Multi-beam sounding system error correction method based on sounding discrepancy value angle correlation

Abstract

The present application belongs to the technical field of marine acoustic equipment, and particularly relates to a multi-beam sounding system error correction method based on sounding non-conformity value angle correlation, to solve the technical problems that the existing multi-beam sounding system sounding data error elimination algorithm does not consider the formation mechanism and characteristics of system error, and the error suppression effect is poor. The present application provides a multi-beam sounding system error correction method based on sounding non-conformity value angle correlation, which is derived from the sounding point three-dimensional coordinate reduction principle and error propagation theory, and according to the correlation between the system error caused by multi-element random error to the sounding and the beam angle, an error-angle correlation model is constructed in the multi-beam cross line area, and then the system error in the sounding data is weakened in the whole area, the applicability of the method to weaken the system error and the sounding precision are improved, the authenticity of the multi-beam sounding data reflecting the underwater topography is improved, and it has important practical significance.

Description

Technical Field

[0001] The present invention belongs to the technical field of marine acoustic equipment, and in particular relates to a multi-beam bathymetric system error correction method based on bathymetric discrepancy angle correlation. Background Art

[0002] Multibeam is a commonly used device for bathymetry. It's an integrated system composed of multiple systems, and its bathymetry accuracy is affected by the combined measurement errors of each component. Each component, such as the sound velocity error generated by the sound velocity profiler, the ship attitude measurement error generated by the attitude sensor, the positioning error generated by the GNSS, the azimuth error derived from the compass, and the installation deviation detection error, all contribute to random measurement errors. However, according to the principle of bathymetry data reduction, these random errors can have a systematic impact on bathymetry, manifesting as systematic bathymetry errors. In a single measurement (a single ping measurement), this manifests as "crying face" or "smiling face" terrain. In the common coverage area of ​​a multibeam bathymetry swath, this manifests as inconsistent bathymetry data, which is difficult to eliminate and seriously affects the bathymetry data's representation of the actual terrain.

[0003] To eliminate the influence of the above factors, two main methods are currently used to suppress the inconsistency of bathymetric data in the common coverage area of ​​different multi-beam strips: the averaging method and the terrain trend method. The averaging method directly averages the bathymetric data from different strips at the same point to eliminate the bathymetric discrepancy between the two. The terrain trend method takes advantage of the high bathymetric accuracy of the central beam of a multi-beam strip, while the bathymetric accuracy of the edge beam is low. It uses the bathymetric data of the central beams of adjacent strips to fit a polynomial model, construct a terrain trend, and combines it with the details of the terrain changes of the edge beams to restore the bathymetric data of the edge beams, thereby eliminating abnormal bathymetric data. Both methods achieve a certain effect in reducing the bathymetric system error under specific conditions, but they mainly reduce the error from the perspective of geometric changes, without considering the formation mechanism and characteristics of the systematic error. As a result, the suppression effect of the multi-beam bathymetric system error is poor. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to address the defects involved in the background technology and provide a multi-beam sounding system error correction method based on the correlation of sounding discrepancy angles, so as to solve the technical problems that the sounding data error elimination algorithm of the existing multi-beam sounding system does not take into account the formation mechanism and characteristics of the system error, and the error suppression effect is poor.

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0006] The present invention provides a multi-beam bathymetric system error correction method based on bathymetric discrepancy angle correlation, comprising the following steps:

[0007] Step S1: For any measuring line L1 and the inspection line L1 that intersects it,ck , establishing a computational grid with a first interval in a common coverage area of ​​the beam strips of the two multi-beam survey lines;

[0008] Step S2, for test L1 and inspection line L ck The wave velocity strips of the two survey lines are located at the sounding points within the calculation grid to respectively construct a first grid and a second grid;

[0009] Step S3, using the depth value of the depth measurement point of the measurement line L1, obtain the first depth value of each grid point in the calculation grid; using the depth value of the depth measurement point of the inspection line L ck The depth value of the sounding point is obtained, and the second depth value of each grid point in the calculation grid is obtained;

[0010] Step S4, obtaining the discrepancy value of the bathymetric data of the grid with the same name;

[0011] Step S5, obtaining the beam angle corresponding to the depth measurement value of the first grid;

[0012] Step S6, fitting the discrepancy value of the bathymetric data of the grid with the same name with the beam angle corresponding to the bathymetric value of the first grid to obtain a bathymetric discrepancy angle correlation model;

[0013] Step S7: correcting the depth values ​​of the measuring points of the survey line using the depth measurement discrepancy angle correlation model.

