Surveying and Digital Restoration of Towering Architectural Heritage in Harsh Environments: a Case Study of the Millennium Ancient Watchtower in Tibet

sustainability

Article Surveying and Digital Restoration of Towering Architectural Heritage in Harsh Environments: a Case Study of the Millennium Ancient Watchtower in Tibet

Siliang Chen 1,2,* ID , Haozhong Yang 1, Shusheng Wang 1,* and Qingwu Hu 3 ID

1 Architecture Department, Xi’an University of Architecture & Technology, No. 13, Yanta Road, Xi’an 710055, China; [email protected] 2 Architecture Department, Chang’an University, No. 161, Chang’an Road, Xi’an 710061, China 3 School of Remote Sensing and Information Engineering, Wuhan University, No. 129, Luoyu Road, Wuhan 430079, China; [email protected] * Correspondence: [email protected] (S.C.); [email protected] (S.W.); Tel.: +86-029-8553-6075 (S.C.)

Received: 12 August 2018; Accepted: 31 August 2018; Published: 3 September 2018

Abstract: Aiming at the problem of difﬁcult data collection and modeling in high-rise ancient buildings with narrow interiors, a method is proposed in this paper for modeling and supporting digital restoration based on unmanned aerial vehicle oblique photogrammetry combined with three-dimensional (3D) laser scanning technology. The ancient watchtower complex in the Tibetan region of China is taken as an example. Firstly, the data is collected using an unmanned aerial vehicle and 3D laser scanner. Secondly, the two types of data are merged to generate a three-dimensional status model. Finally, by analyzing the status model and combining the similar remaining conditions, a virtual restoration scheme is proposed, and a 3D restoration model is established. The results show that virtual restoration based on 3D measurement technology can be used as a new method for the research and protection of towering ancient buildings, asrecorded by adopting targeted technology for digital documentation. It is necessary and effective to adopt a method combining unmanned aerial vehicle oblique photogrammetry and the ground 3D laser scanning technology in harsh environments. The digital model can promote the sustainable utilization of cultural heritage. It is necessary to analyze and make full use of the status model of such ancient buildings based on accurately measured data for the virtual restoration of the damaged ancient buildings. The status model of the ancient buildings can be used for display browsing and disaster recording. The restoration model can be dismantled and used to guide the repair work.

Keywords: digital architectural heritage; virtual restoration; harsh environment; unmanned aerial vehicle aerophotogrammetry; ground 3D laser scanning; watchtower; data fusion

1. Introduction As cultural heritage is the cultural wealth of all mankind, rich and diverse cultural heritage should be protected by adopting targeted technology. With the development of remote sensing technology, a new data capturing method can provide signiﬁcant convenience for the digital recording and display of cultural heritage. Most of the immovable cultural heritage is archaeological sites and ancient buildings. At present, unmanned aerial vehicle (UAV) aerophotogrammetry [1,2] is mainly used for relatively wide and ﬂat archaeological sites and industrial heritage with simple form, for which the quality of the results is higher. Satellite image photogrammetry [3,4] is also used for monitoring and analysis, for which the accuracy of the results is limited. Photogrammetry [5–13], three-dimensional (3D) laser scanning [14–20], or a combination of both methods [21–26] is generally used for ancient

Sustainability 2018, 10, 3138; doi:10.3390/su10093138 www.mdpi.com/journal/sustainability Sustainability 2018, 10, 3138 2 of 23 buildings, for which the surveying and mapping results can be used for post-display, analysis, and evaluation [27–31]. In the above cases, the unmanned aerial vehicle is undoubtedly the best way to achieve the most convenient and cost-effective objectives. However, since ancient buildings are more complex than archaeological sites and the surveying of internal and external geometric data is more susceptible to the surrounding environment, most of the above operations are only suitable for open space or single buildings with simpler structures. For towering ancient buildings with a narrow internal space, there is no perfect technical route on how to do well in surveying and mapping with regards to ensuring the safety of personnel, equipment, cultural relics, and other aspects. Also, it is necessary to investigate and evaluate whether the unmanned aerial vehicle can be effectively applied to such cultural heritage. Moreover, for ancient buildings or ancient architectural complexes with more complicated structures, there are few related studies currently existing on how to effectively integrate the internal and external data [32]. Using a convenient and reliable method to implement the virtual reconstruction of cultural heritage is necessary [33]. As is known to all, China’s Tibet is a world-famous, high-altitude region with very harsh climatic environments, frequent earthquakes, and other natural disasters. There are unprecedented problems for archaeological research and protection in this environment. In April–October 2017, the Chang’an University and the Wuhan University were invited by the Tibet Nyingchi City Bureau of Cultural Relics to make a comprehensive survey and map of an ancient watchtower complex in Tibet. However, the site environment was very dangerous, and the stones might have collapsed and fallen from the top of the towering and fragile watchtower at any time. However, the task was successfully accomplished by using a variety of surveying methods made possible by the unmanned aerial vehicle. On 18 November 2017, a 6.9 magnitude earthquake occurred in Nyingchi City, Tibet. The epicenter was located at 29.75◦ N and 95.02◦ E. Many important cultural heritage sites, including the watchtower, were severely damaged. Fortunately, the data collected previously, with the help of this ancient watchtower complex, may provide an important and unique reference for the research, protection, and restoration of the watchtower after the earthquake. Using the digital model of watchtowers for education and display has substantially promoted the local culture and tourism levels, and also realized sustainable experience and research without destroying the site. It is very signiﬁcant to carry out surveying, mapping, and research on watchtower-type ancient buildings under high-altitude conditions in China for the ﬁrst time. There are more than 1000 watchtowers of the same type in China, and the research methods and experience can be used as an important reference for similar work. In this paper, using the research object (i.e., the Xiuba watchtower complex), the basic principles and working methods for the accurate measurement, three-dimensional modeling, and virtual restoration of the towering architectural heritage in the harsh environments of the plateau are described in detail.

