ARMS10 The ISRM International Symposium for 2018 Method for … · 2019. 3. 19. · ARMS10 10th Asian Rock Mechanics Symposium 29 October to 03 November, 2018, Singapore The ISRM

ARMS10 10th Asian Rock Mechanics Symposium

29 October to 03 November, 2018, Singapore The ISRM International Symposium for 2018

Method for Extracting the Source of Falling Rock from Microtopography Highlight Map Created bBy High-Density Aerial Laser Data

Koki Sakitaa*, Satoshi Nishiyamaa, and Masashi Miyashitab and Yuzo Ohnishic

a Okayama University, Okayama, Japan

b Wesco Co., Ltd, Japan c Kyoto University, Kyoto, Japan * [email protected]

Abstract

In order to conduct disaster-prevention inspections without overlooking falling-rock sources, in this paper, we aim to establish a disaster-prevention inspection method using microtopography highlight maps and verify its effectiveness. First, we create the map using the data obtained from a high-density aerial laser. In our study, measurements were carried out using a measurement helicopter loaded with a laser measuring machine capable of irradiating 80,000 to 400,000 points per second. For creating a microtopographical representation, grid data, contour maps, inclination-amount diagrams that calculate the amount of inclination for each grid datum and change the lightness accordingly to express the topography, and wavelet-analysis diagrams that emphasize the change in the unevenness through wavelet analysis, are generally used. However, it is difficult to extract sources of falling rocks, because it is impossible to express the topographic change point between the contour lines in a contour map. It is also difficult to distinguish between ridges and valleys in the inclination-amount diagram because there is no information indicating the height difference. Furthermore, it is difficult to distinguish the microtopography in the wavelet-analysis diagram, because there is no information indicating the height difference or inclination. Therefore, in this study, we created a microtopography highlight map by overlaying 50 cm of grid data, the inclination-amount map, contour diagram, and wavelet-analysis diagram created from the measured laser data. A field survey verified that, by using this map, it was possible to detect a steep cliff of height 2 m or more, which is a possible source of falling rocks. In our study, we were able to extract sources of falling rocks from a microtopography highlight map.

Keywords: Falling-Rock Source Extraction, Steep Cliff, Aerial Laser Surveying

1. Introduction The necessity of tackling falling-rock projects is increasing, especially, because of the recent heavy

rains and earthquakes in Japan. In September 2017, a man riding a bike in the Hualien County in East Taiwan was hit by a falling stone and died. To avoid such painful accidents, improvements in countermeasure projects for falling rocks are necessary. In Japan's rock-fall countermeasure projects, visual tools (drawings) such as basic forest maps and

aerial photographs are used to investigate slopes and grasp the locations of falling rocks. However, on slopes covered by trees, it is difficult to prepare a plan view by actual measurement. As a background of investigation using these drawings, this is because these can be made relatively easily. However, in the current countermeasure projects, it is difficult to grasp accurately the positions of the falling-rock origins, from the surveys using these drawings. This leads to a decrease in the investigation efficiency, which is caused by poor positioning accuracy during the survey and because of overlooking objects. In addition, there are problems related to the safety of investigators at the time of investigation, because survey routes must be selected using these drawings and reliable research routes cannot be obtained from them. To solve these problems, it is necessary to improve the accuracy of the drawings used for the survey. An aerial laser surveying technique can be used to alleviate these problems. In the measurements by

aerial laser surveying, it is possible to measure a wide area using a laser measuring device mounted on an aircraft. Currently, the technology of aerial laser surveying is applied in various civil engineering fields such as microtopographical interpretation of landslide topography and river inundation supposition (Masuda et al., 2014), (Hasegawa and Ota, 2015), (Kikuchi et al., 2017), (Miyashita et al., 2017), (Gigli et al., 2014), (Tonini and Abellan, 2013), (Mashita et al., 2012), (Chhatkuli et al., 2016).



Furthermore, the performance improvement of aviation laser surveying instruments in the recent years makes high-density measurements possible; therefore, further applications can be expected. In this research, to reduce oversights in the falling-rock countermeasure projects and to enable more

efficient inspection, we created a microtopography highlight map from the point cloud data obtained from a high-density aerial laser, and the usefulness of this drawing was verified. The microtopography highlight map was created by superimposing three drawings providing different information, i.e., a contour map, an inclination-amount map, and a wavelet-analysis map. Although similar research cases exist in Japan and other countries, clear methods have not been established so far. There is no established method for extracting the sources of falling rocks from a map. In this study, we compared the results of a field survey with those from the microtopography highlight map, and verified the usefulness of the map for extracting the source of falling rock.

