], Riegl extended the VUX-1 series of survey-grade lidar instruments in May 2015 [16]. The three subtypes of this series are optimised for different kinematic surveying applications.

VUX-1UAV is optimised for UAV-based laser scanning surveying missions and offers an easy and user-friendly mounting to professional UAS, UAV, RPAS, gyrocopter, etc., VUX-1LR (long range) is ideally suited for airborne surveying missions from manned helicopters, and VUX-1HA (high accuracy) is the right choice for mobile applications easily mountable to whatsoever type of platform for mobile laser scanning, manned or unmanned.

All types offer echo digitisation and subsequent online waveform processing enabling outstanding multiple target capability, facilitating the penetration of even dense vegetation and acquisition of accurate data of the ground.


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To offer a powerful turn-key solution for UAV-based laser scanning, Riegl developed a remotely piloted airborne carrying platform, the RiCopter, a high-performance X8 array octocopter. The VUX-UAV Lidar sensor, the IMU/GNSS unit, a control unit and up to two high-resolution cameras are assembled to build the VUX-SYS, a fully integrated system solution, ready to be mounted on the RiCopter. Equipped with the VUX-SYS, the RiCopter enables high-performance unmanned surveying missions offering a flight endurance of up to 30 minutes at a MTOW (max. take off weight) of slightly less than 25 kg.

Lidar sensor

Airborne laser scanning (ALS) has proven a convincing method in vegetation mapping and habitat monitoring over the last decade. In contrast to photogrammetric techniques laser scanning enables the penetration of vegetation layers resulting in high resolution and high precision 3D data.

With the LMS-Q560, Riegl introduced world's first commercially available airborne laser scanner with echo digitisation for full waveform processing in 2004. From that time on there was a permanent improvement in measurement range and measurement rate leading to the state-of-the-art Riegl LMS-Q1560 dual channel lidar system, an ultra-high performance, fully integrated and calibrated airborne mapping system for large scale, high altitude and complex environment mapping. The advanced technology and knowledge gained through the development of premium airborne long-range lidar sensors [11] marks significant benefits also for the new segment of ULS instruments, typically employed in closer-range environments (survey missions in VLL, very low level, flight altitudes).

There is a long track record of operating Riegl lidar instruments on unmanned aircraft, with first integrations dating back to 2005. Notable examples are Riegl VQ-480-U on Aeroscout Scout B1-100 and Riegl VQ-820-GU on Schiebel Camcopter, as shown in Fig. 1.

Fig. 1: Riegl VQ-480-U on Aeroscout Scout B1-100 (left) and Riegl VQ-820-GU on Schiebel Camcopter (right).

Riegl VUX-1UAV

The new Riegl VUX-1UAV is the first laser scanner of survey-grade measurement quality specifically developed for unmanned airborne laser scanning. The key specifications are given in Fig. 2.


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VUX-1UAV VUX-1UAV

mounted on RiCopter

Eye safety class Laser Class 1

Max. range @ target reflectivity 60 %

920 m

Max. range @ target reflectivity 20 %

550 m

Minimum range 3 m

Accuracy/precision 10 mm/5 mm

Max. effective measurement rate 500 000 meas/sec 350 000 meas/sec

Field of view (FOV) 330° 230°

Max. operating flight altitude AGL 350 m/1,150 ft >500 ft*)

Fig. 2: Key specifications of the Riegl VUX-1UAV. *operational limits for civil unmanned

aircraft according to national regulations to be observed

With regard to the variety of different types of UAS, the scanner is designed to be mounted in any orientation. It is tailored for platforms with limited weight, space, and supply power for payloads. The entire data set of an acquisition campaign is stored onto an internal 240 GB SSD and/or provided in real-time via the integrated LAN-TCP/IP interface for post-processing.

The VUX-1UAV offers high-accuracy laser ranging based on Riegl’s unique echo digitisation and online waveform processing, which enables achieving superior measurement results even under adverse atmospheric conditions, and the evaluation of multiple target echoes. The scanning mechanism is based on a rotating mirror, which provides fully linear, unidirectional and parallel scan lines, resulting in excellent regular point distribution. Employing such cutting edge lidar technology enables operation at an effective measurement rate of up to 500 000 measurements/sec, with a maximum scan speed of 200 scans/sec.

System integration

A full-scale airborne laser scanning system consists of lidar sensor, an IMU, a GPS, a computer for data acquisition control, and optionally a camera and a flight guidance system. For an unmanned aircraft, some of the components for direct user interaction can be omitted. However, besides the lightweight lidar sensor all the other components must also be optimised for UAV with regards to size and weight.

