Methods for installing control points and taking photographs for ground photogrammetry used for three-dimensional measurements

Abstract

To provide methods for installing control points and taking photographs for realizing simplification, acceleration, and labor saving in a cross-sectional view creation method by three-dimensional measurements using photogrammetry at a collapse site necessary for assessing a small-scale collapse site for a disaster.SOLUTION: In photogrammetry called SfM (Structure from Motion) or the like that has been put into practical use in recent years, it is possible to automatically express, as a three-dimensional point cloud shape, a subject appearing in an overlapping portion of photographs captured in an overlapping manner from different positions. Provided is a simple working method capable of eliminating surveying required for installation of a control point and installing a multi-axis coordinate ruler by one person by using a scale engraved on the multi-axis coordinate ruler provided with a plurality of axes for the control point required for correctly giving a scale and horizontal and vertical directions to a three-dimensional point group shape. Provided is a method capable of automatically restoring a three-dimensional point group shape of a photograph photographed in an overlapping manner, and measuring a base line length capable of obtaining an appropriate overlapping degree by gait.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] This invention relates to the work flow shown in Figure 6, in which a cross-section of a small-scale collapsed area is created by three-dimensional measurement using terrestrial photogrammetry, by taking overlapping photographs of the small-scale collapsed area so that there are no actual blank spaces, and in a series of orientation processes that enable three-dimensional measurement from the overlapping photographs, by establishing the orientation points necessary for absolute orientation, which project the photographs into a coordinate system of coordinates provided by the orientation points (Non-Patent Document 5). [Background technology]

[0002] To enable three-dimensional measurement using photography, the subject for which three-dimensional measurement is intended must appear in two or more photographs (Non-Patent Document 4). For this reason, in topographical mapping, which is the most common application of photogrammetry, a camera is pointed vertically from an aircraft and photographs are taken at regular intervals. In terrestrial photogrammetry, which is used for traffic accident investigations, etc., photographs are taken using two cameras pointed in the same direction at a fixed distance. In photogrammetry, known as SfM (Structure from Motion), which has recently come into practical use, when photographs are taken on the ground, the subject is photographed so that it appears in photographs from many directions (Non-Patent Document 1). Given the principle of photogrammetry, which allows three-dimensional measurement with two overlapping photographs, this not only places a heavy burden on the photography but also on post-processing.

[0003] Absolute orientation, which involves projectively transforming a photograph into a coordinate system of coordinates that the orientation point has, involves performing relative orientation (Non-Patent Document 2) using five or more identical locations per pair of overlapping photographs taken in duplicate, and then projecting the relatively oriented photograph into a coordinate system of coordinates that the orientation point has, using orientation points that have been made to appear in multiple photographs in advance.

[0004] The coordinates given to the control point can be two-dimensional or three-dimensional.

[0005] Duplicate photographs, absolutely oriented by control points with given two-dimensional coordinates, are used for three-dimensional measurements, where only the scale needs to be correct. Therefore, when accurate three-dimensional measurement of scale is all that is required, a reference scale shaped like a ruler, whose length can be determined accurately, is used as a control point (Non-Patent Document 4).

[0006] Duplicate photographs, absolutely oriented by control points with given 3D coordinates, are used to measure the shape of objects and their relationships in a specific coordinate system, with well-defined horizontal and vertical directions. In three-dimensional measurements defined in this way, the photographic range is generally wide, so four or more control points are scattered within that range, and coordinates in a unified coordinate system are assigned to the control points by control point surveying (Non-Patent Document 5). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent application 2023-208086 [Non-patent literature]

[0008] [Non-Patent Document 1] Edited by Yasushi Yagi and Hideo Saito, CVIM Cheat Sheet Series - Computer Vision Cutting-Edge Guide 5, Adcom Media Co., Ltd., First Edition, December 5, 2012, pp. 40-42 [Non-patent document 2] Akiyama Minoru, Photogrammetry, Sankaido Co., Ltd., April 30, 2001, pp. 28-35 [Non-patent document 3] Ministry of Agriculture, Forestry and Fisheries, Rural Development Bureau, Disaster Prevention Division, "Restoration Methods for Disaster Recovery Projects for Farmland, Agricultural Facilities, Coasts, etc.", 2014 edition, pp. 86-106 [Non-patent document 4] Kosuke Tsuru and Toshiharu Murai, "Fundamentals of Digital Photogrammetry: How to Perform 3D Measurements with a Digital Camera," Japan Association of Surveyors, March 19, 2017, Revised 1st Edition, pp. 87-116 [Non-Patent Document 5] Japan Association of Surveyors, Survey Technology Center (ed.), "Public Surveying - Work Regulations, Commentary and Application, Topographical Surveying and Photogrammetry Edition, 3D Point Cloud Surveying Edition," Japan Association of Surveyors, 3rd Revised Edition, May 13, 2021, pp. 151-155 Summary of the Invention [Problem to be solved by the invention]

