Method for measuring underwater three-dimensional terrain of pebble bed surface

Abstract

The method for measuring the underwater three-dimensional terrain of the pebble bed surface provided by the present disclosure comprises: building an image acquisition platform for obtaining an underwater image sequence of the pebble bed surface; arranging control points and measuring their coordinates; determining the camera height and the camera moving space interval; taking pictures of the target area by cross-section, obtaining the underwater image sequence of the pebble bed surface; performing spatial calculation on the image sequence based on the motion recovery structure method, obtaining the initial encrypted three-dimensional point cloud and the digital orthographic image; establishing a water surface model and calculating the initial water depth of the rasterized point cloud, performing refraction correction on the initial water depth of each point of the rasterized point cloud, finally calculating the corrected bed surface elevation and performing secondary correction on the bed surface elevation based on the water depth, and obtaining the underwater three-dimensional terrain of the target pebble bed surface. The present disclosure is suitable for direct and continuous measurement of the underwater three-dimensional terrain of the pebble bed surface based on photogrammetry, has high spatial resolution and accuracy, is low in cost, simple in arrangement, and does not interfere with the observation object.

Description

TECHNICAL FIELD

[0001] The present disclosure belongs to the field of environment and water conservancy engineering, river geomorphology and mountain disaster measurement, and particularly relates to a method for measuring underwater three-dimensional terrain of pebble bed surface. BACKGROUND

[0002] Pebble rivers are widely distributed in nature, with a slope between steep bedrock rivers and gentle sandy rivers. Some lakes originating from pebble river ancient channels also have pebble bed surfaces. On the one hand, the pebble bed surface has a large particle size, and on the other hand, it is easy to form various river bed structures, and the three-dimensional terrain characteristics are obvious. There are a large number of pebble rivers and pebble bed surface lakes in the hinterland and the edge of the Qinghai-Tibet Plateau in China. It is of great significance to master the continuous variation characteristics of the terrain of these pebble bed surfaces for infrastructure engineering construction, urbanization development, geological disaster prevention, human settlement improvement, water pollution control, ecological diversity restoration and protection in these regions. However, it is very difficult to directly obtain the underwater three-dimensional terrain of the pebble bed surface whether in field observation or laboratory simulation test. Therefore, the method of stopping water and then measuring the terrain is generally used in the laboratory, but it is easy to cause the change of the bed surface terrain when stopping and starting the water, thereby introducing measurement errors.

[0003] The existing direct measurement methods of underwater three-dimensional terrain of pebble bed surface mainly include acoustic and optical methods. The acoustic method uses the propagation and reflection characteristics of sound waves in water to measure the distance, whether it is single-point or multi-point measurement (such as multi-beam), the spatial resolution is low, and due to the large size of the equipment, it needs to be carried on a ship and cannot be used in shallow water environment. However, pebble rivers with a water depth of not more than 2m are widely distributed in nature, and the water depth condition in the laboratory is generally smaller. Underwater laser scanning can obtain underwater three-dimensional terrain, but due to the rapid attenuation of laser in water, the use range of this measurement method is limited, and the equipment is expensive. There is also a method of measuring underwater three-dimensional terrain by combining depth camera with refraction correction, but the resolution of the depth camera is generally low, which limits the overall spatial resolution of the measurement. If it is necessary to measure from multiple points and then splice the point cloud, a large amount of camera calibration work is needed, and the time resolution of the measurement is very low. In addition, underwater three-dimensional terrain can also be obtained by needle measurement method (including using probe in laboratory and using total station, RTK GPS, etc. in field), but the cost of manpower and time is huge, the spatial resolution is low, and it is an invasive measurement method, which may change the bed surface terrain.

[0004] In view of the foregoing continuous monitoring needs of underwater three-dimensional terrain of pebble bed surface and the problems existing in the prior art, it is necessary to develop a special method for measuring underwater three-dimensional terrain. SUMMARY

[0005] The present disclosure aims to at least solve one of the technical problems existing in the prior art.

[0006] To this end, the method for measuring the underwater three-dimensional terrain of the pebble bed surface provided by the embodiments of the present disclosure has high spatial resolution and precision, is simple to operate, and has low cost, and comprises the following steps:

[0007] (1) An image acquisition platform for acquiring an underwater image sequence of the pebble bed surface is built, the image acquisition platform comprising a camera, and the underwater image sequence comprising a plurality of underwater images of the pebble bed surface;

[0008] (2) Control points are arranged and measured, and the spatial coordinates of the control points are acquired

[0009] The control points are arranged on both sides of the water surface of the target area and comprise a flat plate with a regular geometric pattern drawn on the upper surface and a horizontal slope, the geometric pattern having a clear center point position, the center point position being defined as the coordinate of the control point, and at least two control points being required to be captured in each underwater image of the pebble bed surface;

[0010] The coordinates of the control points are measured by using a measuring device, the position of the measuring device being defined as the origin, the horizontal direction along the water flow or the length direction of the water surface of the target area being defined as the X-axis direction, the horizontal direction perpendicular to the X-axis being defined as the Y-axis direction, and the direction perpendicular to the XY plane being defined as the Z-axis direction;

[0011] (3) The height of the camera and the spatial interval of the camera movement are determined

[0012] (3-1) The height of the camera is determined

[0013] The length direction of the photosensitive element of the camera is arranged along the Y-axis direction, the width direction of the photosensitive element of the camera is arranged along the X-axis direction, then the camera is placed at a certain design height H, and then the actual spatial size covered by each underwater image of the pebble bed surface is calculated according to the size of the photosensitive element of the camera, the focal length of the camera, and the certain design height H, and the calculation formula is as follows:

[0014]

[0015]

[0016]

[0017] wherein L and W are the actual spatial sizes corresponding to the length direction and the width direction of each underwater image of the pebble bed surface, respectively, l c and w c are the length and the width of the photosensitive element of the camera, respectively, h is the image distance, and f is the focal length;

[0018] If the image taken by the camera at the design height H meets the following three requirements: ① the spatial resolution requirement of the measurement result is met, ② the ratio of the water surface width of the target area to the length of the underwater image of the single pebble bed surface where the target area is located should not exceed the first threshold value, and ③ the center point of each control point is clearly visible; it is determined that the height of the camera is H, otherwise, the height of the camera is adjusted until the above three requirements are met at the same time; the spatial resolution Res of the measurement result is calculated according to the following formula:

[0019]

[0020] Wherein, l p is the number of pixels in the length direction of a single camera image;

[0021] (3-2) Determine the spatial interval of camera movement

[0022] According to the overlapping area of adjacent images in the X-axis and Y-axis directions is not less than the second threshold value, the spatial interval L X of the camera moving along the X-axis direction and the spatial interval L Y of the camera moving along the Y-axis direction are determined respectively.