2. Background

2.1. Overview of the Limestone Hall of the Xiuba Ancient Watchtower Watchtowers are tall ancient structures made of natural stones and woods, and usually have stone masonry outside and wooden floors inside. This unique architecture is widely distributed in the plateau areas of Tibet and Sichuan, and is mostly built by Tibetans. According to historical records, this kind of building appeared more than 2000 years ago. There are many kinds of flat forms, but they are all tall towers. The function of the watchtower is not clear. It is generally believed that the Tibetan people built it next to the residential buildings for military defense, and it may also be used for mysterious religious sacrificial activities. In western China, there were at least 10,000 watchtowers in ancient times, many of which collapsed due to weathering and man-made destruction, but there are still more than 1000 left. The ancient watchtowers are usually constructed from processed sheets of rocks in the local mountains, each about 20–60 cm in length, 10–20 cm in width, and generally no more than 10 cm in thickness. After being Sustainability 2018, 10, 3138 3 of 23 processed into a more regular shape, the rocks are easily used to build the walls of the watchtowers. Most ancient watchtowers consist of one tower; occasionally, they consist of two or more towers in parallel. In some of the watchtowers, small fire holes are found, suggesting that users can use the shooting weapons to form cross-defense through the collaboration between the watchtowers. Well-preserved watchtowers are rare in China, and the Xiuba Ancient Watchtowers in Gongbujiangda county, Nyingchi city, Tibet, is a typical case. The Nyingchi Prefecture of Tibet has been an important hub for the exchange between the Central Plains dynasty of China and the Tibetan regime since ancient times. Through this area is where the magnificent Niyang River and the famous Tang–Tibet Ancient Road pass. Gongbo’gyamda County is an important traffic node from Nyingchi to Lhasa. Thousands of years of multi-ethnic exchanges and integration have left many historical and cultural relics in the region. The Xiuba ancient watchtower complex (Figure1) is located in the canyon of Bahe Town, Gongbo’gyamda County. The site covers an area of approximately 4500 m2. A stone wall is built along the outer boundary of the site area, within which there are seven watchtowers with very similar floor plan shapes and building structures. Two of them have destroyed and only have the bottom remaining, and the other five watchtowers still exist, but their tops have collapsed (Figure2). The residual height is approximately 20–27 m. The floor plan is in the shape of a quasi-mandala (i.e., “sub”-shaped). There are 12 corners formed inside and outside the wall, which is hollow without a top. A door is opened in the middle of the wall on one side of the first floor of the interior watchtower. The top of the door is horizontally embedded with several lumps of wood, which serve as the door lintel. The watchtower body is made of flat flaky stones. The appearance is polygonal prismatic in shape. The three to four rectangular lookout holes are vertically open on one side of some of the watchtowers. A circular wooden beam is placed at a certain height inside the watchtower (Figure3). Most of the wooden beams have decayed. It is speculated that an original wooden floor was installed in the interior watchtower. The initial construction age of the watchtower complex is unknown. It is impossible to determine the age of construction by using the stones on the watchtower. However, the age of the wooden beam on the upper part of the watchtower door is determined by using C14 technology (because it is difficult to replace the wood at this section, and it was very likely built at the same time as the watchtower). The determined results show that the lower limit of the construction time of the seven watchtowers is different. The earliest construction time is not later than 780 AD, while the latest construction time is not later than 1470 AD [34]. This spans roughly from the late Tubo Dynasty to the Ming Dynasty. Around the watchtower, there are many low stone walls that are approximately 1 m high and ca 0.6 m thick, and are made of flat stones and slabs conforming to the terrain. A number of architectural spaces are formed by the enclosure of low stone walls, which are usually shaped by connecting several small spaces to a large space. No traces of the pillars are found inside the enclosed space. The upper masonry surface of the low stone wall is also very flat. It indicates that this space is not a remnant of the house, but rather a fence wall matching the watchtower, which may be used as a temporary gathering place for the users of the watchtower. There are also many stone piles with a large lower part and a small upper part scattered around the wall called “Marnyi Stones”, which are the graves of Tibetans (Figure4). Moreover, there are a large number of peach trees with a long history around the watchtower. Some peach trees are at least 500 years old, which can also be used as evidence for the construction era of the watchtower. Sustainability 2018, 10, 3138 4 of 23 Sustainability 2018, 10, x FOR PEER REVIEW 4 of 23

Figure 1. General layout plan of the Xiuba watchtower complex. Figure 1. General layout plan of the Xiuba watchtower complex.

Figure 2.Fivewatchtowers with the main building remaining (looking from north to south). Figure 2.FivewatchtowersFivewatchtowers with with the the main main building building remaining remaining (looking (looking from from north to south). Sustainability 2018, 10, 3138 5 of 23 Sustainability 2018, 10, x FOR PEER REVIEW 5 of 23 Sustainability 2018, 10, x FOR PEER REVIEW 5 of 23

Figure 3. Interior of watchtower No. 4. Figure 3. Interior of watchtower No. 4. Figure 3. Interior of watchtower No. 4.

Figure 4. Structures and plants around watchtower No. 1. Figure 4. Structures and plants around watchtower No. 1. Figure 4. Structures and plants around watchtower No. 1. The Xiuba ancient watchtower complex is of important historical value and outstanding culturalThe value Xiuba in termsancient of watchtowerits construction complex age, bu isilding of important scale, and symbiotichistorical environmentalvalue and outstanding elements The Xiuba ancient watchtower complex is of important historical value and outstanding cultural ascultural a typical value representative in terms of its of construction ancient military age, fortressesbuilding scale, in Tibet. and Thissymbiotic can provide environmental the precious elements and value in terms of its construction age, building scale, and symbiotic environmental elements as a credibleas a typical material representative information of ancientfor understanding military fortresses and studying in Tibet. the This Tibetan can provideand Qiang the watchtowers.precious and Incredible 2009, thematerial Xiuba information watchtower for complex understanding was listed and as thestudying fifth batch the Tibetan of cultural and relicQiang protection watchtowers. units inIn the2009, Tibet the AutonomousXiuba watchtower Region. complex was listed as the fifth batch of cultural relic protection units in the Tibet Autonomous Region. Sustainability 2018, 10, 3138 6 of 23 typical representative of ancient military fortresses in Tibet. This can provide the precious and credible material information for understanding and studying the Tibetan and Qiang watchtowers. In 2009, the Xiuba watchtower complex was listed as the ﬁfth batch of cultural relic protection units in the Tibet Autonomous Region. The Xiuba Watchtower Cluster is an important object of worship for local Tibetan people. It is the symbol of their belief, like snow, mountains, and lake water. Its existence can signiﬁcantly enhance the cohesion of the residents in this area and promote the mutual understanding and solidarity among ethnic groups. At the same time, the Xiuba Watchtower Cluster, as the representative of the archaeological site in Nyingchi, is visited by a large number of non-local tourists every year, which greatly promotes local economic development. As a typical case of the watchtower building, the Xiuba Watchtower Cluster also provides necessary physical evidence for the study of the history and culture of Tibet. However, due to the recent earthquake damage, the status of the watchtower cluster is worrying, so we try to use the collected data for virtual restoration and digital display, in the hope of providing help for later repair work.

2.2. Research Objectives and Surveying Problems

2.2.1. Research Objectives The objectives of this study are to accurately survey and map the seven watchtowers in the Xiuba watchtower complex, within the accuracy of a centimeter; survey and map the overall environment of the watchtower complex, within the accuracy of a decimeter; establish a digital three-dimensional status model that can be used for display and research; establish a three-dimensional virtual restoration model that can be used to guide protection and restoration work; and effectively reveal the appearance and internal structure of the watchtower under good conditions.