[0014] Furthermore, the first grid is specifically a first square area in the beam strip of the survey line L1, with each grid point of the calculation grid as the center and the first interval as the side length;

[0015] The second grid is specifically the inspection line L ck In the beam strip, a second square area is defined with each grid point of the calculation grid as the center and the first interval as the side length.

[0016] Furthermore, in step S3, the first depth value of each grid point in the calculation grid is obtained by using the depth value of the sounding point of the survey line L1, which specifically includes:

[0017] The average of the depth values ​​of the beam points in which the number of the survey lines L1 in each first square area of ​​the first grid is greater than the first preset number is used as the first depth value of each grid point in the calculation grid.

[0018] Furthermore, in step S3, the first depth value of each grid point in the calculation grid is obtained by using the depth value of the sounding point of the survey line L1, which specifically includes:

[0019] The weighted average of the depth values ​​of beam points whose number of survey lines L1 in each first square area of ​​the first grid is greater than a second preset number and the inverse of their distances to the grid points is used as the first depth value of each grid point in the calculation grid.

[0020] Furthermore, in step S3, the inspection line L is used ck The method further comprises: obtaining a second depth value of each grid point in the calculation grid by calculating the depth value of the sounding point; and obtaining a second depth value of each grid point in the calculation grid.

[0021] The inspection line L in each second square area of ​​the second grid ck The average of the depth values ​​of the beam points whose number is greater than the third preset number is used as the second depth value of each grid point in the calculation grid.

[0022] Furthermore, in step S3, the inspection line L is used ck The method further comprises: obtaining a second depth value of each grid point in the calculation grid by calculating the depth value of the sounding point; and obtaining a second depth value of each grid point in the calculation grid.

[0023] The inspection line L in each second square area of ​​the second grid ck A weighted average of the depth values ​​of the beam points whose number is greater than a fourth preset number and the reciprocals of their distances to the grid points is used as the second depth value of each grid point in the calculation grid.

[0024] Furthermore, the discrepancy value of the bathymetric data of the grids with the same name is specifically the difference between the first depth value and the second depth value of the grid points of the calculation network corresponding to the grids at the same position of the first grid and the second grid.

[0025] Furthermore, the step S5 of obtaining the beam angle corresponding to the bathymetric value of the first grid specifically includes:

[0026] Step S51, obtaining beam angles of two adjacent beam points in the same Ping section in each first square area of ​​the first grid;

[0027] Step S52: Using the beam angles of the two adjacent beam points, interpolation is performed to obtain the beam angle corresponding to the depth measurement value of each grid point of the calculation grid.

[0028] Furthermore, the fitting is a quadratic polynomial fitting.

[0029] Furthermore, the first interval is twice the average interval of the sounding points.

[0030] The beneficial effects of the present invention are as follows: The present invention provides a multi-beam bathymetry system error correction method based on the correlation of bathymetry discrepancy angles. Starting from the principle of three-dimensional coordinate reduction of bathymetry points and the error propagation theory, according to the correlation between the systematic error and beam angle brought to bathymetry by multivariate random errors, an error-angle correlation model is constructed in the multi-beam cross-survey line area, thereby achieving full-area weakening of the systematic error in the bathymetry data, improving the applicability of the method for weakening the systematic error and the bathymetry accuracy, and improving the authenticity of the multi-beam bathymetry data in reflecting the underwater terrain, which has important practical significance. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a flow chart of a multi-beam bathymetric system error correction method based on bathymetric discrepancy angle correlation according to the present invention;

[0032] Figure 2 This is a schematic diagram of a computational network construction in a multi-beam bathymetric system error correction method based on bathymetric discrepancy angle correlation according to the present invention;

[0033] Figure 3 This is a schematic diagram of the construction of the first grid and the second grid in a multi-beam bathymetric system error correction method based on bathymetric discrepancy angle correlation of the present invention;