[0023] (4) Based on the parameters determined in step (3), the underwater image sequence of the pebble bed surface is obtained by the image acquisition platform for each cross section of the target area water surface

[0024] During the shooting process, first, move the camera to the vicinity of the most upstream cross section of the target area water surface; for each cross section measurement, fix the focal length of the camera, and the lens of the camera is perpendicular to the water surface, the camera moves from one side of the cross section to the other side of the cross section at a fixed interval L Y After moving a fixed interval L Y , the camera stops shooting a clear image, and then the camera moves a fixed interval L Y , and so on, until a cross section is shot, then move the camera along the X-axis direction by a fixed interval L X , start measuring the next cross section, wherein the cross section position X1 where the camera starts shooting and the cross section position X n where the camera ends shooting should meet: X1≤X min , X max ≤X n , X min and X max are the minimum and maximum values of the target area water surface along the X-axis direction respectively, and n is the number of cross sections.

[0025] All the images shot by the camera form the underwater image sequence of the pebble bed surface;

[0026] (5) based on the motion structure method, the underwater image sequence of the pebble bed surface is modeled to obtain a three-dimensional point cloud, the three-dimensional point cloud is spatially registered by using control points, an initial encrypted three-dimensional point cloud is obtained based on the registered three-dimensional point cloud, and a corresponding digital orthographic image is generated;

[0027] (6) based on the digital orthographic image, a water surface model containing a target area is generated, and the part of the initial encrypted three-dimensional point cloud located below the water surface model is filtered out to obtain a three-dimensional point cloud located in the underwater area, the three-dimensional point cloud located in the underwater area is refracted corrected to obtain a corrected underwater terrain and an initial water depth;

[0028] (7) the corrected underwater terrain is corrected again by using the initial water depth to obtain the underwater three-dimensional terrain measurement result of the pebble bed surface.

[0029] In some embodiments, the image acquisition platform built in step (1) further comprises:

[0030] a longitudinal module comprising longitudinal slides placed on both sides of the water surface of the target area along the direction of the water flow and longitudinal sliders arranged on the longitudinal slides and movable thereon;

[0031] a transverse module comprising transverse slides placed horizontally along the direction perpendicular to the water flow and transverse sliders arranged on the transverse slides and movable thereon, the ends of the transverse slides are connected to the longitudinal sliders, and the position of the transverse slides in the horizontal direction along the water flow is changed by the longitudinal module;

[0032] a vertical module comprising a vertical slide moving with the transverse sliders, a vertical slider arranged on the vertical slide and movable thereon, and a bracket connected to the vertical slider and moving with it, the end of the bracket is installed with a gimbal for fixing the camera; and

[0033] a control module for controlling the movement mode, movement speed and target position of the longitudinal module, the transverse module and the vertical module, and controlling the shooting parameters of the camera and receiving the underwater image sequence shot by the camera.

[0034] Further, the longitudinal module, the transverse module and the vertical module are respectively provided with a positioner for determining the position of each slider, and each positioner is realized by a point laser range finder fixed at one end of each slide to measure the distance and position of the slider.

[0035] In some embodiments, a drone equipped with a camera is used instead of the camera.

[0036] In some embodiments, in step (2), the measuring device uses RTK GPS, total station or laser scanner.

[0037] In some embodiments, in step (4), the number of images of the i-th cross section is m i , satisfying the following relationship: WS i + 6L Y ≤ L + (m i - 1)L Y , i ∈ [1, n], WS i is the maximum water surface width at the i-th cross section.

[0038] In some embodiments, step (5) specifically comprises the following steps:

[0039] spatial calculation is performed on the underwater image sequence based on a motion recovery structure method to reconstruct the camera spatial position and lens orientation corresponding to each image, and a sparse three-dimensional point cloud of the target region is obtained through feature point matching; the center positions of each control point appearing in the underwater image sequence are identified and marked through manual or automatic methods to obtain the image coordinates of the control points, the sparse three-dimensional point cloud is spatially data registered using the corresponding relationship between the real coordinates and image coordinates of the control points, an initial encrypted three-dimensional point cloud is obtained based on the registered three-dimensional point cloud using an encryption method, and a corresponding digital orthographic image is generated.

[0040] In some embodiments, step (6) specifically comprises the following steps:

[0041] In the digital orthographic image, a single point is used to demarcate a water surface boundary point and project it to the initial encrypted three-dimensional point cloud to obtain the spatial coordinates of a series of water surface boundary points, a Delaunay triangular grid is generated based on the water surface boundary points as a water surface model containing the target region; the initial encrypted three-dimensional point cloud is rasterized, the bed elevation value of the raster is the maximum elevation of all point cloud points contained in the raster range, and a rasterized initial three-dimensional point cloud is obtained; the distance from the rasterized initial three-dimensional point cloud to the water surface model is calculated as the initial water depth; three-dimensional points in the rasterized initial three-dimensional point cloud with an initial water depth less than 0 are deleted to obtain raster points located in the underwater region; for the camera position corresponding to each image, the actual spatial range corresponding to the image taken by the camera position is calculated using the camera orientation angle calculated according to the motion recovery structure method and the focal length and size of the camera photosensitive element, and the raster points falling within the actual spatial range are considered to be visible to the camera position;

[0042] For each screened raster point located in the underwater region, it is refracted corrected according to the following formula to obtain the corrected underwater topography and initial water depth:

[0043]

[0044] d a = Z w - Za

[0045]

[0046]

[0047]

[0048] wherein j represents that a certain grid point in the underwater area is visible to the jth camera position, r j is the angle between the camera optical axis and the water surface at the jth camera position, X cj , T cj are the X-axis and Y-axis coordinates of the jth camera position, respectively, H wj is the vertical distance from the jth camera position to the water surface model, X a , Y a , Z a are the X-axis, Y-axis and Z-axis coordinates of the certain grid point, respectively, d a is the initial water depth of the certain grid point, d j is the corrected water depth of the certain grid point calculated according to the jth camera position, is the average corrected water depth calculated according to the camera positions corresponding to all the images containing the certain grid point in the image range, Z w is the elevation of the water surface model corresponding to the horizontal position of the certain grid point, Z p is the corrected bed surface elevation of the certain grid point, n1, n2 are the refractive indexes of water and air, respectively.