In order to spare the user the effort of system integration, Riegl developed the VUX-SYS, a turnkey solution comprising all necessary components for a miniaturised ALS system and also providing standard mechanical and electrical interfaces.

Control unit Optional cameras

IMU Riegl VUX-1 Riegl VUX-SYS

Fig. 3: VUX-SYS.


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The computer for data acquisition control as well as the flight guidance system is positioned on the ground. RIEGL software is operated on the laser scanner’s own internal control computer. It enables remote control of the laser scanner via radio-link from a ground station PC by use of pulse width modulated commands (PWM-commands) and supports features like configuring scan parameters, configuration of interfaces to IMU/GNSS subsystems or optional external digital cameras, and start/stop of data acquisition. Furthermore, vital information like the operational status of the laser scanner, the IMU/GNNS subsystem, and optionally installed camera devices is collected and displayed. A block diagram showing components and cabling of the Riegl VUX-SYS, radio link, and ground station is shown in Fig. 4.

Fig. 4: Block diagram of the VUX-SYS, radio link, and ground station.

UAV platform

Main arguments for the usage of a UAV are safety and efficiency: they are to be employed in dangerous areas or under circumstances that prevent to carry out a safe on-board piloted flight. Efficiency concerns mostly the affordability of aircraft and maintenance costs as the economic benefit gained in quick-response or even repeated surveying missions. A UAV operates due to legal restrictions and limitations in radio-link range at much lower flight altitudes. These operational factors of the aircraft – its low flight altitude and the relatively slow flying speed in combination with the specifications of the sensor – especially the wide field of view of the VUX-1UAV of up to 330° and a high scanning speed – ensure an extremely high point density and almost perfect ground coverage. Fig. 5 shows a detailed comparison between high-altitude ALS data acquired with a LMS-Q780, perfectly suited for efficient wide area mapping, and data from a smaller scale mission carried out by the RiCopter with integrated VUX-SYS.

Fig. 5: High-altitude, wide area LMS-Q780 data (left), low-altitude, high resolution data acquired with VUX-1UAV, carried by the RiCopter (right).


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The VUX-1UAV data has a significantly higher resolution and significantly better coverage of vertical structures due to the reduced ground speed and low flying altitude and the resulting different viewing angles. These advantages are traded for the reduced area coverage compared to conventional airborne laser scanning.

Riegl RiCopter

Fig. 6: RiCopter with sensor payload airborne.

The Riegl RiCopter is an octocopter with a maximum take-off mass (MTOM) of 25 kg. Offering a total payload capacity (sensors and batteries) of up to 16 kg and an effective flight endurance of more than 30 minutes (with full payload) it is ideally suited for typical UAV-based laser scanning (ULS) applications [17]. Structural parts are made out of carbon composite material providing maximum rigidity at low weight. Only scanner, IMU, and optional cameras are mounted on the outside of the UAV, all other components including the control unit of the VUX-SYS are located inside the protective cell of the RiCopter’s body. Batteries reside in two compartments in the front and rear part of the fuselage, resulting in a homogenous weight distribution which has a positive effect on the flight stability and agility.

The VUX-1UAV laser scanner is mounted in such a way that the obstruction of the scanner’s field of view (FOV) by the RiCopter’s body or rotor blades is kept to a minimum. The resulting FOV is 230° with a net measurement rate of 350 000 measurements/s. Fig. 6 shows the RiCopter with sensor payload in the air.

Due to its genuine design with foldable arms, the RiCopter can be stowed for transportation in a very compact and rugged transport box, which fits in most car trunks (see Fig. 7).

Fig. 7: RiCopter with folded arms in transport configuration and stowed in transport box.


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Applications

Precision farming

Crop growth and health are closely monitored in precision farming in order to minimise the use of fertilisers or insecticides and to optimise the amount of irrigation. Airborne laser scanning data enables the observation of plant growth while at the same time displaying changes in ground surface or detecting areas of damage caused by, e.g. hail, storm, or game [7, 10]. The repetitive task of airborne sensing missions by manned aircraft is cost-intensive. Depending on the surveyed area, ULS often represent a cost-effective solution for carrying out these survey missions at much shorter intervals while still at a lower cost. Fig. 8 shows scan-data acquired on different types of crop. The data enables the analysis of growth rates and the detection of areas, where the development of agricultural crops differs, allowing for implementing timely measures and yield estimation.