2.2.2. Problems Encountered Due to the poor preservation of the Xiuba watchtower complex and the complexity of the natural environment, the difficulty in carrying out the related work increased significantly. There were three technical problems faced, which were as follows: Problems encountered when surveying and mapping: Due to low temperature and more turbulent airflow within the plateau gorge area, how can the good operation of the unmanned aerial vehicle and other equipment be ensured? Due to many trees growing around the watchtower, how can occlusion by the trees be avoided to survey and map the exterior of the watchtower? Due to the age of the watchtowers, the structure is very fragile, and the internal space of the watchtower is relatively narrow. If people try to use scaffolding at work, it may cause rocks to tilt or collapse during the removal and construction of the scaffolding, endangering personnel. What’s more, the lower part of the watchtower is large, the upper part is small, and its walls are sloped, making it impossible to keep the crew at an ideal distance from the watchtower when building scaffolding. How can data be collected? Due to the dim light inside the watchtower, how can it be ensured that the collected images meet the requirements? Problems encountered when establishing a model: As the collected data may be obtained by a variety of means, how can the multiple datasets be integrated to generate a status model of the watchtower? How can the virtual restoration model be established with the help of the status model? Problems encountered when studying the virtual restoration model: As the interior of the watchtower has been completely damaged, how can the original appearance of the watchtower be ascertained based on the existing remains? As the top of the watchtower has been collapsed and damaged, how can the height of the watchtower be determined? Sustainability 2018, 10, 3138 7 of 23

3. Research Strategies

3.1. The Technical Flow Chart In this study, the images of the surrounding environment of the watchtower complex were obtained by using an UAV for aerophotogrammetry. The data above and below the exterior of the watchtowerSustainability was 2018 obtained, 10, x FOR PEER by using REVIEW the unmanned aerial vehicle and the three-dimensional 7 (3D) of 23 laser scanner.laser The scanner. data The above data the above interior the interior of the watchtowerof the watchtower were were obtained obtained by usingby using the the unmanned unmanned aerial vehicleaerial via vehicle vertical via take-off vertical and take-off landing and photography landing photography under artiﬁcial under artificial auxiliary auxiliary lighting lighting conditions (by usingconditions high (by brightness using high lights brightness and reﬂectors).The lights and reflectors).The data of the data interior of the lower interior part lower of the part watchtower of the was obtainedwatchtower by was using obtained the 3D by laser using scanner. the 3D laser The coordinatesscanner. The ofcoordinates the indoor of andthe indoor outdoor and control outdoor points werecontrol surveyed points by were using surveyed the total by station;using the the total internal station; andthe internal external and point external clouds point of clouds the watchtower of the werewatchtower combined were by using combined the Geomagic by using the Studio Geomag softwareic Studio (Geomagic software Inc.,(Geomagic Research Inc., TriangleResearch Park, Raleigh,Triangle NC, USA)Park, viaRaleigh, the coordinates NC, USA) of via the the control coordinates points to of generate the control a triangulation points to networkgenerate model.a triangulation network model. A three-dimensional status model with the geometric and color A three-dimensional status model with the geometric and color information of the watchtower was information of the watchtower was generated by texture mapping. Then, the restoration scheme was generated by texture mapping. Then, the restoration scheme was developed by analyzing the status developed by analyzing the status model. The virtual restoration model was recreated with the help model.of the The status virtual model restoration in the 3DS model Max software was recreated (Autodesk with Inc., the San help Rafael, of the CA, status USA). model This is in all the shown 3DS Max softwarein Figure (Autodesk 5. Inc., San Rafael, CA, USA). This is all shown in Figure5.

Figure 5. Work flow chart for data collection, three-dimensional (3D) modeling, and virtual Figure 5. Work flow chart for data collection, three-dimensional (3D) modeling, and virtual restoration restoration of the watchtower complex. of the watchtower complex. 3.2. Specific Implementation Processes 3.2. Specific Implementation Processes 3.2.1. Collection of Data on the Overall Environment of the Watchtower Complex 3.2.1. Collection of Data on the Overall Environment of the Watchtower Complex The unmanned aerial vehicle was used to obtain the image data on the environment where the Theancient unmanned complex is aerial located. vehicle The wasaerophotographic used to obtain control the imagepoint was data where on the the environment ground height where the ancientdifference complex was small is located.and no trees The blocked aerophotographic vision along controlthe route point direction. was The where coordinates the ground of the height differencecontrol was point small wereand surveyed no trees by using blocked the total vision station. along Special the route attention direction. was paid The to coordinatesthe airflow in of the the canyon during flight. The unmanned aerial vehicle must fly at a force of three or below wind control point were surveyed by using the total station. Special attention was paid to the airflow in the speed; otherwise, it is easy to crash. Moreover, as the performance of the unmanned aerial vehicle canyon during flight. The unmanned aerial vehicle must fly at a force of three or below wind speed; battery is significantly reduced due to the low temperature in Tibet, the unmanned aerial vehicle otherwise,must fly it before is easy and to after crash. noon. Moreover, The battery as theneeds performance to be replaced of after the every unmanned sortie. A aerial large vehiclenumber batteryof is significantlybatteries should reduced be dueprepared to the for low replacement. temperature The in Tibet,collected the aerophotographic unmanned aerial images vehicle were must fly processed using the Smart3DCapture software (Acute3D Inc., Sophia Antipolis, France) so that the Digital Surface Model(DSM) point cloud data was generated after automatic system matching, aerial triangulation, and other steps. The Digital Terrain Model (DTM) point cloud data was obtained after Sustainability 2018, 10, 3138 8 of 23 before and after noon. The battery needs to be replaced after every sortie. A large number of batteries should be prepared for replacement. The collected aerophotographic images were processed using the Smart3DCapture software (Acute3D Inc., Sophia Antipolis, France) so that the Digital Surface Sustainability 2018, 10, x FOR PEER REVIEW 8 of 23 Model(DSM) point cloud data was generated after automatic system matching, aerial triangulation, andthe removal other steps. of interference. The Digital TerrainMore detailed Model (DTM)work im pointplementation cloud data methods was obtained can be after referred the removal to in the of interference.literature [8,29]. More detailed work implementation methods can be referred to in the literature [8,29]. The eight-rotor unmanned aerial vehicle was used for surveying and mapping the Xiuba watchtower (Figure6 6);); itsits modelmodel isis DJI DJI S1000 S1000 (DJI-Innovations (DJI-Innovations Inc., Inc., Shenzhen, Shenzhen, China). China). TheThe distancedistance between rotor shaftsshafts isis 104.5104.5 cm.cm. TheThe cameracamera hashas aa lens lens pixel pixel of of 24 24 million million (manufactured (manufactured by by Zoller Zoller + Fröhlich+ Fröhlich Co., Co., Ltd., Ltd., Wangen, Wangen, Germany). Germany). With With consideration consideration for for the the upper upper part part of theof the watchtower, watchtower, the flightthe flight height height was setwas to set 70 m.to 70 The m. image The image resolution resolution was set was to approximately set to approximately 5 cm geometric 5 cm geometric standard deviation.standard deviation. There were There a total were of 12 sorties.a total Theof flight12 sort distanceies. The per flight sortie distance was set toper approximately sortie was 600set m.to Theapproximately duration was 600 set m. to The 15 duration min. There was were set ato total 15 mi ofn. more There than were 2800 a total images. of more The than model 2800 of theimages. total stationThe model is RTS342R6 of the total (Suzhouyiguang station is RTS342R6 Instrument (Suzhouyiguang Co., Ltd., Suzhou, Instrument China). Co., The Ltd., effective Suzhou, surveyed China). distanceThe effective was 600surveyed m in the distance prism-free was mode. 600 m The in accuracythe prism-free of the surveymode. controlThe accuracy point coordinates of the survey was ± withincontrol point2 mm coordinates (Root-Mean-Square was within error). ±2 mm (Root-Mean-Square error).

