工程地质勘测中基于无人机的地图绘制外文文献

Contents lists available at ScienceDirect Engineering Geology journal homepage Technical note Unmanned Aerial Vehicle UAV based mapping in engineering geological surveys Considerations for optimum results O Tziavoua 1 S Pytharoulia J Souterb aDepartment of Civil and Environmental Engineering University of Strathclyde James Weir Building 75 Montrose Street Glasgow G1 1XJ UK bSurvey Solutions Scotland Bilston Glen Industrial Estate 14 Dryden Rd Loanhead EH20 9LZ Scotland UK A R T I C L E I N F O Keywords UAV Fixed wing VTOL Mapping Image resolution Engineering geological survey A B S T R A C T UAVs have been used in engineering for at least two decades mainly focusing on structural health monitoring geological surveys and site inspections especially at cases where a rapid assessment is required for example after a natural disaster While there is a wide range of recognition algorithms for the automatic identifi cation of structural damage structural geological features etc from the acquired images the parameters aff ecting the resolution of these images are often overlooked As a result the UAV technology is not used at its full potential and at times it is even regarded as leading to poor outcomes This paper discusses the main parameters aff ecting the resolution of the images acquired by a UAV We present a case study of the structural geological mapping of a coastal area carried out using two types of UAVs a fi xed wing and a hexacopter A comparison between the structural geological maps based on the orthophotos and one produced using conventional techniques shows that the level of detail is the same and the time spent is at least 5 times less when using a UAV The fi xed wing is faster and therefore can cover large areas while the copter gives better resolution images as it can fl y at lower heights The latter is cost and time eff ective only if it is used for surveys limited to small areas The characterization of some structural geological features has not been possible based solely on the orthophotos We show that in order to achieve the desired accuracy a ground sample distance of at least half that value is required We discuss technical aspects such as the eff ect of topography and UAV orientation on the overlap value the camera ca libration number of control points and lighting conditions that should be taken into account prior to fl ying a UAV and provide recommendations on how to obtain optimum results i e orthophotos that suit the needs of the project 1 Introduction Unmanned Aerial Vehicles UAVs allow for the eff ective mon itoring of large areas of land and existing infrastructure within a very short time compared to conventional techniques a favourable char acteristic especially at cases where urgent intervention is required e g when a natural disaster occurs e g a rock slide Greenwood et al 2016 Tannant et al 2017 2016 Kaikura earthquake New Zealand Erickson 2017 or when inspection is necessary but the site cannot be accessed due to Health and Safety concerns e g 2011 Fukushima earthquake Japan Ackerman 2011 The main principle is that a UAV takes aerial images incorporated with spatial data based on GNSS and or Inertial Measurement Unit IMU over an area to fi nally produce a high resolution 3D point cloud that can be used for a wide range of geological civil mining engineering applications and projects The images are all processed to form a single image mosaic representing the area of interest This image is geometrically corrected orthomo saic and georeferenced and can be used to extract information such as distances and locations in the same way as a map UAVs have signifi cantly developed during the last decades They operate remotely in the form of small platforms carrying cameras and for the majority of applications are available as small or micro aircrafts or Vertical take off and landing VTOL copters with four quadcop ters six hexacopters or more propellers Hackney and Clayton 2015 Jordan 2015 Currently most of them are equipped with GNSS re ceivers and or other sensors e g Inertial System sensors etc Tele metry facilities are frequently deployed for data transmission and or management in almost real time when an immediate reaction is ne cessary Jordan 2015 The leading application of UAVs is undoubtedly 3 dimensional 3D mapping visualisation and modeling thus con tributing to applications such as topographic surveys photogrammetric solutions progressmonitoring disasteranalysis archaeological