[0009] The standard method for creating cross-sectional drawings required for the design of civil engineering structures to restore small-scale collapsed areas is to use pole-based longitudinal and cross-sectional surveying (Non-Patent Document 3). This pole-based longitudinal and cross-sectional surveying requires multiple workers, and because it is carried out using 20-centimeter scales marked in red and white on the surveying pole as a reference, it has low surveying accuracy and cannot be used for construction using the latest digital technology, such as the i-Construction initiative of the Ministry of Land, Infrastructure, Transport and Tourism. Using a method called SfM, photogrammetry can be used to create digital data of terrain consisting of high-density elevation points that can easily be used to create longitudinal and cross-sectional maps. However, to determine the correct scale and horizontal and vertical directions, the photographs must be absolutely oriented using control points. Furthermore, in order to create cross-sectional views using ideal three-dimensional measurements by photogrammetry, a system is needed that makes it easy to determine the position of the camera when photographing small-scale collapsed areas.

[0010] To assign coordinates to the scattered control points, expensive surveying equipment such as a total station that can measure distance and angle simultaneously is required, and specialized skills are required to operate and perform the calculations. In addition, in areas such as collapsed areas, it may be dangerous and difficult for people to enter, making it impossible to set up control points, and in such cases, control points must be set up in a wide, safe area surrounding the collapsed area so that absolute orientation can be performed. At the same time, this means that photography becomes more extensive, and the number of photographs increases, increasing the burden of post-processing.

[0011] In recent years, in photogrammetry (Non-Patent Document 1), known as SfM, which has been put into practical use, when photographs are taken on the ground, the subject is photographed from many directions, which not only places a heavy burden on the photography but also on post-processing. [Means for solving the problem]

[0012] In order to solve such problems, the present invention performs the following processes (1) to (8) in accordance with the workflow shown in FIG. (1) A multi-axis coordinate ruler has coordinate axes that clarify the horizontal and vertical directions, and a scale that clarify the scale. (2) A multi-axis coordinate ruler is installed in an appropriate location within or around a small-scale collapsed area where the multi-axis coordinate ruler is visible, in place of a control point used for absolute orientation of consecutively taken overlapping photographs. (3) The baseline length 18(B) of the stereoscopic photograph is calculated using the following formula from the objective distance 14 from the camera to the subject, the focal length 16 of the lens of the camera, the length 17 of the solid-state image sensor of the camera in the same direction as the subject, and the degree of overlap of the photographs to be taken so that a stereoscopic view can be obtained. The degree of overlap (α) of the photographs is set to a reasonable value that satisfies both ensuring depth accuracy and avoiding gaps in the actual object.

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[0013] The present invention has been invented to enable the creation of cross-sectional maps by three-dimensional measurement of small-scale collapsed areas using photographs that clarify horizontal and vertical directions and are given the correct scale, without requiring expensive equipment or specialized skills, leaving only one worker involved in the work, and without the need to enter dangerous collapsed areas. BEST MODE FOR CARRYING OUT THE INVENTION

[0014] The best mode for carrying out the present invention will now be described with reference to the work flow shown in FIG.

[0015] ===Preparing the 3-axis coordinate ruler=== Prepare a three-axis coordinate ruler 1, a multi-axis right-handed coordinate system in which the angles from the thumb to the index finger and from the index finger to the middle finger are each right angles, with the thumb representing the X-axis, the index finger representing the Y-axis, and the middle finger representing the Z-axis.The length of each axis is about 2 meters, and each axis is shaped like a circular rod, painted white and red every 20 centimeters, so that the coordinates are clear by the borders painted white and red on each axis. A surveying pole is an example of a structure in which each axis is about 2 meters long, has a circular rod shape, and is painted white and red every 20 centimeters.To use a surveying pole as a three-axis coordinate ruler 1, a surveying pole coordinate axis installation stand (Patent Document 1) can be used.

[0016] ===Setting control points=== Instead of a control point to be used for absolute orientation of consecutively taken overlapping photographs, the three-axis coordinate ruler 1 is installed in the middle of the small collapsed area where the three-axis coordinate ruler is captured, and the plane formed by the X-axis and Y-axis is adjusted with a bubble tube so that it is horizontal.