Figure 6. Data collection of the overall environment of the watchtower complex by using a Figure 6. Data collection of the overall environment of the watchtower complex by using a multi-rotor multi-rotor unmanned aerial vehicle. unmanned aerial vehicle.

3.2.2. Layout and Survey of the Target Before surveying each of the watchtowers, eight to 10 pieces of checkerboard target papers were placed in the visiblevisible externalexternal andand internalinternal positionspositions ofof thethe watchtowerwatchtower (Figure(Figure7 7a).a). TheThe blackblack andand white intersection points of the checkerboard target papers were surveyed by using the the total total station. station. The relative coordinate system of the watchtower was established. A total of six pieces of target balls built in thethe FARO-typeFARO-type 3D3D laser laser scanner scanner (FARO (FARO Technologies, Technologies, Inc., Inc., Orlando, Orlando, FL, FL, USA)were USA)were laid laid in the in visiblethe visible area area outside outside the watchtowerthe watchtower (Figure (Figure7b). 7b). Sustainability 2018, 10, 3138 9 of 23 Sustainability 2018, 10, x FOR PEER REVIEW 9 of 23

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Figure 7. (a) Pasting of the target paper inside the watchtower. (b) Layout of the target ball outside Figure 7. (a) Pasting of the target paper inside the watchtower. (b) Layout of the target ball outside the watchtower. the watchtower.

3.2.3. Collection of Data from the Exterior of a Single Watchtower Figure 7. (a) Pasting of the target paper inside the watchtower. (b) Layout of the target ball outside All ofof thethethe personnel personnelwatchtower. wore wore safety safety helmets helmets before befo collectingre collecting the data.the data. Although Although trees weretrees a were shelter a shelter from the exterior of the watchtower, the trees were lower than the height of the watchtower. from the exterior3.2.3. Collection of the of watchtower, Data from the the Exterior trees of were a Single lower Watchtower than the height of the watchtower. Therefore, theTherefore, part of the watchtowerpart of the watchtower above the trees above was the surveyed trees was by surveyed means of by photogrammetry means of photogrammetry by using the by using the manuallyAll of the personneloperated wore and safety controlled helmets multi-rotor before collecting unmanned the data. aerialAlthough vehicle. trees were The a part of the manuallyshelter operated from andthe exterior controlled of the multi-rotorwatchtower, the unmanned trees were lower aerial than vehicle. the height The of the part watchtower. of the watchtower belowwatchtower theTherefore, trees below was thethe scanned part trees of thewas by watchtower using scanned a 3D above by laser using the scanner. trees a 3D was laser Moreover, surveyed scanner. by the means Moreover, position of photogrammetry of the the position target ball of wasthe alsotarget scanned ballby was using as requiredalso the manuallyscanned (Figure operated as 8required). In and order controlled (Figure to prevent multi-rotor 8). In the order unmanned angle to ofprevent elevation aerial vehicle. the from angle The becoming part of elevationof the too largefrom watchtower below the trees was scanned by using a 3D laser scanner. Moreover, the position of the whenbecoming scanning too large the watchtower when scanning and ensure the watchtower the safety of personneland ensure and the equipment, safety of anpersonnel approximate and equipment,target an ballapproximate was also scanned distance as required from (Figurethe watc 8). Inhtower order to was prevent kept the during angle of operation elevation from by means of distance frombecoming the watchtower too large when was scanning kept during the watchtower operation and by meansensure ofthe the safety two of aforementioned personnel and methods. Whenthe two taking aforementionedequipment, photos anwith approximate methods. the unmanned distance When from aerialtaking the watc vehicle,phhtowerotos specialwithwas kept the attention during unmanned operation was aerial paid by means tovehicle, avoiding of special the watchtowerattention wastheand two paid trees.aforementioned to Photosavoiding ofmethods. the prelaidwatchtower When targettaking anph papersdotos trees. with and Photosthe target unmanned of balls the aerial were prelaid vehicle, also target recorded special papers for laterand attention was paid to avoiding the watchtower and trees. Photos of the prelaid target papers and datatarget fusion. balls were also recorded for later data fusion. target balls were also recorded for later data fusion.

Figure 8. 3D laser scanning outside the watchtower. Figure 8. 3D laser scanning outside the watchtower.

Due to the relatively narrow interior of the watchtower, the strong swirling airﬂow generated by the larger UAV interferedFigure with 8. the3D ﬂightlaser scanning of the aircraft. outside the Therefore, watchtower. the small quadrotor UAV was Sustainability 2018, 10, x FOR PEER REVIEW 10 of 23