[0049] Further, in step (6),

[0050] when the grid points in the underwater area obtained after deleting the three-dimensional points in the gridded initial three-dimensional point cloud with initial water depth less than 0 still contain some grid points actually above the water surface model, a spatial fitting plane of the water surface boundary points is further generated, the distance from the gridded initial three-dimensional point cloud to the fitting plane is calculated, and the distance is used for secondary screening of the three-dimensional points above water, different screening thresholds are tested until the grid points above the water surface model are all removed, so that the grid points in the underwater area are finally obtained; and / or

[0051] when the camera focal plane is parallel to the water surface, the following approximate correction method is used to calculate the initial water depth of each grid point in the underwater area screened out:

[0052]

[0053] Z p = Z w -d.

[0054] Further, step (7) specifically comprises:

[0055] the bed surface elevation Z of each point cloud grid of the underwater area after refraction correction p Further correction is made by the following formula to obtain the final corrected bed surface elevation Z p

[0056] ΔZ = -0.0049d - 0.0019

[0057] Z p ' = Z p - ΔZ

[0058] The underwater three-dimensional topographic measurement result of the gravel bed is composed of the X-axis direction coordinate X a , the Y-axis direction coordinate Y a and the final corrected bed surface elevation Z p ' of all grid points.

[0059] The purpose of the application is to overcome the shortcomings and deficiencies of existing underwater three-dimensional topographic measurement technology of gravel bed, and to provide a device and method for underwater three-dimensional topographic measurement of gravel bed based on photogrammetry, which has the advantages of high spatial resolution and accuracy, simple operation and low cost, and has important application prospect. The device and method of the patent can be applied to underwater three-dimensional topographic measurement of gravel bed in laboratory water tank, and can also be applied to underwater three-dimensional topographic measurement of gravel bed in the field, but the prerequisite is that the bed surface is clearly visible from above the water surface, so it is mainly suitable for water bodies with shallow depth, high transparency and calm surface.

[0060] The beneficial effects of the present disclosure are:

[0061] 1. The underwater three-dimensional topography of the gravel bed with high spatial resolution and accuracy can be obtained; wherein the reconstructed underwater three-dimensional topography of the gravel bed has high spatial resolution up to sub-millimeter level based on the visible light and images used; the overall error is controlled based on the control points, especially when the coordinates of the control points with high accuracy can be obtained, the error can be further reduced (such as using a total station to measure the coordinates of the control points, the error is millimeter level), in addition, the disclosure further corrects after refraction correction, and removes the elevation error introduced by refraction correction and bed surface material, therefore, the above two reasons make the reconstructed underwater three-dimensional topography of the gravel bed have high accuracy up to millimeter level;

[0062] 2. The test process does not need to stop water measurement, so that the gravel bed under water condition can be directly measured;

[0063] 3. No waterproof shooting equipment is needed, the cost is low and the arrangement is simple;

[0064] ​4. The non-invasive measurement method does not interfere with the observation object. BRIEF DESCRIPTION OF DRAWINGS

[0065] Figure 1 FIG. 1 is a flow chart of the measurement method of the underwater three-dimensional terrain of the pebble bed surface provided by the embodiments of the present disclosure.

[0066] Figure 2 FIG. 2 is a structural schematic diagram of the image acquisition platform built in the measurement method provided by the embodiments of the present disclosure, wherein (a) is a side view, (b) is a front view, and (c) is a top view.

[0067] Figure 3 FIG. 3 is a structural schematic diagram of the control point in the image acquisition platform shown in FIG. 2. Figure 2

[0068] Figure 4 FIG. 4 is a digital elevation model diagram of the underwater three-dimensional terrain of the pebble bed surface obtained by the measurement method provided by the embodiments of the present disclosure, wherein (a) is the underwater digital elevation model of the pebble bed surface before refraction correction, and (b) is the digital elevation model after refraction correction.

[0069] In the figure, 1 is a camera, 2-1 is a longitudinal sliding rail, 2-2 is a longitudinal sliding block, 3-1 is a transverse sliding rail, 3-2 is a transverse sliding block, 3-3 is a transverse positioner, 4-1 is a vertical sliding rail, 4-2 is a vertical sliding block, 4-3 is a support, 4-4 is a vertical positioner, 5 is a control module, 6 is a control point, 6-1 is a frame, 6-2 is a flat plate, 6-3 is a ground nail, 6-4 is a rotating shaft, and 6-5 is a support rod. DETAILED DESCRIPTION

[0070] In order to make the purpose, technical scheme and advantages of the present application more clear, the present application is further described in detail below in combination with the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and do not limit the present application.

[0071] On the contrary, the present application covers any alternative, modification, equivalent method and scheme made on the essence and scope of the present application as defined by the claims. Further, in order to make the public have a better understanding of the present application, some specific details are described in detail in the following detailed description of the present application. The present application can also be completely understood without the description of these details by those skilled in the art.

[0072] ​In the description of the present disclosure, it needs to be understood that the orientations or positional relationships indicated by the terms "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise" and the like are based on the orientations or positional relationships shown in the drawings, and are only for the convenience of describing the present disclosure and simplifying the description, and do not indicate or imply that the bases or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present disclosure. In addition, the terms "first", "second" are only for the purpose of description, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined as "first", "second" can explicitly or implicitly include one or more of the features. In the description of the present disclosure, the meaning of "a plurality of" is two or more, unless otherwise explicitly specified and limited.

[0073] In the description of the present disclosure, it needs to be noted that, unless otherwise explicitly specified and limited, the terms "mounting", "connecting", "connection" should be understood broadly, for example, it can be fixed connection, or detachable connection, or integrally connected; it can be mechanical connection, or electrical connection; it can be directly connected, or indirectly connected through an intermediate medium, it can be the internal communication of two elements or the interaction relationship between two elements. For those skilled in the art, the specific meaning of the above terms in the present disclosure can be understood according to the specific circumstances.