https doi org 10 1016 j enggeo 2017 11 004 Received 8 June 2017 Received in revised form 5 November 2017 Accepted 5 November 2017 Corresponding author 1Deceased on 5th March 2017 E mail address stella pytharouli strath ac uk S Pytharouli Engineering Geology 232 2018 12 21 Available online 07 November 2017 0013 7952 2017 Elsevier B V All rights reserved T mapping agriculture and forestry e g Remondino et al 2011 Niethammer et al 2012 Draeyer and Strecha 2014 Cryderman et al 2015 A detailed discussion on the evolution of UAVs and the state of the art of this technology is given in a review work by Watts et al 2012 and Colomina and Molina 2014 The latter conclude that the majority of commercial applications is supported by UAVs the market of which is progressively developed the UAV production the civil commercial applications and the research on UAVs have increased by 68 78 and 55 respectively between 2005 and 2013 Colomina and Molina 2014 Table 1 p 80 With the ever increasing use of UAVs there have been a number of studies focusing on the effi ciency of UAVs in geotechnical geological and other engineering applications Siebert and Teizer 2014 used a UAV technology for the estimation of position errors and volume un certainty estimation in construction and other civil engineering projects e g roadworks excavation mining works etc Error in heights was determined at the level of about 4 cm when the fl ight level was at 30 m and the diff erence in a volume of 440 m3was found to be approxi mately 9 as compared to the fi gure obtained by conventional sur veying using GPS Similar results and techniques are also reported in Draeyer and Strecha 2014 for the determination of stockpile volumes and in Raeva et al 2016 for the case of mining in quarries Engineering geology mapping surveys include detailed mapping of the outcrop annotation of all features names dips and strikes that allow for the characterization of the site UAVs can produce a very detailed image of the outcrop e g Peng et al 2017 Mart nez Mart nez et al 2017 but in most cases there is the need for someone to go on site for reconnaissance Cawood et al 2017 New developments on algorithms and image processing techniques permitted the automatic identifi cation of types of rocks faults dip and strike measurements so manual work can signifi cantly be reduced e g Michlethwaite et al 2012 Stumpf et al 2013 Vasuki et al 2014 Bemis et al 2014 Based on the reviewed literature there are no easily accessible guidelines available regarding the choice of some of the parameters that greatly aff ect the quality of photos and consequently the orthomosaic obtained from a UAV e g overlap between photos fl ight height light conditions specifi cations of the lens and camera and weather condi tions As a result if the user is not experienced or does not have a basic knowledge on surveying and photogrammetry quite common con sidering the wide range of UAV user backgrounds a poor quality or thomosaic is produced on which an automated image algorithm can do little This frequently leads to the misconception that a poor outcome is always due to limitations of the UAV technology This paper focuses on the use of UAVs for engineering mapping surveys and makes re commendations on the parameters that should be considered prior to fl ying a UAV in order to achieve optimum resolution of the obtained images A case study on the mapping of structural geological features at an outcrop along the coast of South Ayrshire Scotland is also pre sented 2 Principles of photogrammetry and choice of fl ight parameters There are a number of technical parameters that need to be con sidered prior to fl ying a UAV the required resolution of the ortho photos the fl ight height the overlap between the photos the lens and camera characteristics These parameters are important because they not only aff ect the resolution of the obtained images but also the time spent on site and the post processing time and eff ort for the production of the point cloud and the orthomosaic In addition the resolution of the fi nal orthomosaic signifi cantly aff ects the amount of information that can be extracted and any results obtained from the use of auto matic feature detection algorithms For engineering geological mapping surveys a spatial resolution of less than 10 cm is generally good This translates to a requirement of maximum 10 cm pixel i e the Ground Sample Distance GSD the distance on the ground between the centres of two adjacent