Slope Gradient Analysis at Different Resolution Using Terrestrial Laser Scanner

Wan Abdul Aziz Wan Mohd Akib *1, Khairul Nizam Tahar #2, Anuar Ahmad *1 *1 Department of Geoinformatics, Faculty of Geoinformation Science & Engineering

Universiti Teknologi Malaysia 81310 UTM Johor Bahru, Johor, Malaysia wanaziz@utm.my; anuarahmad@utm.my

#2 Department of Surveying Science & Geomatics, Faculty of Architecture, Planning & Surveying Universiti Teknologi MARA

40450 Shah Alam, Selangor, Malaysia nizamtahar@gmail.com

Abstract - Slope instability in landslide prone area is a costly problem, which can lead to travel disruption, property damages, and injury or loss of life. For many years, slope monitoring activities are largely carried out by conventional survey instruments such as Total Station, levelling and Global Positioning System (GPS). However, these techniques is a time consuming, hazardous, and costly process, and it can be difficult to ensure that problems are recorded and handled in a consistent manner. Terrestrial laser scanners (TLS) find rapidly growing interest in remote sensing and photogrammetry field as efficient tools for fast and reliable three‐dimensional (3D) point cloud data acquisition. The TLS technology is based on the reflectorless and contactless acquisition of a point cloud of the topography using the time-of-flight distance measurement of an infrared laser pulse. This paper describes the capabilities of TLS in slope mapping studies. The main objective of this study is to evaluate the slope mapping pattern at different resolution. This study has been conducted at Cameron Highland, Malaysia, which approximately located at latitude 4d26’36” and longitude 101d23’4”. Several data acquisitions on selected cut slopes were performed using the Topcon GLS1500 laser scanner. Ground control points were established using Real Time Kinematic GPS to provide a local coordinate system on laser scanning data. Resolution of a laser scanner determines a density of point cloud during data acquisition. In this experiment, there is a different number of resolution has been set during data acquisition in order to evaluate the result of slope mapping pattern. It was found that, resolution gives an effect in slope mapping pattern. Keywords: Terrestrial Laser Scanner, Resolution, Accuracy, Slope Analysis

I. INTRODUCTION Recently, Terrestrial laser scanner (TLS) has been used for

data collection in monitoring landslide incident. TLS can

provide a high density of point cloud data in high resolution. Reference [1] has proposed a new procedure in land deformation using TLS. In this study, laser scanner was proven in contribute a high accurate result in land deformation. In theory, TLS supplied a very high data redundancy which allows some movement in land deformation can be detected in centimeter level. One of the major factors in landslide deformation is a slope gradient. Slope gradient played an important role in landslide risk analysis. There are many previous techniques has been used for landslide movement detection such as close range photogrammetry, real time kinematic global positioning system, satellite image, leveling, airborne laser scanner and total station [3]. All of these techniques offer a different result in terms of accuracy. A landslide phenomenon is usually caused by mass movement at 0.01mm to 10mm daily [4]. Therefore, TLS is a solution in landslide movement detection, and it can provide a data until millimeter level [2]. The main product that can be produced directly from TLS is the digital surface model (DSM). DSM is including any features and object that are available during scanning operation.

The aim of this study is to determine the effect of TLS resolution in mapping slope. A different scanning resolution has been set during scanning operation and each of them was used in generating the slope maps for the study area. The study was conducted at the prone area in Cameron highland. Fig. 1 shows, the location study area and the condition of slope at the study area in Cameron Highlands.

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Fig. 1 Study Area

II. INSTRUMENTS

In this study, data collection solely obtained from TLS, which involved different number of scanning operation at one selected study area. The distance between TLS and slope area is fixed at 100 meter and four epoch of observation were carried out in this study. TLS is a robust scanner and it send out a laser beam that captures data at 30000 points per second

at range about 150m or 500ft. This scanner was installed with 2.0 mega pixel digital camera, which allows collecting sharp and detailed images from scanning location. Fig. 2 illustrated TLS instrument and target point that has been set up for data collection.