[0074] In the present disclosure, unless otherwise explicitly specified and limited, the "upper" or "lower" of the first feature to the second feature can include that the first and second features are in direct contact, or that the first and second features are not in direct contact but are in contact through another feature between them. Moreover, the "upper", "above" and "on" of the first feature to the second feature includes that the first feature is directly above and obliquely above the second feature, or only indicates that the horizontal height of the first feature is higher than that of the second feature. The "below", "under" and "under" of the first feature to the second feature includes that the first feature is directly below and obliquely below the second feature, or only indicates that the horizontal height of the first feature is less than that of the second feature.

[0075] Referring to Figure 1 The first aspect of the present disclosure provides a method for measuring the underwater three-dimensional terrain of the pebble bed surface, comprising:

[0076] (1) Building an image acquisition platform

[0077] Referring to Figure 2 The built image acquisition platform comprises:

[0078] Camera 1 is used to capture underwater image sequences of the target area. During the shooting process, the camera's focal length is fixed, and the camera lens is oriented basically perpendicular to the water surface. It can remotely control the shooting and set shooting parameters such as aperture, shutter speed, and ISO. It preferably supports remote data transmission.

[0079] The longitudinal module includes longitudinal slide rails 2-1 placed on both sides of the target area along the direction of water flow (one longitudinal slide rail 2-1 is provided on each side), and a longitudinal slider 2-2 provided on the longitudinal slide rails 2-1 and movable thereon.

[0080] The horizontal module includes a horizontal slide rail 3-1 that is generally placed horizontally along the direction of water flow, and a horizontal slider 3-2 that is set on the horizontal slide rail 3-1 and can move along it. The end of the horizontal slide rail 3-1 is connected to the vertical slider 2-2. The horizontal position of the horizontal slide rail 3-1 along the direction of water flow is changed by the vertical module.

[0081] The vertical module includes a vertical slide rail 4-1 that moves with and is connected to the horizontal slider 3-2, a vertical slider 4-2 that is mounted on the vertical slide rail 4-1 and can move along it, and a bracket 4-3 that is connected to and moves with the vertical slider 4-2. A gimbal for fixing the camera 1 is mounted at the end of the bracket 4-3.

[0082] The control module 5 is used to control the movement mode (including stepping and constant speed) of the longitudinal module, the transverse module and the vertical module, as well as the movement speed and target position. It can also remotely control the shooting parameters of the camera 1 and receive the underwater image sequence captured by the camera 1.

[0083] Control point 6, see Figure 3 (a) to (d) include a frame 6-1 fixed on both sides of the target area and a plate 6-2 rotatably connected to the frame 6-1. The slope of the plate 6-2 should be kept horizontal. The upper surface of the plate 6-2 is painted with a pattern composed of black and white colors. The pattern is generally a regular geometric shape and has a clear center point position. This position is defined as the coordinate of the control point. The side length of the control point 6 should ensure that the center point position of the control point 6 can be clearly identified in the image of the camera 1 (or the drone). Figure 3 In the diagram, (a) shows the shape of the control point when it is placed horizontally, (b) shows the shape of the control point when it is placed on a slope, (c) shows the bottom view of the control point, and (d) shows the ground stake of the control point, which is fixed to the outer frame of the control point by a screw segment.

[0084] Furthermore, the longitudinal module, transverse module, and vertical module are each equipped with a positioner for determining the position of their respective sliders. Figure 2 The diagram only shows the horizontal locator 3-3 and the vertical locator 4-4. Each locator uses a point laser rangefinder fixed to one end of its respective slide rail to achieve slider distance measurement and positioning functions.

[0085] Further, the control point 6 is a solid rectangular (mostly square) plastic flat plate with a thickness of 5-10 mm, and the length of the side is determined according to the height of the camera 1 (or the unmanned aerial vehicle), and the principle of the value is that the center point position of the control point 6 can be clearly identified in the image of the camera 1 (or the unmanned aerial vehicle). The flat plate 6-2 of the control point is connected to one side of the frame 6-1 through the rotating shaft 6-4, and the flat plate 6-2 is connected to the other side of the frame 6-1 through the support rod 6-5, and the frame 6-1 is left with a screw port for installing the ground nail 6-3. The control point 6 is generally installed on the shore, and the ground nail 6-3 is embedded into the bed surface to fix the control point 6, and then the slope of the flat plate 6-2 is adjusted to be substantially horizontal through the rotating shaft 6-4, so as to facilitate the camera 1 to clearly shoot from the top.

[0086] It should be noted that if there are installation conditions, the aforementioned image acquisition platform is recommended to be used, and if there are no conditions for installing the transverse module (such as no guide rail or power supply conditions in the field or laboratory measurement), the image acquisition can be performed by manually holding the vertical support with the camera to obtain the underwater three-dimensional terrain of the target bed surface. Thus, the vertical slide rail is replaced by manually fixing the camera position, and the transverse slide rail 3-1 is replaced by the movement of the measurer, and at this time, the laser range finder fixed on the shore can still be used to determine the measurement position and the movement interval. In addition, a drone with a camera can also be used to replace the camera for shooting in the field measurement, and the camera lens is kept perpendicular to the water surface during the shooting process to reduce the water surface reflection.

[0087] (2) Arranging and measuring control points to obtain the spatial coordinates of the control points

[0088] The control points are arranged on the water-free ground around the water surface of the target area. The control point 6 should be as close to the water surface as possible, and the position interval of the adjacent two control points 6 on the same shore is relatively uniform, and the interval is related to the height and focal length of the camera 1, and the arrangement principle is that at least 2 control points 6 can be shot in each camera image, and preferably more than 4 control points 6 can be shot at the same time. The interval of the adjacent two control points 6 on the same shore is preliminarily determined by the following formula, and regardless of whether the flow direction of the target area can be clearly determined, the control point interval can also be preliminarily selected according to the following standard. It should be noted that after the control points 6 are preliminarily arranged, the camera 1 needs to be installed to test whether it can meet the standard that at least 2 control points 6 can be shot in each camera image, and if not, the number and interval of the control points 6 need to be adjusted until the requirements are met.

