The Usage of Terrestrial Laser Scanner for 3d Laser Scanning of Complex-shaped Structure

Nowadays, assessment of existing structures for structural stability becomes important task for safety of people. Different methods and instruments have been used for assessment of structures for different purposes. In this study, usage of terrestrial laser scanner for 3D laser scanning of complex-shaped structure such as Standing Buddha Statue on hill top is discussed. Computational Fluid Dynamic (CFD), commonly used by wind engineering researchers for various wind engineering research purposes, requires stereolithography (STL) file format to analyze the data to stimulate wind flow or to calculate wind loading on structures. Therefore, generation of STL file from 3D scanned data is also conducted during this study. Finally, steps, tips and recommendations for a successful 3D scan project plan for outdoor- type large complex-structure is included.Keywords: Terrestrial Laser Scanner; Complex-Shaped Structure; Stereolithography; Wind Loads; Buddha Statue

Introduction

People like to construct new and distinct structures even though they don’t have much interest in proper maintenance of these structures. However, in real life, even the smallest structure requires proper periodical and annual structural maintenance for safety as time passed by. Factor of safety calculation and numerical analysis like finite element analysis are some of the popular methods used in the structural assessment. Among them, structural configuration is believed to be included in one of the most important topics in the calculation process for the safety of the structure such as structural deformation or structure defects. Various methods and equipment can be used to determine the configuration of the structures.

In this research, medium range ground based terrestrial laser scanner, Faro Focus 3D X330 is used to record the exterior figure of the Laykyun Sekkya Standing Buddha Statue. Laykyun Sekkya Standing Buddha Statue is located on Po Khaung Mountain, Khatakan Taung Village, Monywa City, Sagaing Region, Myanmar. The location of Buddha Statue is shown in Figure 1. This hollow type Buddha statue has a total of 31 stories with area of 131ft (39.93m) and 44ft (13.41m) with storey height of 12ft (3.66m). The statue is constructed by using reinforced concrete, steel and fibre at 1996 and completely finished 2008. According to Wikipedia, Laykyun Sekkya Standing Buddha Statue is counted as the second tallest statue in the world.

Three Dimensional Laser Scanning

During three dimensional laser scanning process, laser scanner sends an infrared laser beam into the centre of its rotating mirror first. Then, this beam is deflected by the mirror on a vertical rotation. This beam is scattered around the proposed scanned object. The scattered light from surrounding of scanned object is reflected back into the scanner after they made contact with the proposed object. Finally, scanned image is obtained. Phase shift technology is used to determine the distance between the proposed scanned object and scanner. Phrase shift means changes in the phrase of a waveform from the usual position. From mirror rotation and horizontal rotation of the laser scanner, Polar coordinates of δ, α and β which represent distance, vertical angle and horizontal angle are obtained. After that, these Polar coordinates are transformed into Cartesian coordinate for further calculations.

Three dimensional Cartesian coordinate data give the location of each point. These set of points represent the exterior shape of the scanned object. These points are known as point cloud which is the final output of the three dimensional laser scanner. Scanner locationThree dimensional laser scanner is located within the distance range of 303.742 m from the base edge of the Buddha Statue. Scanner locations within this range are further divided into three different groups which are categorized as top viewpoint, middle viewpoint and base viewpoint. Top viewpoint aims to capture the head and shoulder parts of the Buddha Statue.

Middle viewpoint plans to capture the body parts of the Buddha Statue. Base viewpoint intends to capture the base parts of the Buddha Statue such as leg, foot and stage. Therefore, a total of 36 viewpoints, which consists of 11 top viewpoints, 14 middle viewpoints and 11 base viewpoints, were used to scan the exterior figure of the Standing Buddha Statue. Locations of these viewpoints are shown in Figure 3 roughly. Two viewpoints are deleted due to high obstructions.

SettingScanning parameters play an important role in getting a good scan. A good scan provides precise point cloud which can generate exact STL of scanned object. Point cloud data might get some error if the three dimensional laser scanner is located far away from the scanned object. During on-site measurement for this study, resolution of ½, quality of 3x, range of 303.742 m and colour scan was used. Four types of sensors such as global position system, compass, inclinometer and altimeter were also used. Targetless scan is carried out. As a result, outdoor: far distances altered profile is used to capture three dimensional exterior figure of the Laykyun Setkkar Standing Buddha Statue.

ParameterScanner speed 976,000 points/sec

Measurement range 330 m

Possible error 2 mm at 25 m from scanner

Horizontal field of view 360˚

Vertical field of view 330˚

Weather Condition

Rain and dirt produce noise in scans. Moreover, high temperature has negative impact on laser scanning process. Recommended charging temperature of laser scanner is between 10˚C (50˚F) and 30˚C (86˚F). Recommended operating temperature of laser scanner is between 5˚C (41˚F) and 40˚C (104˚F). As a result, scanning schedule is completely depending on the weather condition of the selected scanning area.Monywa is famous for its very high temperature as it has hot semi-arid climate. According to the Myanmar Times which was released on 17 May 2010, Monywa has temperature record of 45.8˚C (114.4˚F) on 13 May 2010. Therefore, August is chosen to perform on-site laser scanning because early monsoon months, April to July, are extremely hot. Between August 6 to August 8, Monywa is partly cloudy, dirt free and slight rain with temperature between 27.8˚C (82˚F) and 37.8˚C (100˚F).