pixels should be 10 cm pixel or less For a certain GSD the fl ight height de pends on the focal length FL the sensor width Swand the number of pixels per photo width PN He et al 2012 FGSD F P S HL N W 1 where FH is the fl ight height m GSDis the ground sample distance m FLis the focal length mm PNis the number of pixels per image width Swis the sensor width mm From eq 1 it is evident that keeping the fl ight height number of pixels per image width and sensor width the same and increasing the focal length results in a better GSD i e spatial resolution For example for a fl ight height of 75 m a sensor width of 35 9 mm and 7360 number of pixels per image width a lens with 15 mm focal length gives a GSD of 2 4 cm pixel This value becomes equal to 1 5 cm pixel and 1 cm pixel for a lens with focal length of 25 mm and 35 mm respectively However the GSD and the camera used are not the only parameters that should be considered before choosing the fl ight height Other factors to be accounted for are the fl ight time and the number of images required to cover a specifi c area Both depend on the overlap percen tage i e the percentage of the same area on the ground covered by adjacent images as shown in Fig 1b In general an overlap value of more than 60 for the forward overlap and at least 20 for the side overlap is considered adequate in photogrammetry in order for an or thomosaic to be created In practice for UAVs a higher overlap value e g 80 85 would minimize the possibility of gaps in the ortho mosaic and is recommended Campbell and Wynne 2011 However it might not always be achievable due to camera triggering limitations and the fl ight parameters For example in the absence of wind a UAV that fl ies at 23 5 m s equipped with a camera that has a triggering limitation of 1 Hz will need a fl ight height at which 23 5 m represents 20 of the along track image footprint to achieve an 80 forward overlap For a sensor size of 24 mm and a focal length of 35 mm it needs 175 m above ground level to achieve an along track image footprint that lets the camera capture 80 forward overlap This fl ight height might not allow for the desired image resolution Furthermore there might be additional limitations e g the maximum fl ight height in the UK is 500 ft approximately 152 m Civil Aviation Authority 2016 which means that a fl ight height of 175 m is not permitted From trigonometry the Ground distance GDx footprint perpendi cular to the fl ight line is related to the fl ight height FH the focal length FLand the sensor width Swas see Fig 1a for reference G F F S DxHLw 2 Similarly the Ground distance GDy footprint along the fl ight line is given by G F F S DyHLL 3 where SL is the sensor size in the direction perpendicular to the fl ight line Table 1 Main technical characteristics of TRIMBLE UX5 HP and ZX5 UX5 HPZX5 TypeFixed WingRotary Wing Dimensions100 65 10 5 cm85 49 cm CameraSony A7R 36 MPOlympus E PL7 16 MP Image dimensions7360 4912 pixels4608 3456 pixels Focal length15 mm14 mm Sensor size39 5 mm 24 mm17 3 mm 13 mm O Tziavou et al Engineering Geology 232 2018 12 21 13 The fl ight line spacing FLS Fig 1b is fi ven by FG 1 side overlap LSDx 4 while the number of fl ight lines NFLis equal to NW F FLLS 5 where W is the width of the surveyed area The distance between the images Diis given by DG 1 forward overlap iDy 6 The number of images per fl ight line of length L is N LG 2 D iDyi 7 and the total number of images per fl ight is equal to NNN TiFLi 8 Forexample theSonyA7Rcamerahasasensorsize 39 5 mm 24 mm If the area to be surveyed is 200 m 100 m using a lens with a focal length of 15 mm and a fl ight height of 75 m would resultinagrounddistance footprint foreachphotoof 197 5 m 120 m Eqs 2 and 3 respectively The spacing between the fl ight lines for an overlap of 80 0 8 is 39 5 m Eq 4 and the number of fl ight lines required for an area of width 100 m is 3 rounded up from Eq 5 The distance between the images is 24 m Eq 6 and the number of images per fl ight line length equal to 200 m is 11 rounded up from Eq 7 This brings the total number of acquired images for this area to 33 Eq 8 For the Sony A7R camera mounted on Trimble UX5 HP fi xed wing Fig 2 summarises how the GSD the fl ight time and the number of acquired images change with the fl ight height and the focal length lens for an area of 1 km 1 km Numbers in Fig 2a and c have been calculated using Eqs 1 8 while Fig 2b numbers were calculated usingtheTrimbleFlightCalculator calculator From Fig 