[0089] L G ≤0.5W

[0090] Wherein, L G is the interval of the adjacent two control points on the same shore, and W is the actual spatial size corresponding to the width direction of a single camera image (the smaller value of the two side lengths of the image).

[0091] After the control point arrangement is determined, the coordinates of the center points of the control points are measured using a device with high measurement accuracy, such as an RTK GPS, a total station, or a laser scanner. Taking the position of the RTK GPS, the total station, or the laser scanner as the origin, the horizontal direction in the direction of the water flow is defined as the positive direction of the X axis, the horizontal direction perpendicular to the left thereof is defined as the positive direction of the Y axis, and the vertical direction upward is defined as the positive direction of the Z axis. When the direction of the water flow is not clear, the arrangement direction of the transverse slide rail 3-1 is defined as the Y axis direction, the horizontal direction perpendicular thereto is defined as the X axis direction, and the positive directions of the X axis and the Y axis are determined according to the measurement direction.

[0092] (3) Determining the height of the camera and the spatial interval of camera movement

[0093] (3-1) Determining the height of the camera and the resolution of the camera image: The length direction of the photosensitive element of the camera is arranged along the Y axis direction, and the width direction is arranged along the X axis direction. Then, the camera is set at a certain design height, and the spatial size covered by each camera image is calculated according to the size of the photosensitive element of the camera, the focal length of the camera, and the certain design height. The calculation formula is as follows:

[0094]

[0095]

[0096]

[0097] wherein L is the actual spatial size corresponding to the length direction of a single camera image, l c and w c are the length and width of the photosensitive element of the camera, respectively, H is the design height of the camera, h is the image distance, and f is the focal length.

[0098] If the image taken by the camera at the design height H meets the following three requirements at the same time: ① the spatial resolution requirement of the measurement result is met, ② the ratio of the water surface width of the target region to the length of the single image in which it is located should not exceed a first threshold value (in this embodiment, the first threshold value is taken as 70%), and ③ the center point of each control point is clearly visible and easy to identify, then the height of the camera is determined as H. Otherwise, the height of the camera is adjusted until the above three requirements are met at the same time. The spatial resolution Res of the measurement result is calculated according to the following formula:

[0099]

[0100] wherein l p is the number of pixels in the length direction of a single camera image.

[0101] The height of the camera and the resolution of the image (such as the number of pixels in the length direction) are determined based on the above requirements and the spatial resolution requirement of the measurement result.

[0102] (3-2) Determine the spatial interval of camera movement: the spatial interval of camera movement is divided into X-axis direction (horizontal direction along the water flow) and Y-axis direction (horizontal direction perpendicular to the water flow), which is mainly calculated according to the camera height, focal length, photosensitive element size and the requirement of the overlap rate between adjacent images along the X-axis and Y-axis directions, and the overlapping area of adjacent images in the X-axis and Y-axis directions is required to be not less than a second threshold, it should be noted that the overlapping area requirement of adjacent images in the X-axis and Y-axis directions can be the same or different, in this embodiment, the same requirement is taken, and the second threshold is 80%, so the spatial interval of camera movement can be determined according to the following formula:

[0103] L X ≤0.2W

[0104] L Y ≤0.2L

[0105] Wherein, L X and L Y are the spatial intervals of camera movement along the X-axis and Y-axis directions respectively.

[0106] (4) Based on the parameters determined in step (3), take pictures for each cross section to obtain a sequence of underwater images of the gravel bed surface

[0107] During the shooting process, first move the camera to the vicinity of the most upstream cross section (parallel to the YOZ plane) of the target area, and when there is no clear water flow direction, move to the vicinity of one end of the target area along the specified X-axis direction, and mark the X-axis direction coordinate of the one end of the target area as X min . For each cross section measurement, the camera moves from one side of the cross section to the other side at a fixed motion interval L Y , stops and takes a clear image after moving a fixed interval, then moves the camera by another fixed interval, and so on, until the cross section is shot, then move the camera along the X-axis direction by a fixed interval L X to start measuring the next cross section. The cross section position X1 where the camera starts shooting and the cross section position X n where the camera ends shooting should satisfy:

[0108] X1≤X min

[0109] X max ≤X n

[0110] Wherein, X min and X max are the minimum and maximum values of the water surface along the X-axis direction of the target area, X1 is the cross section position with the minimum X-axis direction coordinate, and X n is the cross section position with the maximum X-axis direction coordinate, and n is the number of cross sections.

[0111] To ensure that the number of projections of the underwater area in the Y direction in all the captured images is not less than 4, the number of cross-section images m i satisfies the following relationship:

[0112] WS i+ 6L Y ≤L+(m i -1)L Y

[0113] where i is the cross-section number, i∈[1,,n], WS i is the maximum water surface width at the i-th cross-section.

[0114] The camera shooting parameters (image size, image format, aperture, shutter, ISO, etc.) are unchanged during the measurement process, and the motion speed is unchanged. The camera pause time at the cross-section measurement is determined by whether a clear image can be obtained, and is generally maintained at 1s to eliminate the influence of camera motion stop shaking on shooting. If the lighting conditions are good during shooting, the shutter speed can be maintained above 1 / 500s under the premise of clear imaging, then the camera can be kept at a uniform speed without pausing, and the motion speed U is set in the same way as the imaging is clear and the edges are sharp. If uniform motion is used, the camera shutter needs to be set to shoot at fixed time intervals, and the time interval Δt is calculated and set as follows:

[0115]

[0116] All the images captured by the camera are arranged in chronological order to form an underwater image sequence of the pebble bed surface, denoted as P={P1,P2,…,P l ,…P q}, P l is the l-th underwater image of the pebble bed surface captured by the camera, l∈[1,q]. q is the total number of images,

[0117] (5) Based on the Structure from Motion (SfM) method, the underwater terrain modeling is performed to obtain a three-dimensional point cloud, and the spatial data registration is performed by using the control points.