Fig. 8: Lidar data of different agricultural features: corn (top left), sunflowers displaying inhomogeneous growth, possibly due to irrigation issues (top right), regular sunflower field with recognisable and quantifiable gaps (bottom left), and wheat field with nutrient oversupply due to wash out in uneven terrain. (bottom right).

Regular lidar survey allows for both growth monitoring over a single season and long-term observation of terrain changes in order to detect critical areas. The results can be used for decision making in plant cycle

determination and as a basis for cropland development support programs.

Forest inventory

Because of their potential in providing digital terrain models, detection of deadwood, biomass and understory estimation, as well as canopy change monitoring, airborne laser scanning data have proven significantly relevant for the forest industry [1, 7, 10]. Yet in difficult-to-approach areas such as narrow valleys where it would be dangerous or impossible to employ conventional aircraft or for applications where point densities higher than achievable with ALS are required, ULS comes into play. The large FOV of the Riegl VUX-1 Series provides a comprehensive scan of such environments. Fig. 9 gives an impression of the wealth of information that can be retrieved from high-resolution ULS data.


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Fig. 9: Layers extracted from forestry ULS dataset (from top to bottom): vegetation, ground conditions,

growth monitoring/tree height, shrub layer and deadfall, erosion.

Power line inspection and infrastructure monitoring

The current status of power lines and towers is subject of periodical monitoring. Until now, this has been carried out by helicopters typically flying above the structures at low speed and at lowest possible altitude to achieve high density images and point clouds [5]. This inspection is thus not only dangerous, but also very time consuming, and, due to the high demand for staff and equipment, extremely expensive. Furthermore, in some areas the time for inspection is restricted by noise abatement regulations, a fact that further complicates surveying missions.


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The advanced technology provided by UAS equipped with a survey-grade laser scanner, is therefore considered a convincing economic benefit for this kind of application. By adjusting the scan pattern of the Riegl VUX-1UAV, the capability to detect even very fine features, such as cables, is augmented. The dataset displayed in Fig. 10 is an example of step-by-step filtering of raw data and the facilitated or even automated extraction of features. The additional information contained in the scan data serves to identify and to classify targets. Power line monitoring is therefore considered a very promising ULS application not only enhancing data acquisition but making use of the full potential of data analysis.

Fig. 10: Power line inspection: point cloud with the point brightness according to reflectance (left), point cloud with RGB color information from cameras and classified powerline highlighted in yellow

(middle), analysis of vegetation encroachment (right).

Topography in open-pit mining areas

Laserscanning in open-pit mining areas is an efficient and cost effective method to generate data for volume calculation and the analysis of stockpiles and sediments [3]. It has been used for years, mainly by means of terrestrial (tripod-based) laser scanning but also airborne laser scanning. Surveying the topography of mining areas by ULS facilitates the generation of complete and high-resolution datasets which can easily be generated repeatedly or on demand. Comparison of repeatedly surveyed areas serves as a basis for the mine planning model.

Specific software solutions for mining applications allow automated surface extraction and feature modeling as well as breakline extraction out of the scan data, thus providing the standard results to mining customer requirements. Examples of data relevant for mining applications are shown in Fig. 11.

Fig. 11: ULS in mining applications: point cloud with the point brightness according to reflectance (left), surface modeling (middle), feature extraction (right).

Archeology/cultural heritage

The ability of 3D laser scanning to capture complex objects in their entity with high resolution and high accuracy without the necessity of direct, physical access proved to be extremely valuable [6][8]. Many landmarks, excavation sites, or ground structures bearing the remainder of human activities have been surveyed by terrestrial or airborne laser scanning for conservation, education, and/or preservation purposes.

When it comes to large buildings or structures in areas not easily accessible the use of multiple scan positions from the ground can become a cumbersome or even dangerous business. In such cases, ULS may be used to complement or even replace terrestrial laser scanning. Flexibility of vantage point and surveying speed on one hand and the possibility to realise extremely high resolution are in favor of ULS for archeological / cultural heritage applications.


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In spring 2015 Castle Vianden in Luxembourg was surveyed by using a VUX-SYS on RiCopter (see Fig. 12) in order to complete a meticulous surveying project carried out over several years by different surveying methods by the ArcTron 3D, a renowned German specialist for archaeological survey. The results of the ULS survey, data acquired in just one day, highlight the great potential of ULS for completing existing datasets of comparably complex and inaccessible environments.