Stimulation Of Scan Data

Point cloud which contains a set of points with location coordinates are transfer into SCENE. Then, scan processing, registration, generation of high quality data and exporting them into STL format are carried out. Faro SCENE is commercial software designed for Faro Focus Scanner and it can evaluate captured scan data in Virtual Reality (VR) environment.

Import

Firstly, draft scans which contain location coordinates of the Buddha Statue are imported. Draft scan image from viewpoint number 0ProcessingProcessing of each scan for a total of 34 scans is carried out. Three dimensional coordinate data of the panorama 360 degree scans are divided and sorted out for each scan. In general, full colour high quality image data and scan point clouds are generated from the draft scan image. Furthermore, filters such as dark scan point filter and distance filter are applied. Dark scan point filter removes scan points by adjusting reflectance threshold. This filter is required because dark surfaces reflected only small amount of light and these reflections decrease accuracy of scan points. Distance filter deletes unnecessary points which are outside the specified distance range in order to avoid overlapping of scan points in outdoor scans.

Registration

Registration aligns coordinate system of all 34 scans into one global coordinate system by using common reference planes between each scan. During this study, combination of auto registration and manual registration is done. This is achieved by combining images from targetless three dimensional laser scan. Data from the sensors are also used during registration in order to improve accuracy of each point. At first, scans are divided into four clusters such as front, back, right and left cluster. After that, auto registration is carried out under top base and cloud to cloud registration mode. Afterwards, manual registration is done by manually selecting common points and planes between each scan. Error of 23.4 mm is present during this laser measurement.

Export STL format

Project point cloud for the whole Buddha Statue is created after successful registration. Some of the duplicate points are deleted during creation of project point cloud. Next, project point cloud is meshed into maximum of 40,000,000 triangles. After the mesh is successfully finished, it is ready to transform into STL format. Finally, STL file is produced from project point cloud data.

Finalization

Finalizing is done by using FreeCAD software. Since black surface is difficult to capture with three dimensional laser scanner, FreeCAD software is used to manually draw the hair part of the Buddha Statue as it colour is black. Firstly, SCENE output STL file is imported into FreeCAD. Then, the normals of it are harmonized into one direction as many as possible. After refining this, it is ready to be added a new hair. Refining shape will also clean residual points or edges of the original STL file. Finally, fully detailed drawn exterior figure of Laykyun Sekkya Standing Buddha Statue is successfully obtained.

Conclusions

Faro Focus 3D X330 cannot scan through object and only operates on line of sight. The selected scanner’s location must not have large or huge obstruction. Therefore, when we locate scanner position, we need to check whether or not obstruction on selected position can affect the target structure’s configuration during registration process. Otherwise, these can become hindrance in registration of scan for global coordinate. In this study, two scans are discarded because of human and tree obstacles. These bring difficulty in recognition process later on. So, it can be concluded that it is important to careful about obstruction degree.

During this study, three dimensional exterior figure of the complex shaped structure is able to capture. Moreover, STL file of file of this structure is successfully generated by using terrestrial type three dimensional laser scanner. Currently, Terrestrial laser scanner has been widely used to capture three dimensional exterior figures of structures. However, it usage on complex-shaped structure is still limited. From this study, it is found out that the output STL file has some missing data on some parts. As a result, data refining in other third party software is required to produce water tight 3D printable stage. To overcome this, it is recommended to perform necessary data treatment when scanning complex-shaped structures.

Acknowledgement

The author would like to express deeply thanks to her late supervisor Dr. Hiromichi Shirato, professor and head of Bridge Engineering Laboratory, Department of Civil and Earth Resources Engineering, Kyoto University, for his guidance and advices. The author also wishes to express her deep gratitude to Dr. Khin Than Yu, pro-Rector of Yangon Technological University and Dr. Kimiro Meguro, professor of International Centre for Urban Safety Engineering, Institute of Industrial Science, University of Tokyo, for their permission to use the 3D laser scanner of the SATREPS-JICA project. The author is thankful to director Dr. Sao Hone Pha and assistant director Dr. Kyaw Zaya Htun, Remote Sensing and GIS Research Centre, Yangon Technological University, for their helpful suggestions on laser scanning project. The author is grateful to president Engr. Aung Kyaw Myint and Engr. Thant Zin Win, Myanmar Engineering Society, Monywa for their interest and help during laser scanning trip.

References:

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