[0034] Figure 4 Schematic diagram of the discrepancy value of the sounding data in the multi-beam sounding system error correction method based on the correlation of the sounding discrepancy value angle of the present invention;

[0035] Figure 5 1 is a flow chart of step S5 in a multi-beam bathymetric system error correction method based on bathymetric discrepancy angle correlation according to the present invention;

[0036] Figure 6 The present invention is a multi-beam sounding system error correction method based on the correlation of sounding discrepancy angles, which uses interpolation to obtain the beam angle corresponding to the sounding value of each grid point in the calculation grid. DETAILED DESCRIPTION

[0037] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0038] The embodiment of the present invention provides a multi-beam bathymetric system error correction method based on bathymetric discrepancy angle correlation, the process is as follows: Figure 1To address the technical issues of existing multi-beam bathymetry systems' bathymetry data error elimination algorithms failing to consider the formation mechanism and characteristics of systematic errors, resulting in poor error suppression, the present invention, based on the principle of three-dimensional coordinate reduction of bathymetry points and error propagation theory, and based on the correlation between the systematic error and beam angle caused by multivariate random errors in bathymetry, constructs an error-angle correlation model in the multi-beam cross-survey line area, and proposes a multi-beam bathymetry system error correction method based on the correlation of bathymetry discrepancy angles.

[0039] A multi-beam bathymetry system error correction method based on bathymetry discrepancy angle correlation in this embodiment includes the following steps:

[0040] Step S1: For any measuring line L1 and the inspection line L1 that intersects it, ck A computational grid with a first interval is established in a common coverage area of ​​the beam strips of the two multi-beam survey lines.

[0041] Step S2: for the measuring line L1 and the inspection line L ck The beam strips of the two survey lines are located at the sounding points within the calculation grid to respectively construct a first grid and a second grid;

[0042] Step S3, using the depth value of the depth measurement point of the measurement line L1, obtain the first depth value of each grid point in the calculation grid; using the depth value of the depth measurement point of the inspection line L ck The depth value of the sounding point is obtained, and the second depth value of each grid point in the calculation grid is obtained;

[0043] Step S4, obtaining the discrepancy value of the bathymetric data of the grid with the same name;

[0044] Step S5, obtaining the beam angle corresponding to the depth measurement value of the first grid;

[0045] Step S6, fitting the discrepancy value of the bathymetric data of the grid with the same name with the beam angle corresponding to the bathymetric value of the first grid to obtain a bathymetric discrepancy angle correlation model;

[0046] Step S7: correcting the depth values ​​of the measuring points of the survey line using the depth measurement discrepancy angle correlation model.

[0047] This embodiment is a multi-beam sounding system error correction method based on the correlation of sounding discrepancy angle. The algorithm principle is to rely on the multi-beam survey line L1 and the inspection line L1 intersecting with it. ck , a computational grid is constructed on the strip terrain formed by the two multi-beams, and the terrain belonging to the survey line L1 and the inspection line L ckGrids are constructed for the sounding points of each grid, and the sounding discrepancies of the grids with the same name are calculated. Based on the principle of sounding, the corresponding beam angles of the sounding values ​​of the grids with the same name on the multi-beam survey line are obtained. The depth discrepancy value and the corresponding beam angle of each grid point with the same name are fitted to obtain the sounding discrepancy angle correlation model. Finally, the sounding discrepancy angle correlation model is used to eliminate the error of the sounding system.

[0048] Step S1 is mainly to divide the computational grid. The schematic diagram of computational grid construction is as follows: Figure 2 As shown, for any multi-beam survey line L1 and the inspection line L that intersects it ck In a preferred embodiment, the multi-beam measuring line L1 and the inspection line L ck Orthogonal. Between the measuring line L1 and the inspection line L ck The beam strips of these two multi-beam lines ( Figure 2 A calculation grid with a first interval is established in the common coverage area (shaded area) of the calculation grid. The size of the first interval of the calculation grid can be set to twice the average interval of the sounding points, thereby forming a regular grid.

[0049] Step S2: for the measuring line L1 and the inspection line L ck The beam strips of the two survey lines are located at the depth measurement points within the calculation grid to respectively construct a first grid and a second grid, thereby forming respective DDMs (Digital Depth Models).