Due to the relatively narrow interior of the watchtower, the strong swirling airflow generated Sustainabilityby the larger2018 UAV, 10, 3138 interfered with the flight of the aircraft. Therefore, the small quadrotor UAV10 was of 23 used for surveying and mapping inside the watchtower. Its model is DJI Phantom3 (DJI-Innovations Inc., Shenzhen, China). The camera was manufactured by the aircraft manufacturerused for surveying themselves, and mapping and had inside a lens the ofwatchtower. 16 million pixels. Its model To prevent is DJI Phantom3 the aircraft (DJI-Innovations propeller from touchingInc., Shenzhen, the watchtower, China). The a propeller camera protection was manufactured ring must by be the installed aircraft onto manufacturer the aircraft.themselves, The model ofand the had 3D a lenslaser ofscanner 16 million was pixels.FAROFocus3D To prevent X330. the aircraftIn order propeller to ensure from the touching safety of the personnel watchtower, and equipment,a propeller protectionduring UAV ring control must beand installed 3D laser onto scanni the aircraft.ng, the operators The model should of the be 3D in laser the scannercentral area was ofFAROFocus3D the interior X330.of the In watchtower. order to ensure The theduration safety ofof personnel3D laser andscanning equipment, on each during watchtower UAV control was approximatelyand 3D laser scanning, 30 min. theThe operators duration should of the be unmanned in the central aerial area vehicle of the interioraerophotogrammetry of the watchtower. was Theapproximately duration of 30 3D min. laser scanningThe number on each of captured watchtower images was approximatelywas approximately 30 min. 300–350. The duration The image of the resolutionunmanned was aerial approximately vehicle aerophotogrammetry 2 cm. was approximately 30 min. The number of captured images was approximately 300–350. The image resolution was approximately 2 cm. 3.2.4. Collection of Data from the Interior of a Single Watchtower 3.2.4. Collection of Data from the Interior of a Single Watchtower The lower part of the interior of the watchtower was scanned using a 3D laser scanner (Figure 9a). TheBefore lower scanning, part of thethe interior three ofinternal the watchtower target balls was were scanned moved using into a 3D the laser watchtower. scanner (Figure When9a). surveying,Before scanning, the positions the three of internal the three target internal balls wereor external moved target into the balls watchtower. were scanned. When Due surveying, to the excessivethe positions angle of theof threeelevation, internal the orupper external part target of the balls interior were scanned.of the watchtower Due to the excessivewas surveyed angle by of meanselevation, of thephotogrammetry upper part of the with interior a camera of the carried watchtower in the was unmanned surveyed byaerial means vehicle of photogrammetry via manually withoperated a camera vertical carried take-off in theand unmanned landing in aerialthe watc vehiclehtower via (Figure manually 9b). operated Moreover, vertical the target take-off papers and placedlanding inside in the the watchtower watchtower (Figure were9b). also Moreover, required the to be target photographed. papers placed In insideorder theto ensure watchtower the smooth were integrationalso required of to internal be photographed. and external In orderdata, toattent ensureion the was smooth focused integration on collecting of internal the anddata externalon the door-openingdata, attention parts was focusedat the junction on collecting between the the data inte onrior the and door-opening exterior of partsthe watchtower at the junction during between data collection.the interior and exterior of the watchtower during data collection. The equipmentequipment used used for for the the collection collection of data of fromdata thefrom interior the interior of a single of watchtowera single watchtower is exactly the is exactlysame as thethe equipmentsame as the used equipment for the collection used offor data the from collection the exterior of data of the from watchtower. the exterior The duration of the watchtower.of 3D laser scanning The duration on each of watchtower3D laser scanning was approximately on each watchtower 20 min. was The approximately scanning speed 20 of min. 3D laser The × scanning speed was 976,000 of 3D laser points/s, scanning and was its optical976,000 resolutionpoints/s, and was its 600 optical1200 resolution dpi. The was duration 600 × 1200 of dpi.the UAVThe duration aerophotogrammetry of the UAV wasaerophotogrammetr approximatelyy 30 was min. approximately The number of30 capturedmin. The imagesnumber was of capturedapproximately images 300–350. was approximately 300–350.

Figure 9. (a) 3D laser scanning inside the watchtower, (b) manipulation of the unmanned aerial Figure 9. (a) 3D laser scanning inside the watchtower, (b) manipulation of the unmanned aerial vehicle vehicle inside the watchtower. inside the watchtower.

3.2.5. Data Processing and Fusion and the Generation of Status Model All of thethe datadata onon the the overall overall environment environment of of the the watchtower watchtower complex complex can can be collectedbe collected using using the theunmanned unmanned aerial aerial vehicle, vehicle, which which can be can automatically be automa processedtically processed by Agisoft by PhotoScanAgisoft PhotoScan v1.2.6 software v1.2.6 (Agisoftsoftware LLC., (Agisoft St. Petersburg,LLC., St. Petersburg, Russia) to generateRussia) to the generate point cloud the data.point Bycloud generating data. By the generating triangulation the triangulationnetwork and texturenetwork mapping, and texture a three-dimensional mapping, a thre digitale-dimensional model based digital on themodel real coordinatebased on the system real and reﬂecting the overall environment of the watchtower can be generated. This ﬁle can be opened and edited using the 3D max software. Sustainability 2018, 10, x FOR PEER REVIEW 11 of 23

Sustainability 2018, 10, 3138 11 of 23 coordinate system and reflecting the overall environment of the watchtower can be generated. This file can be opened and edited using the 3D max software. The data onon aa singlesingle watchtowerwatchtower waswas collectedcollected using the unmannedunmanned aerial vehicle and the 3D laserlaser scanner.scanner. Fusion of the two kindskinds of datadata waswas oneone difficultydifficulty in thisthis task.task. Firstly, thethe imageimage datadata acquired by thethe unmannedunmanned aerialaerial vehiclevehicle andand thethe positionposition informationinformation obtainedobtained by thethe measuringmeasuring targettarget pointpoint were were imported imported into into the the Agisoft Agisoft PhotoScan PhotoScan software software (Agisoft (Agisoft LLC., St.LLC., Petersburg, St. Petersburg, Russia). AfterRussia). pre-processing After pre-processi by theng software, by the software, the point the cloud point data cloud (in data the .xyz(in the format) .xyz format) with the with control the pointcontrol information point information was obtained. was obtained. After the After data the acquired data acquired by the 3D by laser the 3D scanner laser andscanner the positionand the informationposition information obtained obtained by the measuring by the measuring target point targ andet point the target and ballthe weretarget processed ball were byprocessed Faro Scene by v5.1Faro softwareScene v5.1 (FARO software Technologies, (FARO Technologies, Inc., Orlando, In FL,c., USA),Orlando, the engineeringFL, USA), the file engineering (in the .fws format)file (in couldthe .fws be format) obtained, could and be the obtained, point cloud and data the (inpoint the cloud .xyz format) data (in with the the.xyz control format) point with information the control couldpoint beinformation exported. could After pre-processing,be exported. After the twopre-processing, sets of point the cloud two data sets (in of the point .xyz cloud format) data could (in bethe acquired, .xyz format) which could share be aacquired, set of control which point share information. a set of control After point both information. were successively After both imported were intosuccessively the GeomagicStudio imported into software the GeomagicStudio (Geomagic Inc., software Research (Geomagic Triangle Park,Inc., Research Raleigh, NC,Triangle USA), Park, the twoRaleigh, sets NC, of point USA), cloud the datatwo sets were of automaticallypoint cloud da combinedta were automatically into a complete combined 3D point into cloud a complete model (Figure3D point 10 clouda,b). Inmodel translucent (Figure pattern 10a,b). (FigureIn transluc 10b),ent we pattern can clearly (Figure see 10b), the outerwe can and clearly internal see wall the surfacesouter and the internal watchtowers wall surfaces (Figure the 10c), watchtowers and the wooden (Figure beams 10c), (Figureand the 10 woodenc) inserted beams into (Figure the interior 10c) wall.inserted In thisinto way,the interior not only wall. can In the this relationship way, not only between can the the relationship wooden beams between inside the the wooden watchtower beams andinside the the external watchtower shape insideand the the external watchtower shape be in analyzedside the through watchtower the comparison be analyzed of internalthrough andthe externalcomparison images, of internal the number and ofexternal floors insideimages, the the watchtower number of can floors be vaguely inside the imagined watchtower also from can thebe exteriorvaguely ofimagined the watchtower also from (similar the exterior to X-ray of radiation), the watchtower which will(similar facilitate to X-ray the subsequentradiation), restorationwhich will research.facilitate the The subsequent texture map restor generatedation research. by the unmanned The texture aerial map vehicle generated data processedby the unmanned by the Agisoft aerial Photoscanvehicle data software processed was by directly the Agisoft mapped Photoscan onto the software 3D model was in thedirectly GeomagicStudio mapped onto software. the 3D model Thus, thein the 3D GeomagicStudio model (in the .obj software. format) and Thus, the the texture 3D model map (in (in the the .mtl .obj format) format) could and bethe directly texture exported map (in andthe .mtl opened format) and could edited be by directly using the exported 3D max and software. opened and edited by using the 3D max software. The accuracy of aa single-watchtowersingle-watchtower model waswas higher, while the accuracy of thethe globalglobal environmental model of of the the watchtower watchtower complex complex wa wass slightly slightly lower. lower. In the In the3D 3Dmax max software, software, the theprecise precise model model of each of eachwatchtower watchtower could could be embedded be embedded into the into global the global environmental environmental model modelof the ofwatchtower the watchtower complex complex according according to its actual to its actualdistribution distribution position position so that so the that 3D the status 3D statusmodel model could couldbe obtained. be obtained.