[0118] The underwater image sequence P is calculated based on a structure from motion (SfM) method, the spatial positions and lens orientations of the cameras corresponding to the images are reconstructed, and a sparse three-dimensional point cloud of the target region is obtained through feature point matching. The center positions of the control points appearing in the underwater image sequence P are identified and marked by manual or automatic methods, and the image coordinates of the control points are obtained. The manual method is to visually identify the positions of the control point centers in the images. The automatic method is to identify the control point mark area in the image through image processing (such as edge detection) or training a convolutional neural network (CNN), and then to calculate the image coordinates of the control point centers by approximating the set rule geometric shape (such as a circle, an ellipse, a rectangle, etc., depending on the actual shape of the control point pattern) and the centroid. The sparse three-dimensional point cloud is spatially registered using the corresponding relationship between the true coordinates and the image coordinates of the control points, and an initial dense three-dimensional point cloud is obtained based on the registered three-dimensional point cloud using an encryption method (such as multi-view stereo, MVS), and a corresponding digital orthographic image is generated.

[0119] (6) The initial dense three-dimensional point cloud is corrected for refraction to obtain the corrected underwater terrain and water depth.

[0120] In the digital orthographic image containing the target region, a series of water surface boundary points are manually calibrated using single-point calibration, the planar spatial coordinates of the water surface boundary points are obtained, and the vertical coordinates of the water surface boundary points are obtained by projecting the planar spatial coordinates onto the initial dense three-dimensional point cloud. A Delaunay triangular mesh is generated based on the water surface boundary points as a water surface model. The initial dense three-dimensional point cloud is rasterized, and the raster size is determined by the spatial resolution requirement of data analysis and should be smaller than the spatial resolution required by data analysis. The bed elevation of the raster is the maximum elevation of all the point cloud points within the range of the raster, and the rasterized initial three-dimensional point cloud is obtained. The distance between the rasterized initial three-dimensional point cloud and the water surface model is calculated as the initial water depth. The three-dimensional points in the rasterized initial three-dimensional point cloud with an initial water depth less than 0 can be deleted, i.e., the raster points located above the water surface model are deleted. If the raster points selected based on the above screening criteria still contain some raster points actually located above the water surface model, a spatial fitting plane of the water surface boundary points is generated, the distance between the rasterized initial three-dimensional point cloud and the fitting plane is calculated, and the water surface three-dimensional points are screened again using this distance. Different screening thresholds are tested until all the raster points located above the water surface model are removed, and thus the raster points located in the underwater region are obtained. For the camera positions corresponding to each image, the actual spatial range corresponding to the images taken by the camera positions is calculated using the camera orientation angles (pitch angle, roll angle, and yaw angle) calculated by the SfM method, the focal length of the camera, and the size of the camera photosensitive element. The raster points falling within the actual spatial range are considered to be visible to the camera positions, and each raster point is generally visible to multiple camera positions.

[0121] For each of the screened grid points located in the underwater area, the initial water depth thereof is corrected according to the following formula:

[0122]

[0123] d a = Z w - Z a

[0124]

[0125]

[0126]

[0127] wherein j represents that a certain grid point located in the underwater area is visible to the jth camera position, r j is the included angle between the camera optical axis at the jth camera position and the water surface, X cj , Y cj are the X-axis direction and Y-axis direction coordinates of the jth camera position respectively, H wj is the vertical distance from the jth camera position to the water surface model, X a , Y a , Z a are the X-axis direction, Y-axis direction and Z-axis direction coordinates of the certain grid point respectively, d a is the initial water depth of the certain grid point, d j is the corrected water depth of the certain grid point calculated according to the jth camera position, is the average corrected water depth calculated according to the camera positions corresponding to all the images containing the certain grid point in the image range, Z w is the elevation of the water surface model corresponding to the horizontal position of the certain grid point, Z p is the corrected bed surface elevation of the certain grid point, n1, n2 are the refractive indexes of water and air respectively.

[0128] When the measurement accuracy requirement is low, or the camera focal plane and the water surface can be strictly parallel, the following approximate correction method can also be used to calculate the initial water depth of each of the screened grid points in the underwater area:

[0129]

[0130] Z p = Z w - d

[0131] The embodiment of the present disclosure eliminates the influence of the refraction of visible light through the water surface on the SfM-based three-dimensional reconstruction result through step (6).

[0132] (7) The corrected underwater terrain is again modified by the initial water depth, and the underwater three-dimensional terrain measurement result of the gravel bed surface is obtained

[0133] The bed surface elevation Z of each point cloud grid in the refractive corrected underwater area p The final corrected bed surface elevation Z needs to be further corrected by the following formula (length unit is m) p :

[0134] ΔZ = -0.0049d - 0.0019

[0135] Z p ' = Z p -ΔZ

[0136] The underwater three-dimensional terrain measurement result of the gravel bed surface is composed of the X-axis direction coordinate X a , the Y-axis direction coordinate Y a and the final corrected bed surface elevation Z p '.

[0137] The embodiment of the present disclosure further eliminates the error in the refractive corrected bed surface elevation, which is related to the refractive correction itself and the bed surface material, through step (7). This error has a linear relationship with the water depth, so it can be corrected by the provided linear formula.

[0138] In one embodiment of the measurement method of the present disclosure, the underwater three-dimensional terrain of the gravel bed surface is measured in an indoor water tank. The gravel particles constituting the bed surface have a particle size range of 9mm-50mm, and the median particle size is about 15mm. The bed surface width is 2.2m, and the measurement length along the path is 4m. The test uses a single camera scheme (Canon 80D SLR), and the camera distance from the bed surface is about 3.4m. The camera focal length is fixed at 18mm, the photosensitive element size is 22.3x14.9mm 2 , and the photo resolution is 6000x4000pix 2 . Four control points are arranged in the measurement area, two on each bank, and the control points are made of PVC plastic plates with a side length of 10cm, located on the bank near the water surface, and the two control points on the same bank are spaced 2m apart in the flow direction (X-axis direction). The true coordinates of the control points are measured using a total station, with an accuracy of 3mm. This embodiment illustrates the measurement and data processing process of the underwater three-dimensional terrain of the gravel bed surface.

[0139] The measurement process is as follows:

[0140] 1. Turn on the sink circulation system and set the inflow flow rate to 40 L / s. After the flow rate stabilizes, start measuring. The horizontal and vertical moving space intervals of the camera are both 0.5 m. Set the slider of the horizontal sliding rail to step motion, and stop for 1 s between adjacent steps. Set the aperture to 4.0, the shutter to higher than 1 / 80 s, and ISO to 1600-2000 during the shooting process. Collect a total of 101 images of the target area.