[0050] The first grid is specifically a first square area in the beam strip of the measurement line L1, with each grid point of the calculation grid as the center and the first interval as the side length; the second grid is specifically a first square area in the beam strip of the measurement line L1, with each grid point of the calculation grid as the center and the first interval as the side length; ck In the beam strip of , a second square area with each grid point of the calculation grid as the center and the first interval as the side length is formed, such as Figure 3 As shown. Thus, the measuring line L1 and the inspection line L ck The beam strips of the two multi-beam survey lines respectively construct the first grid and the second grid consisting of a square area with the first interval as the side length, that is, the first grid and the second grid coincide with the calculation network and have the same grid size. The only difference is that the first grid is located in the beam strip of the survey line L1, and the second grid is located in the beam strip of the inspection line L ck beam strips.

[0051] Step S3: Use the measuring line L1 and the inspection line L ck The depth value of the sounding point is obtained, and the first depth value of each grid point in the calculation grid measured on the survey line L1 and the first depth value measured on the inspection line L ck The second depth value is measured, thereby obtaining two sets of bathymetric data for the grid with the same name.

[0052] Specifically, using the depth value of the sounding point of the survey line L1 to obtain the first depth value of each grid point in the calculation grid includes:

[0053] The average of the depth values ​​of the beam points in which the number of the survey lines L1 in each first square area of ​​the first grid is greater than the first preset number is used as the first depth value of each grid point in the calculation grid.

[0054] Alternatively, the weighted average of the depth values ​​of beam points whose number of survey lines L1 in each first square area of ​​the first grid is greater than a second preset number and the inverse of their distances to the grid points is used as the first depth value of each grid point in the calculation grid.

[0055] Using the inspection line L ck The method further comprises: obtaining a second depth value of each grid point in the calculation grid by calculating the depth value of the sounding point; and obtaining a second depth value of each grid point in the calculation grid.

[0056] The inspection line L in each second square area of ​​the second grid ck The average of the depth values ​​of the beam points whose number is greater than the third preset number is used as the second depth value of each grid point in the calculation grid.

[0057] Alternatively, the weighted average of the depth values ​​of beam points whose number of inspection lines Lck in each second square area of ​​the second grid is greater than a fourth preset number and the inverse of their distances to the grid points is used as the second depth value of each grid point in the calculation grid.

[0058] like Figure 3 As shown, for each grid point in the computational grid, located at the center point of the first grid / second grid, the first depth value and the second depth value of the grid point can be respectively obtained using the depth values ​​of the beam points of the survey line / inspection line within the square area within the first grid / second grid. Specifically, the depth value of the grid point can be obtained by taking the average of the depth values ​​of a number of beam points greater than a preset number within the first square area / second square area, or by taking the weighted average of the depth values ​​and the inverse of the distances from the corresponding grid points. Obviously, other algorithms can also be used to obtain the depth value of the grid point, and the present invention does not impose any limitation thereto.

[0059] The weighted average is calculated as shown below.

[0060]

[0061] Among them, h0 is the depth value of the center point of the grid to be calculated (first grid / second grid), h i is the depth value of the i-th beam point in the grid to be calculated, p iis the inverse of the distance from the i-th beam point to the corresponding grid point in the grid to be calculated, and n is the number of preset beam points. The first preset number, the second preset number, the third preset number, and the fourth preset number can be the same or different.

[0062] After obtaining the first depth value and the second depth value of each grid point in the calculation grid, the discrepancy value of the bathymetric data of the grid with the same name can be obtained, such as Figure 4 As shown. The grids with the same name in the present invention refer to the grids at the same position in the first grid and the second grid. Specifically, it is the difference between the first depth value and the second depth value of the grid points of the calculation network corresponding to the grids at the same position in the first grid and the second grid, that is:

[0063]

[0064] Where Δh j 0, h j 0-1 and h j 0-ck L1 and L ck The bathymetric discrepancy value of the j-th grid with the same name, the bathymetric value of L1 and L ck The depth value of .

[0065] After obtaining the discrepancy value of the bathymetric data of the same grid, the water depth correction value ΔZ can be calculated based on the depth value difference. i , but missing θ i The effective information of the first grid can be obtained by using the bathymetric principle to obtain the wave velocity angle corresponding to the bathymetric value of the first grid. Figure 5 The process of step S5 is shown, which specifically includes:

[0066] Step S51: Obtain beam angles of two adjacent beam points in the same Ping section in each first square area of ​​the first grid.