Figure 10. Opaque pattern (a), translucent pattern (b), and partial enlarged drawing (c) of the 3D Figure 10. Opaque pattern (a), translucent pattern (b), and partial enlarged drawing (c) of the 3D status status model of watchtower No. 4 in the .obj format. model of watchtower No. 4 in the .obj format.

Sustainability 2018, 10, 3138 12 of 23

3.2.6. Analysis of the 3D Status Model and Proposal of the Virtual Restoration Scheme

BySustainability analyzing 2018 the, 10, 3Dx FOR status PEER REVIEW model, combined with the relevant literature and other 12 ofexamples, 23 the virtual restoration of the watchtower was studied. Similar methods have been used in the case of the Limestone3.2.6. Analysis Hall inof Shaanxithe 3D Status province, Model and China Proposal [30]. of The the original Virtual Restoration appearance Scheme of the damaged parts of the watchtowerBy analyzing (including the the3D status internal model, wooden combined structure, with the top relevant appearance literature or and style, other and examples, other elements) was emphasizedthe virtual andrestoration analyzed. of the watchtower was studied. Similar methods have been used in the case of the Limestone Hall in Shaanxi province, China [30]. The original appearance of the damaged Internalparts Wood of Constructionthe watchtower Practices (including and the Floorinternal Distribution wooden structure, Rules top appearance or style, and other elements) was emphasized and analyzed. There is still a relatively complete wooden structure on the first floor of Xiuba watchtower No. 7, whichInternal can provide Wood Construction an important Practi basisces and for Floor restoration. Distribution It canRules be seen from Figure 11 that the floor slab of the watchtowerThere is still a isrelatively composed complete of a mainwooden beam, structure secondary on the first beam, floor andof Xiuba plank. watchtower The main No. beam is penetrated7, which into can the provide interior an in important parallel basis and for inserted restoration. into It the can interior be seen offrom the Figure stone 11 wall. that the The floor secondary beam isslab divided of the intowatchtower three sections, is composed which of a main are stacked beam, secondary in the vertical beam, and direction plank. The of the main main beam beam. is The penetrated into the interior in parallel and inserted into the interior of the stone wall. The secondary main and secondary beams are made from logs. The plank is laid on the secondary beam. Moreover, beam is divided into three sections, which are stacked in the vertical direction of the main beam. there isThe a square main and stair secondary hole on onebeams side are of made each from floor logs. of the The watchtower, plank is laid andon the the secondary wooden stairbeam. frame is set on theMoreover, main beam there is or a onsquare the stair wall hole for on access. one side Thus, of each a complete floor of the wooden watchtower, structure and the building wooden system inside thestair watchtower frame is set on is the formed. main beam In combination or on the wall with for access. the data Thus, measured a complete by wooden the 3D structure laser scanner, accordingbuilding to the system restoration inside the scheme, watchtower the is diameter formed. Inof combination the main with beam theis data set measured to an average by the 3D of 27 cm. laser scanner, according to the restoration scheme, the diameter of the main beam is set to an The diameter of the secondary beam is set to 12 cm. The thickness of the plank is set to 3 cm. The width average of 27 cm. The diameter of the secondary beam is set to 12 cm. The thickness of the plank is of the watchtowerset to 3 cm. The is setwidth to of 96 the cm. watchtower is set to 96 cm.

Figure 11. Residual wooden beam, plank, and wooden staircase inside watchtower No. 7. Figure 11. Residual wooden beam, plank, and wooden staircase inside watchtower No. 7. Since the Xiuba watchtower has not been repaired for many years, there are different degrees Sinceof damage the Xiuba on the watchtower top of the watchtower. has not been During repaired restoration, for many the years,total height there and are the different number degrees of of internal floors of the building should can speculated according to the status of the surviving damage on the top of the watchtower. During restoration, the total height and the number of internal buildings. It is worth noting that the top of watchtower No. 4 has barely collapsed, and the internal floors ofwooden the building beams are should relatively can completely speculated preserved, according which to can the accurately status of reflect the surviving the information buildings. on It is worth notingall of the that floors the of top the ofbuilding. watchtower The distribution No. 4 has of barelythe wooden collapsed, beams inside and thethe watchtower internal wooden can be beams are relativelyclearly completelyseen through preserved, the vertical whichsection canof the accurately 3D status reflectmodel theof watchtower information No. on 4 (Figure all of the 12). floors of the building. The distribution of the wooden beams inside the watchtower can be clearly seen through the vertical section of the 3D status model of watchtower No. 4 (Figure 12). From this, the position of the floor and the number of floors can be determined. By measuring the height of each floor, it is found Sustainability 2018,, 10,, 3138x FOR PEER REVIEW 1313 of of 23

From this, the position of the floor and the number of floors can be determined. By measuring the thatheight there of each is some floor, randomness it is found in that the there distances, is some but randomness the overall trend in the is distances, decreasing. but The the average overall height trend ofis decreasing. floors decreased The average by approximately height of floors 45 mm. decreased The total by height approxim of theately watchtower 45 mm. The can total be calculated height of accordingthe watchtower to this can decreasing be calculated law. according The highest to levelthis decreasing of the restored law. watchtowerThe highest level 4 is 2.12 of the m, restored and we believewatchtower that the4 is other 2.12 watchtowersm, and we believe should that follow the aother similar watchtowers pattern, with should the highest follow levela similar approaching pattern, 2.12with m,the regardless highest level of the approaching number of 2.12 floors. m, Inregardle addition,ss of judging the number from theof floors. later watchtowers In addition, injudging other partsfrom ofthe Tibet, later itwatchtowers is rare for the in interiorother parts to have of Tibet, more it than is rare 14 floors.for the Basedinterior on to the have above more principles, than 14 forfloors. other Based watchtowers, on the above the number principles, of floors for other and the watchtowers, total height the of each number watchtower of floors (Table and 1the) can total be calculatedheight of accordingeach watchtower to the slope (Table of the1) wall,can be the calculated position of according the wooden to beam, the slope and theof appearancethe wall, the of theposition watchtowers. of the wooden beam, and the appearance of the watchtowers.