[0017] ===Calculating the baseline length of a stereophoto=== The baseline length 18(B) of the stereoscopic photograph shown in Figure 4 is calculated using the following formula from the objective distance 14 from the camera to the innermost part of the small-scale collapsed area, the focal length 16 of the lens of the camera, the length 17 of the solid-state image sensor of the camera in the same direction as the subject, and the degree of overlap of the photographs to be taken so that a stereoscopic view can be obtained. The degree of overlap (α) of the photographs is set to 0.6, which is the most reasonable value that satisfies both ensuring depth accuracy and avoiding blank areas in the actual body.

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[0018] === Calculating the number of steps taken to measure the baseline length and taking photographs of small-scale collapsed areas === The number of strides (N) of the photographer at the baseline length 18 of the stereoscopic photograph is calculated to one decimal place using the following formula, and by walking along the subject for each stride 11 and operating the camera towards the subject, a stereoscopic photograph of the small collapsed area to be used for three-dimensional measurement is taken along with a three-axis coordinate ruler.

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[0019] ===Photographing the blank areas of the entity=== Depending on the topography of the small-scale collapsed area, if a photograph in which the camera faces the small-scale collapsed area directly produces a blank area, a photograph should be taken from a different direction from the one in which the camera faces the small-scale collapsed area directly, so that the blank area can be seen as a blank area.

[0020] ===Measurement of control points=== On a three-axis coordinate ruler, the center of the red and white border at the tip of each axis is used as the control point for absolute orientation of the photograph, and is measured using any stereophotograph that shows the three-axis coordinate ruler.

[0021] ===Generating 3D coordinates=== The photographs taken are subjected to absolute orientation using bundle adjustment, using the photographic coordinates of the measured orientation points and the corresponding coordinates on a three-axis coordinate ruler, and then the correspondence between all pixels within the overlapping range of the photographs is extracted to calculate three-dimensional point cloud data (Non-Patent Document 4).

[0022] ===Creating a cross section=== From the calculated three-dimensional point cloud data, cross sections of the locations required for disaster assessment are created by three-dimensional measurement (Non-Patent Document 6). [Brief explanation of the drawings]

[0023] [Figure 1] This is a perspective view of a three-axis coordinate ruler that is expected to become standard. [Figure 2] This is a front view of the collapsed area with a three-axis coordinate ruler installed. [Figure 3] This is a side view of the collapsed area with a three-axis coordinate ruler installed. [Figure 4] This is a plan view showing the relationship between the parameters for photographing a collapsed area from the ground. [Figure 5] This is a method for determining the position to hold the camera by pacing. [Figure 6] This is the workflow for creating a cross-sectional view by installing a three-axis coordinate ruler and using photogrammetry to measure three dimensions. [Explanation of symbols]

[0024] 1 3-axis coordinate ruler (perspective view) 2. 3-axis coordinate ruler (front view) 3. Collapsed slope (front view) 4. 3-axis coordinate ruler (side view) 5. Slope of collapsed area (side view) 6. Camera solid-state imaging element 7. Principal point of camera lens 8. Slope of collapsed area (plan view) 9 Slope adjacent to the collapsed area (plan view) 10 Shooting Locations 11 Photographer's stride (W) 12 Integer part of the number of strides between shooting points 13 Range of stride length to the first decimal place between shooting locations 14. Number of strides (N) to one decimal place indicating the baseline length 15 Object distance from camera to subject (L) 16. The focal length (f) of the lens on the camera 17. The length (d) of the solid-state image sensor in the camera in the same direction as the subject 18 Baseline length of the stereophotograph (B), which is the length of the overlapping area of ​​the photographs you want to take so that they can be viewed stereoscopically

Claims

1. A method for setting a control point in which a multi-axis coordinate ruler, which has multiple axes with scales indicating one or more coordinates, is placed in front of a subject, and multiple photographs are taken so that the subject and the multi-axis coordinate ruler appear overlapping in the photographs as shown in Figure 4, thereby enabling absolute orientation of the photographs to the coordinate system provided by the multi-axis coordinate ruler.

2. This method of photography involves dividing the baseline length (18) of the stereoscopic photograph by the photographer's stride length from the objective distance (14) from the camera to the subject, the focal length (16) of the lens of the camera, the length (17) of the solid-state image sensor of the camera in the same direction as the subject, and the degree of overlap of the stereoscopic photograph, to determine the number of strides of the photographer, and then taking photographs that are taken along the subject and aimed at the subject at intervals of that number of strides, as shown in Figure 5, so that they can be viewed as a stereoscopic photograph.

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