[0067] According to the principle of ray tracing, based on the beam angle corresponding to the original actual sounding point in the grid point with the same name, the beam angle corresponding to the depth value of the grid center is obtained by interpolation method.

[0068] In the cross section formed by a multi-beam measurement (Ping cross section), if the initial incident angle θ and beam duration t are known, the interpolation area of ​​the beam point can be determined during interpolation, and the horizontal displacement y and depth z of the beam point can be obtained by interpolation. Similarly, if the position (y, z) of the beam point P is known, the initial incident angle of the beam can also be calculated in reverse. The basic principle is shown in the attached figure. Figure 6 shown.

[0069] Step S52: Using the beam angles of the two adjacent beam points, interpolation is performed to obtain the beam angle corresponding to the depth measurement value of each grid point of the calculation grid.

[0070] If there are two adjacent points k and k+1 in the Ping section, the corresponding coordinates are (0, y, z) k 、(0,y,z) k+1 , then use the corresponding angle θ between points k and k+1 k and θ k+1 , use depth interpolation to get the angle corresponding to the depth value h0 of each grid point of the calculation grid:

[0071]

[0072] Where θ0 is the beam angle corresponding to the grid center depth h0, and the definitions of the other parameters are the same as before.

[0073] Step S6, fitting the discrepancy value of the bathymetric data of the grid with the same name to the beam angle corresponding to the bathymetric value of the first grid, to obtain a bathymetric discrepancy angle correlation model. In a preferred embodiment, the fitting is a quadratic polynomial fitting, that is, constructing a quadratic polynomial model, as shown in the following formula:

[0074] Δh0=f(θ0)=a(θ0) 2 +bθ0+c

[0075] Using the depth discrepancy Δh0 and beam angle θ0 on all grids with the same name, we can form the following equations:

[0076]

[0077] Where n is the number of grids with the same name, and a, b, and c are model parameters.

[0078] Its matrix form is:

[0079] l=HX

[0080] Solve the model parameters X = (a, b, c):

[0081] X=(H T H) - H T l

[0082] The parameters a, b, and c can be obtained using numerical algorithms.

[0083] Step S7: correcting the depth values ​​of the measuring points of the survey line using the depth measurement discrepancy angle correlation model.

[0084] After obtaining the above model parameters, the sounding discrepancy angle correlation model can be used to correct the angle correlation system error for each sounding point in the survey line L1 to eliminate the influence of the systematic error.

[0085] The calculation formula for the depth value of the measuring point of the corrected survey line is:

[0086] h′=h-Δh0=h-(aθ 2 +bθ+c)

[0087] Where h′ is the corrected depth of the sounding point, h is the original sounding point, Δh is the correction value, and θ is the beam incident angle corresponding to the sounding point.

[0088] At this point, the error correction of the multi-beam sounding system is completed.

[0089] In summary, the present invention starts from the principle of three-dimensional coordinate reduction of sounding points and the error propagation theory, and according to the correlation between the systematic error and beam angle brought by multivariate random errors to sounding, by constructing an error-angle correlation model in the multi-beam cross-survey line area, provides a multi-beam sounding systematic error correction method based on the correlation of sounding discrepancy angles, realizes the full-area weakening of systematic errors in sounding data, improves the applicability of the method for weakening systematic errors and sounding accuracy, and improves the authenticity of multi-beam sounding data in reflecting underwater terrain, which has important practical significance.

[0090] Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions, and substitutions are possible, and therefore, the scope of the present invention should not be limited to the above-described embodiments. It will be apparent to those skilled in the art that several modifications may be made without departing from the principles of the present invention, and such modifications should also be considered within the scope of protection of the present invention.