Figure 12. Distribution of floors and residual wooden beams inside watchtower No. 4. Figure 12. Distribution of ﬂoors and residual wooden beams inside watchtower No. 4.

Table 1. The survived number of floors and total height as well as virtual recovered number of Table 1. The survived number of ﬂoors and total height as well as virtual recovered number of ﬂoors floors and total height of seven watchtowers. and total height of seven watchtowers. Watchtower Number of Number of Virtual Remaining Virtual Recovered NumberWatchtower SurvivedNumber Floors of NumberRecovered of VirtualFloors RemainingHeight Height (m) VirtualTotal Recovered Height (m) Number1 Survived12 Floors Recovered 12 Floors (m) 27.41 Total Height28.40 (m) 2 110 12 12 12 27.41 21.66 28.4029.01 3 212 10 12 13 21.66 25.11 29.0128.97 4 312 12 13 12 25.11 27.24 28.9728.81 5 412 12 12 12 27.24 29.50 28.8130.15 6 51 12 12 12 29.50 3.27 30.1529.01 7 62 1 12 12 3.27 7.76 29.0132.48 7 2 12 7.76 32.48

Sustainability 2018, 10, 3138 14 of 23

Style ofSustainability the Roof 2018 of a, 10 Watchtower, x FOR PEER REVIEW 14 of 23 TheStyle planar of the styleRoof of of a Watchtower the Tibetan watchtower consists of shapes such as tetragons, octagons, dodecagons,The and planar quasi-mandalas. style of the Tibetan The style watchtower of the roof cons ofists the of watchtowershapes such isas closelytetragons, related octagons, to the ﬂoor planar styledodecagons, of the watchtower.and quasi-mandalas. The Shade The style watchtower of the roof (Figureof the watchtower 13), which is wasclosely built related 800 to years the ago in Kangdingfloor County, planar style Ganzi of the Tibetan watchtower. Autonomous The Shade Prefecture, watchtower Sichuan,(Figure 13), has which a similar was built ﬂoor 800 years planar style as the Xiubaago in watchtower.Kangding County, The Ganzi roof Tibetan of the ShadeAutonomous watchtower Prefecture, is relativelySichuan, has well a similar preserved, floor planar which can style as the Xiuba watchtower. The roof of the Shade watchtower is relatively well preserved, which be usedcan as abe key used basis as a forkey thebasis restoration for the restoration of the of roof theof roof the of Xiuba the Xiuba watchtower. watchtower. The The top top ofof thethe virtual recoveredvirtual Xiuba recovered watchtower Xiuba watchtower penetrates penetrates the interior the interior with a with wooden a wooden beam. beam. The The wooden wooden ceiling is laying onceiling the is indoor laying woodenon the indoor beam. wooden The beam. wooden The beamwooden protrudes beam protrudes out of out the of watchtower the watchtower by 25 cm. The woodenby 25 cm. beam The iswooden covered beam and is covered enclosed and with enclos woodened with wooden boards boards and stones, and stones, and and makes makes a slopea by rammingslope the by earth ramming for drainage. the earth for In orderdrainage. to expressIn order respectto express for respect the gods, for the the gods, wall the at thewall bottom at the of the bottom of the wooden beam on the top of the watchtower is built with red and white aggregated wooden beam on the top of the watchtower is built with red and white aggregated rocks. rocks.

Figure 13. Polygonal watchtower located in Shade Township, Kangding County (photographed by Figure 13.FrederiquePolygonal Darragon). watchtower located in Shade Township, Kangding County (photographed by Frederique Darragon). Sustainability 2018,, 10,, 3138x FOR PEER REVIEW 1515 of of 23

3.2.7. Re-Establishment of the Restoration Model According to the Virtual Restoration Scheme 3.2.7. Re-Establishment of the Restoration Model According to the Virtual Restoration Scheme In the 3ds Max software (Autodesk Inc., San Rafael, CA, USA), with the help of the 3D status model,In the 3dswatchtowers Max software were (Autodeskremodeled Inc.,one Sanby one. Rafael, The CA, wall USA), was repaired with the completely help of the according 3D status tomodel, the set the height watchtowers of the werewatchtower. remodeled Then, one the by one.wooden The wallbeam, was floor repaired slab, floor completely body, accordingceiling, and to otherthe set components height of the were watchtower. added Then,inside the the wooden watchtower beam, flooraccording slab, floorto the body, set ceiling, dimensions and other of components (Figure were added 14), so inside that the the three-dimens watchtowerional according restoration to the model set dimensions of the single of componentswatchtower (Figurecould be 14 obtained.), so that the three-dimensional restoration model of the single watchtower could be obtained. Moreover, the enclosure wall, Marnyi Stones, and other structures around the watchtower were also reconstructed in the model. After After modeling modeling the the re relatedlated structures, structures, the the figures, figures, trees, trees, flying flying birds, birds, and otherother entouragesentourages were were added added to to the the three-dimensional three-dimensional model model with with the properthe proper dimensions, dimensions, in order in orderto provide to provide an intuitive an intuitive scale referencescale reference for the for watchtower the watchtower and increase and increase the fidelity the fidelity of the of scene. the scene.

Figure 14. Scenario for fabricating the watchtower components in the 3ds Max software. Figure 14. Scenario for fabricating the watchtower components in the 3ds Max software.

4. Results and Related Discussion

4.1. Current Status Model and Virtual Recovery Status Model The 3D 3D status status model model and and the the 3D 3D restoration restoration model model of ofthe the Xiuba Xiuba watchtower watchtower complex complex (both (both are generatedare generated after after processing) processing) (Figure (Figure 15) 15 can) can be berendered rendered by by the the 3Ds 3Ds Max Max software software to to output a high-resolution rendering image. The environmental elements, such as snow, mountains, and rivers where the watchtower complex is located, can be added to the rendering image of the restoration model afterafter preciseprecise processing processing using using the the Photoshop Photoshop software software (Adobe (Adobe Systems Systems Incorporated, Incorporated, San Jose,San Jose,CA, USA)CA, USA) in order in order to obtain to obtain the more the realisticmore realistic renderings renderings (Figures (figures 16–18). 16–18). The 3D The restoration 3D restoration model modelcan also can allow also for allow a vertical for a vertical cut display cut display (Figure (Fig 19)ure and 19) structural and structural split display split display (Figure (Figure20). 20). Sustainability 2018, 10, 3138 16 of 23 Sustainability 2018, 10, x FOR PEER REVIEW 16 of 23 Sustainability 2018, 10, x FOR PEER REVIEW 16 of 23

Figure 15. Rendering generated by the 3D status model of the Xiuba watchtower complex. Figure 15. Rendering generated by the 3D status model of the Xiuba watchtower complex. Figure 15. Rendering generated by the 3D status model of the Xiuba watchtower complex.