Fig. 2 Terrestrial Laser Scanner and Target Point

III. RESEARCH METHODOLOGY

The methodology can be divided into three phases includes data collection, data processing and data analysis. Data collection involves one-day observation at Cameron highland. One slope has been selected near to the mosque and this slope has slide one month ago. Therefore, it required

monitoring data observation to determine slope behavior in daily. TLS has been set up approximately 100m from the slope. Real Time Kinematic (RTK) was used to obtained TLS coordinate station and one target point was established near to the slope area. Four different observations were conducted in this study and each observation took about 1 hour to complete. Each observation was captured using different resolution at

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the same slope area. All TLS data were downloaded into a computer for data processing stage. In data processing, all point clouds were registered to the local coordinate using control point coordinate. Scan master software has been used to edit point clouds before generating surface model of study area. Editing session needs professional opinion to remove noise during scanning processed. After filter all noise from point cloud data, the outputs were exported in ArcGIS format for data analysis. These steps were repeated for all observation data. Data analysis was covered generation of the digital surface model until the slope map. Each slope map

represented of each observation has been compared to determine the effect of scanning resolution in mapping slope. Analysis results were discussed in detail at analysis section.

IV. RESULTS AND ANALYSIS TLS provided point cloud data of the slope area based on

signal returned to the scanner. Point cloud data record every feature that is available during scanning processed. Fig. 3 shows an example of point cloud data and triangulated irregular network (TIN) at the slope area.

Fig. 3 An example of point cloud data and triagulated irregular network (TIN)

All scanning point clouds were processed to fulfill the

objective of this study. There are four different types of data has been analyzed to determine the effect of scanning resolution in mapping slope. Four different resolutions were obtained from field observation such as 10mm,

30mm, 50mm and 70mm. As mentioned before, each observation takes approximately one hour to complete. However, the time of scanning can be increased when the resolution value is decreased. Fig. 4 shows slope map for each observation based on different scanning resolution.

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(a) (b)

(c) (d) Fig. 4 Slope result for four different resolution, (a) 10mm;(b) 30mm; (c) 50mm; (d)70mm

Based on the slope results (Fig. 4), it was found that,

scanning resolution did affect the slope map products. However, the different between each resolution did not significant and can be used for different types of application. The accuracy of the slope map was depending on the high

density of point cloud during scanning processed. Hence, the scanning resolution can be determined based on the application needs. These results were described in the graph in Fig. 5.

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Fig. 5 Slope classsification at different scanning resolution

Fig. 5 shows the pattern of the slope maps are quite similar

to each different resolution except for 10mm resolution, which record more accurate data in slope result.

V. CONCLUSIONS This paper has discussed the comparison results based on different resolution using TLS. In conclusion, it was found that scanning resolution effected slope map result. Therefore, the scanning resolution must be set according to the application needs. TLS is one of the faster data collections in slope area and it provided the high-accuracy data. The density of point cloud also depends on the resolution of point cloud. Scanning resolution might cause time delay in slope observation using the laser scanner. In future, the study will be expanding to analyze the effect of the aspect map results in different scanning resolution. The scanning resolution can be changed in centimeter level in order to see the effect in slope and aspect map result.

ACKNOWLEDGEMENTS Research Management Centre, Universiti Teknologi

Malaysia (UTM) and Ministry of Higher Education (MOHE) are greatly acknowledged because providing the fund Vot 78570 to enable this study is carried out.

REFERENCES

[1] O. Monserrat and M. Crosetto, “Deformation measurement using terrestrial laser scanning data and least squares 3D surface matching,” ISPRS Journal of Photogrammetry & Remote Sensing (63), pp. 142–154, 2008.

[2] L. Sui, X. Wang, D. Zhao and J. Qu, ” Application of 3D laser scanner for monitoring of landslide hazards,” The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B1. Beijing, 2008.

[3] M. Tsakiri, D. Lichti and N. Pfeifer, “Terrestrial Laser Scanning For Deformation Monitoring,” 3rd Iag / 12th Fig Symposium, Baden, 2006.

[4] T. Miyazaki, K. Kinoshita and M. Takagi, “Accurate Geometric Transformation of Laser Scanner Data for Landslide Monitoring,” Infrastructure Systems Engineering, Kochi University of Technology Tosayamada-city, Kami-shi, Kochi, pp. 782-8502, Japan, 2008.

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