Fig. 1. Example of aerial photograph and basic forest map. 2. Aerial laser surveying 2.1 Overview

Aerial laser surveying is a mobile measurement method in which measurement is carried out using a nonprism-type laser range finder mounted on an aircraft. The irradiated laser pulse strikes the ground surface and is reflected. The distance is measured by acquiring the returned laser pulse. By repeating this process during the entire flight duration, three-dimensional measurement becomes possible.

Fig. 2. Example of aerial laser survey. 2.2 Types of aerial laser data

The data generated from the results of aerial laser measurements are shown in Fig. 3. The data obtained from the measurement include noise. The original data are obtained by omitting this noise. However, the original data include details such as the vegetation on sloping lands, which are not necessary to express microtopography. Therefore, to remove these, filtering is performed on the original data, and only the data on the ground surface are extracted. These data are called ground data. Furthermore, in order to facilitate handling of point clouds, the grid data are obtained by making irregular triangular meshes from the ground data, covering them with equally spaced meshes, and interpolating the heights of the mesh points by linear approximation.



Fig. 3. Data generated from the results of aerial laser measurements. 3. Map overview 3.1 Microtopography highlight map The contour map, inclination-amount diagram, wavelet-analysis diagram, and advanced-step drawing, are included in the drawing proposed as a microtopography highlight expression method created from laser data. However, it is difficult to extract the source of falling rocks from these individual drawings, because of certain problems in each map. Therefore, a microtopography highlight map created by transparently combining these maps is used. This map is created using three microtopographically emphasized expression methods—a contour map, an inclination-amount map, and a wavelet-analysis map—and it is expected that each disadvantage will be supplemented while retaining the characteristics of each of these maps. Similarly, researches are also being conducted by overlapping advanced charts and inclination maps (Sasaki and Mukoyama, 2009).

3.2 Contour map A contour map is a drawing of a locus connecting points of the same altitude. It is possible to discriminate the slope gradient from the density of the contour lines and the ridge valley from the unevenness, and express the height difference. However, it is not possible to express clearly the terrain change between contour lines and the topography of a steep slope.

Fig. 4. Contour map



3.3 Inclination-amount map The inclination-amount map is a diagram expressing the slope inclination with lightness, according to the inclination amount in the maximum inclination direction of the plane that best explains the central point from nine adjacent points of the calculated grid data (Kamita et al., 1999).

Fig. 5. Inclination-amount map and method of calculating the inclination amount 3.4 Wavelet-analysis map Eq. (1) is a two-dimensional wavelet analysis. The wavelet-analysis map can be obtained by performing the convolution integral on a function ψ called a mother wavelet and a coordinate value z of a slope with arbitrary coordinates (a, b) and a shift amount s. The wavelet-analysis map is a diagram expressing the undulation of the terrain, by color, according to the wavelet coefficient C. However, since there is no information indicating the height difference or slope, it is difficult to read the microtopography.

C(s, a, b) = 1

𝑠∫ ∫ 𝑧(𝑥, 𝑦)𝜓(

𝑥−𝑎

𝑠,

𝑦−𝑏

𝑠) 𝑑𝑥 𝑑𝑦

∞

−∞

∞

−∞ (1)

In this study, the mother wavelet adopts the second derivative of the Gaussian function and takes a shape similar to a Mexican Hat function (Eq. (2)). The wavelength in the function is given by Eq. (3), and the figure is for a 1.0 m mesh size and s = 1.

ψ(x, y) = (2 − 𝑥2 − 𝑦2)exp {−1

2(𝑥2 + 𝑦2)} (2)

λ ≒ 4s (3)

Fig. 6. Wavelet analysis map and the Mexican Hat function 3.5 Advanced-step map In the advanced-step map, an arbitrary color is assigned for an altitude, and the difference in elevation is expressed continuously by hue and contrast differences. Although the overall tendency of altitude changes is easy to understand, it is difficult to express the micro topography with only hue.



4. Research outline 4.1 Measurement In this study, the Okayama Prefecture General Route 53, Okayama City Kita Ward Mistu Kuso was set as the research site. To increase the point density in the measurement, the measurement was carried out using a helicopter equipped with a laser measuring machine capable of emitting 80,000 to 400,000 irradiations per second. On general aviation laser surveying in Japan, Data of map information level 2500 (lattice interval is within 2 m) based on the rule of public surveying work standard issued by the Geographical Survey Institute of Japan is mainstream. For this reason, the density of the mountains is low and the topography can’t be represented correctly. However, by using helicopters for measurement, this problem can be solved and high-density measurement is possible. Table 1 shows the performance of the aeronautical laser system used in this study.