[0141] 2. Use the SfM method to reconstruct the obtained images in three dimensions to obtain a sparse three-dimensional point cloud. Manually mark the center positions of the control points in each image as the image coordinates of the control points. Use the correspondence between the real coordinates and the image coordinates of the control points to spatially register the sparse three-dimensional point cloud. Based on the registered three-dimensional point cloud, use the MVS method to encrypt and calculate to obtain an initial encrypted three-dimensional point cloud. See Fig. 1(a), and generate the corresponding digital orthographic image with a spatial resolution of 0.65 mm / pix. Figure 4

[0142] 3. In the digital orthographic image, manually calibrate the water surface boundary points using single-point calibration to obtain the spatial coordinates of 195 water surface boundary points and export them. Use the Cloud Compare software to generate a Delaunay triangular mesh based on the water surface boundary points as a water surface model. Perform 0.005 m gridding processing on the initial encrypted three-dimensional point cloud. The bed elevation value of the grid is the maximum elevation of all point cloud points contained in the grid range. Obtain the gridded initial three-dimensional point cloud, and calculate its distance to the water surface as the initial water depth. Delete the grid points located above the water surface model according to the standard that the initial water depth is less than 0. Since there are problems in calculating the distance of the triangular mesh at the boundary of the water surface model, generate a spatial fitting plane of the water surface boundary points, test different distance thresholds for secondary screening, and finally delete all grid points above the water surface to obtain grid points located in the underwater area. At this time, the data of the grid points includes grid point coordinates X a , Y a , Z a , water surface elevation Z w , and initial water depth d a .

[0143] 4. Export the grid points as a csv format. Export the camera position (three-dimensional coordinates and three angles representing the orientation) data from the SfM method, and prepare the camera's own photosensitive element data. Based on the above three data, correct the refractive error of the underwater grid points to obtain the corrected water depth of each grid point and the corrected bed elevation Z p . Finally, further correct the bed elevation Z p according to the empirical formula to obtain the final corrected bed elevation Z p ', and generate the underwater three-dimensional terrain. See Fig. 1(b). Figure 4 .​

[0144] In the description of the present specification, the description of the terms "one embodiment", "some embodiments", "exemplary embodiment", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the exemplary description of the above terms does not necessarily mean the same embodiment or example. Also, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0145] Although the embodiments of the present application have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions, and alterations can be made thereto without departing from the principles and spirit of the application, the scope of which is defined by the claims and their equivalents.