2 it is evident that the focal length of the camera plays a signifi cant role on the fl ight height as it can result in the same or even better resolution at twice the fl ight height to the one achieved by a lens with a smaller focal length Fig 2a Choosing a higher fl ight height reduces the fl ight time Fig 2b and the post processing time since the number of acquired images covering the same area is signifi cantly smaller When the camera and focal length lens do not change the impact of the fl ight height on the image resolution the fl ight time and number of images is more prominent Fig 3 a and b show the eff ect of the fl ight height on the change of the GSD the number of images acquired and the fl ight time calculated using the Trimble Flight Calculator for a survey area of 1 km2and 0 01km2 respectively The results refer to an Olympus E PL7 camera with a 14 mm lens mounted on Trimble ZX5 a b Fig 1 a Schematic diagram not in scale showing the relationship between the fl ight height the focal length the sensor size and the ground distance b Schematic representation of the fl ight lines red solid arrow lines above the area of interest dashed line the forward and side overlap and the fl ight line spacing The blue rectangles represent the footprint of images on the ground Figure not in scale For interpretation of the references to colour in this fi gure legend the reader is referred to the web version of this article Fig 2 Change of a the GSD b the fl ight time and c the number of acquired images with the fl ight height and the focal length lens for a survey area of 1 km 1 km as obtained for the Sony A7R camera mounted on Trimble UX5 HP fi xed wing The dashed horizontal line in b denotes the threshold of 35 mins which is the maximum time per fl ight for the UX5 HP O Tziavou et al Engineering Geology 232 2018 12 21 14 hexacopter This allowed for a wider range of fl ight height values compared to those for the fi xed wing As can be seen from Fig 3a the resolution of the images GSD value could be better than 0 1 cm pixel however this would require 326 fl ights or at least 4 5 days for a maximum time of 20 min per fl ight for the ZX5 Even if the fl ight time was acceptable the total number of acquired images 1 550 000 would have made the post processing impossible A relatively manageable number of images i e less than 8000 for a commonly used computer would translate to a fl ight height of 75 m or less for a 1 km2 survey area But the required fl ight time is still quite high at 435 min more than 7 h resulting in the need of 22 fl ights Even at a fl ight height of 150 m the required number of fl ights to cover the 1 km2area would be 10 These numbers reduce by at least 2 orders of magnitude if the area to be surveyed is smaller as shown in Fig 3 b While the value of the GSD does not change for the same fl ight height between Fig 3 a and b the diff erences in the required fl ight times and number of acquired images are signifi cant An area of 0 1 km 0 1 km can be surveyed with a single fl ight 16 min at 25 m fl ight height resulting in a GSD value smaller than 1 cm pixel The choice in the range of values used for the fl ight height in Figs 2 and 3 was dictated by aviation regulations In the UK the maximum fl ight height above ground level AGL and the maximum horizontal distance from the person in charge are defi ned by the Civil Aviation Authority CAA as 122 m 400 ft and 500 m Visual Line Of Sight VLOS and 152 m 500 ft and 750 m Extended VLOS EVLOS re spectively In other European countries the fl ight height is 150 m in the US it is 400 ft 122 m There are also limitations due to the UAV technology itself For example for the fi xed wing and the hexacopter used in this study the minimum fl ight height is 75 m and 20 m for an autonomous fl ight respectively 3 Case study structural geological mapping of a sedimentary outcrop in South Ayrshire Scotland We tested the UAV technology on a project demanding high re solution the structural geological mapping of a fault zone outcrop in Scotland s south west coast The fi eld area is located on Whitehouse Shore a rocky beach a few miles south of the town of Girvan South Ayrshire The outcrop has well exposed sedimentary and structural geological features Fig 4 and is located within an Ordovician inlier in the Midland Valley Terrane McCay 2014 The area has been mapped in detail as part of previous projects Lawson and Weedon 1992 McCay 2014 and therefore constituted a favourable site that allowed