Table 1 Measurement specifications

Fig 7. Helicopter for measurement

Fig 8. Flight plan in measurement 4.2 Map creation The survey area is divided into three areas: 1, 2, and 3, based on the undulations in the topography of the Ridge Valley, and a microtopography highlight map is created for each area. Some parameters must be set in order to create microtopography highlights. The mesh size when creating grid data from the original data obtained from aviation laser survey is set to 50 cm. Furthermore, in the wavelet analysis, the scales in the x and y directions are set to be the same, and the dependence of the analysis direction is not considered. Since the wavelength on the mother wavelet has a mesh size of 0.5 m, λ = 2.0 m is obtained from Eq. (3) by setting the shift amount s = 0.5. This is shown in Fig. 9.



Fig 9. Microtopography highlight map of the survey site 4.3 Experiment contents In the microtopographically highlighted map, ridges and valleys are expressed. Fine terrain such as rockfall sources are also expected to be expressed. Therefore, it must be verified whether the extraction of falling-rock origins are possible from the map. Details of the verification are described below. 4.3.1 Trial on ground extraction In the extraction trial, referring to existing disaster-prevention charts, the parts that are considered to be sources of falling rocks are marked. 4.3.2 Field survey In the field survey, in order to verify whether the result of desk extraction is correct or not, we conduct investigations on the site, referring to the marked microtopography highlight map, and confirm the accuracy. From these results, the errata for the extraction of falling-rock origins in the maps of all areas are prepared. 4.3.3 Evaluation using point group To quantitatively evaluate the possibility of extracting the sources of falling rocks, from the map, we use the grid data used for creating the map to examine the factors that affect the availability of extraction. We extract, from the grid data, the parts where extraction is feasible in the extraction and field survey results, and summarize this data for each factor. As the factor, we calculate the inclination angle and the relative height of the falling stone source, respectively, using the coordinates of the steep cliff portion of the grid data.

Fig 10. Calculation of relative height and inclination angle from obtained data.



5. Experimental results 5.1 Extraction trial

Based on the existing disaster-prevention record chart, we marked the sources of falling rocks, which were dangerous places, on the created microtopography highlight map. The results are shown in Fig. 11 below. From the results, falling-rock sources are found to be concentrated mainly on the part displayed in red and black, and the part from blue to green is not marked.

Fig 11. Result of extraction trial and existing disaster-prevention record chart 5.2 Field survey results

Based on the results of marking on the micro topography highlight map in the previous section, we conducted a field survey and verified whether the sources of falling rocks actually existed. In the actual survey, some of the falling stone sources could not be extracted in the map. A summary of the results of the field surveys are shown in Fig. 12, and 13 and 14. Because of the amount of data, I’ll show you the result on only area 1. In the figure, the orange circle is the falling-stone generation source that could be extracted at the desk. The light blue circle is not a falling-rock origin but was extracted at the desk. The yellow circle is a falling-rock generation source that could not be extracted at the desk. The results are summarized in Table 2, which shows the possibilities of extraction in areas 1 to 3 and the survey results.

Fig 12. Summary of extraction trial and field survey.



Fig 13. Some examples of field survey.



Fig 14. Images of falling rock that could’t extract in field survey

Table 2. Summary of survey results

5.3 Evaluation by point group

Based on the results in the previous section, we calculated the inclination angle and relative height of the part from the grid data of the part corresponding to the falling-rock origin and summarized it. As a result, rockfall sources with relative heights of 1.0 m or more and inclination angles of 60 degrees or more could be extracted from the microtopography highlight map, and those with a relative height less than 1.0 m and inclination angles less than 60 degrees could not be extracted.



Fig 154. Relationship between relative height and inclination angle in the extracted place

6. Conclusion and future work In this research, we created a microtopography highlight map as a tool for efficient rock-fall countermeasure projects and verified its usefulness. From desktop extraction and field survey, it was confirmed that approximately 90% of the falling-stone sources could be extracted from the microtopography highlight chart. The inclination angles of the extracted sources had to be 60 degrees or more and their relative heights had to be 1.0 m or more. In addition to the sources of falling rocks, valley topographic features could also be extracted. In the future, we will analyze the results of the wavelet analysis and, at that time, we will aim to establish a more efficient method by automating the desk extraction that is currently done by human hand. References Chhatkuli, S., Kawamura, K., Manno, K., Satoh, T., and Tachibana, K., 2016, An approach to automatic

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Documents

ARMS10 The ISRM International Symposium for 2018 Method for … · 2019. 3. 19. · ARMS10 10th Asian Rock Mechanics Symposium 29 October to 03 November, 2018, Singapore The ISRM