Claims

1. A method of measuring the underwater three-dimensional topography of a gravel bed surface, characterized by, The method comprises: (1) setting up an image acquisition platform for obtaining a sequence of underwater images of a gravel bed surface, the image acquisition platform comprising a camera, the sequence of underwater images comprising a plurality of underwater images of the gravel bed surface; (2) arranging and measuring control points to obtain the spatial coordinates of the control points The control points are arranged on both sides of the water surface of the target area, and comprise a flat plate with a regular geometric pattern drawn on the upper surface and a horizontal slope, the geometric pattern having a clear center point position, the center point position being defined as the coordinate of the control point, and at least two control points being required to be visible in each underwater image of the gravel bed surface; The coordinates of the control points are measured by a measuring device, the position of the measuring device being defined as the origin, the horizontal direction along the water flow or the length direction of the water surface of the target area being defined as the X-axis direction, the horizontal direction perpendicular to the X-axis being defined as the Y-axis direction, and the direction perpendicular to the XY plane being defined as the Z-axis direction; (3) determining the height of the camera and the spatial interval of the camera movement (3-1) determining the height of the camera The length direction of the photosensitive element of the camera is arranged along the Y-axis direction, and the width direction is arranged along the X-axis direction, and then the camera is placed at a certain design distance from the bed surface height H, and the actual spatial size covered by each underwater image of the gravel bed surface is calculated according to the size of the photosensitive element of the camera, the focal length of the camera and the certain design height H, and the calculation formula is as follows: Wherein, L and W are the actual spatial dimensions corresponding to the length direction and the width direction of the underwater image of the single pebble bed surface, respectively, l c and w c are the length and the width of the camera photosensitive element, respectively, h is the image distance, and f is the focal length. If the image taken by the camera at the design height H meets the following three requirements at the same time: ① the spatial resolution requirement of the measurement result is met, ② the ratio of the water surface width of the target area to the length of the underwater image of the single gravel bed surface on which the target area is located should not exceed a first threshold value, and ③ the center point of each control point is clearly visible, then the height of the camera is determined as H, otherwise, the height of the camera is adjusted until the above three requirements are met at the same time; the spatial resolution Res of the measurement result is calculated according to the following formula: wherein, l p is the number of pixels in the length direction of the single camera image; (3-2) determining the spatial interval of the camera movement According to the overlapping area of the adjacent images in the X-axis and Y-axis directions being not less than a second threshold, respectively determine the spatial interval L of the camera moving along the X-axis direction X and the spatial interval L of the camera moving along the Y-axis direction Y ; (4) taking images of the water surface of the target area by cross-section based on the parameters determined in step (3), to obtain a sequence of underwater images of the gravel bed surface During the shooting process, first, the camera is moved to the vicinity of the uppermost cross section of the target area water surface; for each cross section measurement, the focal length of the camera is fixed, and the lens of the camera is perpendicular to the water surface, the camera is moved at a fixed interval L Y from one side of the cross section to the other side of the cross section Y , and then the camera is stopped to shoot a clear image, and then the camera is moved by a fixed interval L Y , and so on, until the shooting of one cross section is completed, and then the camera is moved by a fixed interval L X along the X-axis direction, and the shooting of the next cross section is started, wherein the cross section position X n at which the camera starts to shoot and the cross section position X min at which the camera stops to shoot should satisfy: X max ≤X n , X min and X max are the minimum value and the maximum value of the water surface of the target area along the X-axis direction respectively, and n is the number of cross sections. All the images taken by the camera form a sequence of underwater images of the gravel bed surface; (5) obtaining a three-dimensional point cloud by modeling the underwater terrain based on the sequence of underwater images of the gravel bed surface using a motion recovery structure method, and performing spatial data registration on the three-dimensional point cloud using the control points, obtaining an initial encrypted three-dimensional point cloud based on the registered three-dimensional point cloud, and generating a corresponding digital orthographic image; (6) generating a water surface model of the target area based on the digital orthographic image, filtering out the part of the initial encrypted three-dimensional point cloud located below the water surface model to obtain a three-dimensional point cloud located in the underwater area, and performing refraction correction on the three-dimensional point cloud located in the underwater area to obtain a corrected underwater terrain and an initial water depth; (7) correcting the corrected underwater terrain again using the initial water depth to obtain the underwater three-dimensional terrain measurement result of the gravel bed surface; Step (6) specifically comprises the following steps: In the digital orthographic image, a single point is used to calibrate the water surface boundary point and project it to the initial encrypted three-dimensional point cloud to obtain a series of spatial coordinates of the water surface boundary points, and a Delaunay triangular grid is generated based on the water surface boundary points as a water surface model containing the target area; the initial encrypted three-dimensional point cloud is rasterized, and the bed elevation value of the grid is the maximum elevation of all point cloud points contained in the grid range, to obtain a rasterized initial three-dimensional point cloud; the distance from the rasterized initial three-dimensional point cloud to the water surface model is calculated as the initial water depth; the three-dimensional points in the rasterized initial three-dimensional point cloud with an initial water depth less than 0 are deleted to obtain grid points located in the underwater area; for each camera position corresponding to an image, the actual spatial range corresponding to the image taken by the camera position is calculated according to the camera orientation angle calculated by the motion recovery structure method and the focal length and size of the camera photosensitive element, and the grid points falling within the actual spatial range are considered to be visible to the camera position; For each filtered grid point located in the underwater area, the initial water depth is corrected according to the following formula to obtain the corrected underwater topography and initial water depth: d a = Z w - Z a wherein j represents a certain grid point located in the underwater area is visible to the jth camera position, r j is the angle between the camera optical axis and the water surface at the jth camera position, X cj , Y cj are the X-axis direction and Y-axis direction coordinates of the jth camera position, H wj is the vertical distance from the jth camera position to the water surface model, X a , Y a , Z a are the X-axis direction, Y-axis direction and Z-axis direction coordinates of the certain grid point, d a is the initial water depth of the certain grid point, d j is the corrected water depth of the certain grid point calculated according to the jth camera position, is the average corrected water depth calculated according to the camera positions corresponding to all the images containing the certain grid point, Z w is the elevation of the water surface model corresponding to the horizontal position of the certain grid point, Z p is the corrected bed surface elevation of the certain grid point, n1, n2 are the refractive indexes of water and air respectively; In step (6), when the grid points located in the underwater area obtained after deleting the three-dimensional points in the rasterized initial three-dimensional point cloud with an initial water depth less than 0 still contain some grid points actually located above the water surface model, a spatial fitting plane of the water surface boundary points is generated, the distance from the rasterized initial three-dimensional point cloud to the fitting plane is calculated, and the water surface three-dimensional points are secondarily screened using this distance, different screening thresholds are tested until the grid points located above the water surface model are completely removed, thereby finally obtaining the grid points located in the underwater area; and / or When the camera focal plane is parallel to the water surface, the following approximate correction method is used to calculate the initial water depth of each grid point in the underwater area: Z p = Z w - d; Step (7) specifically includes: The bed elevation Z of each point cloud grid in the underwater region after refraction correction. p The final corrected bed elevation Z is obtained by further correction using the following formula. p ': ΔZ = -0.0049d-0.0019 Z p ' = Z p - ΔZ The underwater three-dimensional topography of the gravel bed is constituted by the X-axis direction coordinate X of all the grid points a , the Y-axis direction coordinate Y a , and the final corrected bed surface elevation Z p .

2. The measurement method according to claim 1, characterized in that, The image acquisition platform built in step (1) further includes: A longitudinal module including longitudinal slides placed on both sides of the water surface of the target area in the direction of the water flow and longitudinal sliders arranged on the longitudinal slides and movable thereon; A transverse module including a transverse slide placed horizontally in the direction perpendicular to the water flow and a transverse slider arranged on the transverse slide and movable thereon, the end of the transverse slide is connected with the longitudinal slider, and the position of the transverse slide in the horizontal direction along the water flow is changed by the longitudinal module; A vertical module including a vertical slide moving with the transverse slider, a vertical slider arranged on the vertical slide and movable thereon, and a bracket connected with the vertical slider and moving with it, the end of the bracket is installed with a gimbal for fixing the camera; and A control module for controlling the movement mode, movement speed and target position of the longitudinal module, the transverse module and the vertical module, and controlling the shooting parameters of the camera and receiving the underwater image sequence shot by the camera.

3. The measurement method according to claim 2, characterized in that, The longitudinal module, the transverse module and the vertical module are respectively provided with positioners for determining the positions of the sliders, and each positioner is realized by a point laser range finder fixed at one end of the corresponding slide rail.

4. The measurement method according to claim 1, characterized by, The camera is replaced by a drone provided with a camera.

5. The measurement method according to claim 1, characterized by, In step (2), the measuring device is an RTK GPS, a total station or a laser scanner.

6. The measurement method of claim 1, wherein, In step (4), let the number of images taken at the i-th cross section be m i , and satisfy the following relationship: WS i + 6L Y ≤ L + (m i - 1)L Y , i ∈ [1, n], WS i is the maximum water surface width at the i-th cross section.

7. The measurement method according to claim 1, characterized by, Step (5) specifically comprises the following steps: The underwater image sequence is calculated based on a motion recovery structure method, the spatial positions of the cameras corresponding to the images and the orientations of the lenses are reconstructed, a sparse three-dimensional point cloud of a target region is obtained through feature point matching, the central positions of each control point appearing in the underwater image sequence are identified and marked through an artificial or automatic method, the image coordinates of the control points are obtained, the sparse three-dimensional point cloud is spatially data-registered by using the corresponding relationship between the real coordinates and the image coordinates of the control points, an initial encrypted three-dimensional point cloud is obtained based on the registered three-dimensional point cloud by using an encryption method, and a corresponding digital orthographic image is generated.

Images