Figure 16. Aerial view rendering generated by the 3D restoration model of the Xiuba watchtower Figure 16. Aerial view rendering generated by the 3D restoration model of the Xiuba watchtower complex.Figure 16. Aerial view rendering generated by the 3D restoration model of the Xiuba complex. watchtower complex. Sustainability 2018, 10, 3138 17 of 23 Sustainability 2018, 10, x FOR PEER REVIEW 17 of 23 Sustainability 2018, 10, x FOR PEER REVIEW 17 of 23

Figure 17. Facade rendering of the Xiuba watchtower complex. Figure 17. Facade rendering of the Xiuba watchtower complex. Figure 17. Facade rendering of the Xiuba watchtower complex.

Figure 18. Local scene rendering of the Xiuba watchtower complex. Figure 18. Local scene rendering of the Xiuba watchtower complex. Figure 18. Local scene rendering of the Xiuba watchtower complex. Sustainability 2018, 10, 3138 18 of 23 Sustainability 2018, 10, x FOR PEER REVIEW 18 of 23

Figure 19. Single building section of the Xiuba watchtower complex. Figure 19. Single building section of the Xiuba watchtower complex. Sustainability 2018, 10, 3138 19 of 23

Sustainability 2018, 10, x FOR PEER REVIEW 19 of 23

Figure 20. Single building structure split diagram of the Xiuba watchtower complex. Figure 20. Single building structure split diagram of the Xiuba watchtower complex.

4.2. Discussion of the Results

4.2.1. Accuracy of the Status Model After comparing the single 3D status model of the Xiuba watchtower with the real object for inspection, it was found that the accuracy of model was greater than 2 cm. The gap between the stones Sustainability 2018, 10, 3138 20 of 23 on the watchtower was clearly presented. The model map could more realistically reflect the current status of the watchtower. The colors of the rocks, ground soil, and trees were very close to that of the real environment. The accuracy of the global environmental model of the watchtower complex was greater than 5.5 cm. However, because it was difficult for the unmanned aerial vehicle to generate a model for the scattered tree leaves, a spherical image integrated by the tree leaves is presented in the final results, which will not significantly impact the display and preservation of cultural relics. Moreover, since the entire site was surveyed using the total station, the elevation error was not greater than 6 cm.

4.2.2. Use of the Status Model and the Restoration Model The 3D status model can be used not only for the display and virtual tour of the current status of the watchtower, it can also be used for disaster recording and analysis and providing assistance in making any protection decisions by the cultural relics protection department. For example, after an earthquake occurred in Nyingchi Prefecture in November 2017, the top of the seven watchtowers collapsed partially. We found by comparing the status model (made from the data collected in October 2017) with the current image of the watchtower that the top of watchtower No. 5 had collapsed most severely, and that there was an urgent need for emergency protection. At present, relevant countermeasures have been taken by the Nyingchi City Bureau of Cultural Relics. The 3D restoration model is a new model that is regularized and idealized on the basis of the status model, reflecting the sound condition of the watchtower. By comparing the restoration model with the status model, the settlement and offset of a certain watchtower can be identified and measured in terms of its appearance. This provides the basis and guidance for possible repair work in the future. The restoration model cannot only be disassembled according to requirements, it can also be used to export the plan, elevation, and section of the watchtower in the CAD (Computer-Aided Design) data. This provides an important reference for subsequent scientific research, repair construction, and other operations. Moreover, the restoration model can be used to create animations related to the watchtower, which can provide assistance in cultural propaganda and knowledge popularization.

4.2.3. Necessity and Limitations of the Unmanned Aerial Vehicle Applied in Harsh Environments In this work, the unmanned aerial vehicle has played an important and irreplaceable role. The watchtower is too high to directly measure the height of the watchtower or transport heavier equipment to the top of the watchtower. Moreover, the rock on the top of the watchtower is extremely fragile. If scaffolds are erected outside or inside the watchtower, it is likely to cause damage to or even collapse of the watchtower. There are often stones falling from the inner edge of the watchtower. Workers can only stay in the inner central position; therefore, there should not be too many people working in the watchtower. The application of an unmanned aerial vehicle does not only effectively overcome the above difficulties, it also ensures the safety of the workers to the greatest possible extent. Of course, there are also certain limitations to unmanned aerial vehicles. For example, if there are more trees beside the watchtower, the flight will be affected. It is difficult to collect and produce data on the tree leaves. The lower part of the watchtower must be recorded with a 3D laser scanner and other equipment for operation. However, although it also played a significant role, the 3D laser scanner can be replaced by ground photogrammetry, which will have a certain impact on the accuracy of the results.

5. Conclusions The following conclusions were obtained.

1. Virtual restoration based on 3D scanning technology can be used as a new method for the research and protection of towering ancient buildings; it can be used for reference to survey and research more than 1000 watchtowers in western China. Sustainability 2018, 10, 3138 21 of 23

2. The digital model is of great help to the sustainable use of cultural heritage, the popularization of education, and scientific research. Meanwhile, it can support the adoption of different virtual restoration schemes for the site and evaluation the site without damaging it. 3. It is necessary and effective to adopt a method combining unmanned aerial vehicle oblique photogrammetry and ground 3D laser scanning technology in harsh environments, especially in a dangerous working environment. In the operation and control process of unmanned aerial vehicle control, special attention should be paid to the influence of airflow and temperature on the aircraft. 4. The virtual restoration of ancient buildings needs to be judged by analyzing the remains of the components in the status model and combining the conditions of the same type of ancient buildings. Accurate restoration cannot be guaranteed without accurate measurement data. 5. The status model generated by the surveying and mapping data can be used for display browsing and disaster recording, while the restoration model made by virtual repair based on the status model can be disassembled and used to guide repair work. It has been widely used in this manner.

Author Contributions: S.C. conceived the study and wrote the paper, did the field data collection and data processing, and implemented 3D modeling. S.W. implemented the 3D rendering and provided some methods. H.Y. assisted S.C. in analyzing the results and provided some valuable advice. Q.H. also conceived the study. Funding: The authors would like to thank “The Compass Plan” supported by the State Administration of Cultural Heritage. This research was also supported by the National Natural Science Foundation of China (Grant No. 41271452), Key Technologies R&D Program of China (Grant No. 2015BAK03B04), major theoretical and practical research projects in the social sciences of Shaanxi Province (Grant No. 2017C024), and The Fundamental Research Funds for the Central Universities (Chang’an University) (Grant No. 310841161002). Acknowledgments: Thanks to the Tibet Nyingchi City Bureau of Cultural Relics for their help. Thanks for assistance of Huai Cui. Conflicts of Interest: The authors declare no conflict of interest.

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