Claims

1. A multi-beam bathymetric system error correction method based on bathymetric discrepancy angle correlation is characterized by: The following steps are involved: Step S1: For any measuring line L1 and the inspection line L1 that intersects it, ck , establishing a computational grid with a first interval in a common coverage area of ​​the beam strips of the two multi-beam survey lines; Step S2: for the measuring line L1 and the inspection line L ck The beam strips of the two survey lines are located at the sounding points within the calculation grid to respectively construct a first grid and a second grid; Step S3, using the depth value of the depth measurement point of the measurement line L1, obtain the first depth value of each grid point in the calculation grid; using the depth value of the depth measurement point of the inspection line L ck The depth value of the sounding point is obtained, and the second depth value of each grid point in the calculation grid is obtained; Step S4, obtaining the discrepancy value of the bathymetric data of the grid with the same name; Step S5, obtaining the beam angle corresponding to the depth measurement value of the first grid; Step S6, fitting the discrepancy value of the bathymetric data of the grid with the same name with the beam angle corresponding to the bathymetric value of the first grid to obtain a bathymetric discrepancy angle correlation model; Step S7: correcting the depth values ​​of the measuring points of the survey line using the depth measurement discrepancy angle correlation model.

2. The multi-beam bathymetric system error correction method based on bathymetric discrepancy angle correlation according to claim 1, characterized in that: The first grid is specifically a first square area in the beam strip of the survey line L1, with each grid point of the calculation grid as the center and the first interval as the side length; The second grid is specifically the inspection line L ck In the beam strip, a second square area is defined with each grid point of the calculation grid as the center and the first interval as the side length.

3. The multi-beam bathymetric system error correction method based on bathymetric discrepancy angle correlation according to claim 2, characterized in that: In step S3, the first depth value of each grid point in the calculation grid is obtained by using the depth value of the sounding point of the survey line L1, which specifically includes: The average of the depth values ​​of the beam points in which the number of the survey lines L1 in each first square area of ​​the first grid is greater than the first preset number is used as the first depth value of each grid point in the calculation grid.

4. The multi-beam bathymetric system error correction method based on bathymetric discrepancy angle correlation according to claim 2, characterized in that: In step S3, the first depth value of each grid point in the calculation grid is obtained by using the depth value of the sounding point of the survey line L1, which specifically includes: The weighted average of the depth values ​​of beam points whose number of survey lines L1 in each first square area of ​​the first grid is greater than a second preset number and the inverse of their distances to the grid points is used as the first depth value of each grid point in the calculation grid.

5. The multi-beam bathymetric system error correction method based on bathymetric discrepancy angle correlation according to claim 2, characterized in that: In the step S3, the inspection line L is used ck The depth value of the sounding point is obtained, and the second depth value of each grid point in the calculation grid is obtained, specifically including: The inspection line L in each second square area of ​​the second grid ck The average of the depth values ​​of the beam points whose number is greater than the third preset number is used as the second depth value of each grid point in the calculation grid.

6. The multi-beam bathymetric system error correction method based on bathymetric discrepancy angle correlation according to claim 2, characterized in that: In the step S3, the inspection line L is used ck The depth value of the sounding point is obtained, and the second depth value of each grid point in the calculation grid is obtained, specifically including: The weighted average of the depth values ​​of beam points whose number of inspection lines Lck in each second square area of ​​the second grid is greater than a fourth preset number and the inverse of their distances to the grid points is used as the second depth value of each grid point in the calculation grid.

7. The multi-beam bathymetric system error correction method based on bathymetric discrepancy angle correlation according to claim 2, characterized in that: The discrepancy value of the bathymetric data of the grids with the same name is specifically the difference between the first depth value and the second depth value of the grid points of the calculation network corresponding to the grids at the same position of the first grid and the second grid.

8. The multi-beam bathymetric system error correction method based on bathymetric discrepancy angle correlation according to claim 7, characterized in that: The step S5 of obtaining the beam angle corresponding to the bathymetric value of the first grid specifically includes: Step S51, obtaining beam angles of two adjacent beam points in the same Ping section in each first square area of ​​the first grid; Step S52: Using the beam angles of the two adjacent beam points, interpolation is performed to obtain the beam angle corresponding to the depth measurement value of each grid point of the calculation grid.

9. The multi-beam bathymetric system error correction method based on bathymetric discrepancy angle correlation according to claim 7, characterized in that: The fit is a quadratic polynomial fit.

10. The multi-beam bathymetric system error correction method based on bathymetric discrepancy angle correlation according to any one of claims 1 to 9, characterized in that: The first interval is twice the average interval of the sounding points.

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