Fig. 12: Detail of Castle Vianden in Luxembourg, data captured by RiCopter equipped with VUX-SYS in 2015.

Conclusions

UAV-based laser scanning (ULS) is a new segment complementing the existing applications for lidar technology airborne, mobile, and terrestrial laser scanning. It shows high potential for various applications.

With the VUX-1UAV together with the RiCopter and supporting peripheral hardware and dedicated software solutions, Riegl provides a tailored toolbox for this emerging field. A number of use cases have been demonstrated and are reported in this article.

References

[1] E Naesset, T Gobakken: “Estimating forest growth using canopy metrics derived from airborne laser scanner data”, Remote Sensing of Environment, 453-465, 2005.

[2] H Hirschmüller: “Stereo processing by semiglobal matching and mutual information,” IEEE Transactions on pattern analysis and machine intelligence 30, pp. 328-341, 2008.

[3] B Höfle, M Rutzinger: “Topographic airborne Lidar in geomorphology: A technological perspective”, Zeitschrift für Geomorphologie/Annals of Geomorphology 55 (Supplementary Issue 2), 1-29, 2011.

[4] N Haala, M Rothermel: “Dense multi-stereo matching for high quality digital elevation models”, PFG Photogrammetrie, Fernerkundung, Geoinformation. Jahrgang 2012 Heft 4 (2012), p. 331-343, 2012.

[5] M Ritter, W Benger, “Reconstructing power cables from lidar data using eigenvector streamlines of the point distribution tensor field”, Journal of WSCG 20, 223-230, 2012.

[6] K Challis, A Howard: “The role of lidar intensity data in interpreting environmental and cultural archaeological landscapes.” In: Rachel S. Opitz und David Cowley (Hg.): Interpreting archaeological topography. Airborne laser scanning, 3D data and ground observation. Oxford: Oxbow Books (Occasional Publication of the Aerial Archaeology Research Group, 5), S. 161-170, 2013.

[7] W Mücke, A Zlinszky, S Hasan, M Pfennigbauer, H Heilmeier, N Pfeifer: "Small-footprint full-waveform airborne LiDAR for habitat assessment in the ChangeHabitats2 project"; Europen Lidar Mapping Forum (ELMF), Amsterdam, The Netherlands, November 2013.

[8] C Briese, M Pfennigbauer, A Ullrich, M. Doneus, "Radiometric information from airborne laser scanning for archaeological prospection", CHNT, Vienna, 2014.

[9] P Amon, U Riegl, P Rieger, M Pfennigbauer: “Introducing a new class of survey-grade laser scanning by use of unmanned aerial systems (UAS)”, FIG Congress 2014, Kuala Lumpur, June 2014.


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[10] U Riegl, P Rieger, M Pfennigbauer, P Amon: “UAS based laser scanning for forest inventory and precision farming”, Int. Workshop “Remote Sensing and GIS for Monitoring of Habitat Quality, TU Wien, September 2014.

[11] A Ullrich: “Radiometric calibration of multi-wavelength airborne LIDAR data “, the International LiDAR Mapping Forum 2015, Denver, Colorado, USA, February 2015.

[12] T Gaisecker, M Pfennigbauer, P Rieger, U Riegl, P Amon: „UAS-Laserscanning im praktischen Einsatz: Systemintegration – Datenaufnahme – Datenanalyse”, Oldenburger 3D Tage 2015, Oldenburg, February 2015.

[13] U Riegl, P Amon, P Rieger, M Pfennigbauer: “Meeting Urgent Demands within Large Scale ALS/MLS Projects with UAV-based Laser Scanning”, Workshop Uni Köln“Applications in Laser Scanning”, Köln, March 2015.

[14] P Amon, U Riegl, P Rieger, M Pfennigbauer: “UAV-Based laser scanning for monitoring applications and challenging, complex surveying tasks”, Geosiberia 2015, Serbia, April 2015.

[15] Datasheet: “RIEGL VUX-1: new lightweight ALS sensor”, www.riegl.com/products/uasuav-scanning/

[16] Infosheet: “RIEGL VUX-1 SERIES”, www.riegl.com/products/newriegl-vux-1-series/

[17] Infosheet: “RiCOPTER “, www.riegl.com/products/uasuav-scanning/new-riegl-ricopter-with-vux-sys/

Contact Philipp Amon, Riegl Laser Measurement Systems, [email protected]

uav-based laser scanning to meet special challenges in ......uav-based laser scanning to meet...

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