A method for retrieving water depth by using airborne laser radar sea surface point cloud

Abstract

The application discloses a method for retrieving water depth by using airborne laser radar sea surface point cloud. The method carries out pretreatment on the original point cloud data obtained by the airborne laser radar, and extracts the sea surface signal point cloud; carries out two-dimensional Fourier transform on the sea surface signal point cloud, and extracts the wave length and wave direction information of the sea surface at different positions; establishes the relationship between the wave length, period and water depth of the sea surface based on the linear wave theory, estimates the wave period through the wave characteristics of the deep water area; and calculates the water depth result of the near-shore water area through the dispersion relationship by using the tracked near-shore wave length and wave direction information. Since the laser energy is rapidly attenuated by turbid water, the traditional laser depth radar cannot directly detect the water bottom signal in the turbid water area, the sea surface point cloud data obtained by the airborne laser radar is used to calculate the sea surface wave parameters and then retrieve the water depth, and the method is not affected by water quality, and can effectively fill the underwater topographic data blank in the turbid water area.

Description

Technical Field

[0001] This invention belongs to the field of laser remote sensing technology, specifically relating to a method for inverting water depth using airborne lidar point clouds over the sea surface. Background Technology

[0002] Nearshore bathymetry is a fundamental dataset for planning marine engineering projects, developing hydrodynamic models, investigating coastal ecosystems, and other coastal applications. Traditional methods of bathymetry primarily utilize bathymetry equipment (such as sounding rods, sounding hammers, echo sounders, and multibeam echo sounders) and positioning devices (such as sextants, radar locators, and GPS) mounted on survey vessels to create a network of measurement points in the bathymetry area, thereby obtaining underwater topographic data for the study area. In recent years, airborne lidar bathymetry systems have emerged, using blue-green band laser sources with low water attenuation coefficients to obtain the water depth at the measurement location based on the pulse intervals returned from the water surface and bottom. Currently, airborne bathymetry systems can measure water depths up to 50 meters in clear water. Compared to shipborne bathymetry systems, airborne systems offer advantages such as higher measurement accuracy, higher measurement point density, higher efficiency, greater mobility, and greater measurement continuity, making them more suitable for shallow water depth measurements in complex terrain areas such as islands, reefs, and coastlines. However, in turbid water areas, even with blue-green light wavelengths, laser energy is rapidly attenuated by the water, making it impossible to detect underwater signals and hindering the measurement of water depth and underwater topography.

[0003] Currently, many wave theories describing the propagation and evolution of ocean surface waves in ocean and coastal waters have been developed. During propagation from the deep sea to the nearshore, ocean waves are influenced by various factors such as currents, seabed topography, and bottom friction, resulting in physical phenomena such as shallow-water deformation, refraction, reflection, diffraction, and energy dissipation. Their propagation speed, wavelength, and wave surface shape all undergo significant changes, leading to alterations in a series of characteristic wave parameters. Due to the rapid changes in underwater depth, nearshore waters generate extensive surface features, causing waves to refract and eventually align parallel to the coastline. A significant change is that the wavelength of ocean gravity waves decreases significantly upon reaching shallow waters, and this wavelength change exhibits a strong correlation with seabed topography. This provides the possibility of estimating underwater topography through ocean surface wave characteristics.

[0004] Previous studies have combined ocean linear wave theory with SAR data to observe wavelengths and estimate water depth. Compared to the nonlinear imaging of ocean waves by SAR, airborne lidar data provides high-resolution three-dimensional information of the ocean surface, thus offering more accurate wave characteristics, with a more pronounced advantage in shallow water areas. Based on airborne lidar data, the wave direction and wavelength variation trends of wave propagation in coastal areas can be visually observed. According to the water wave dispersion relationship of linear wave theory, water depth can be inverted by analyzing wavelength variations in near-shore areas, obtaining high-resolution underwater topography of near-shore areas. This method is unaffected by water quality and can effectively fill the data gaps in underwater topography in turbid water areas. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for inverting water depth using airborne lidar point clouds over the sea surface, comprising the following steps:

[0006] Step 1: Extract the three-dimensional signal point cloud of the ocean surface from the three-dimensional point cloud data acquired by the airborne lidar, and resample it;

[0007] Step 1.1: Preprocess the raw point cloud data acquired by the airborne lidar to extract the ocean surface signal point cloud;

[0008] Step 1.2: Resample the ocean surface signal point cloud extracted in Step 1.1;

[0009] Step 2: Perform a two-dimensional Fourier transform on the ocean surface signal point cloud to extract wave wavelength and wave direction information at different locations on the sea surface. Based on linear wave theory, establish the relationship between sea surface wave wavelength, period and water depth. Estimate the wave period through the wave characteristics of deep water areas. Utilize the tracked nearshore wave wavelength and wave direction information to calculate the nearshore water depth through dispersion relation.

[0010] Step 2.1: Perform a two-dimensional Fourier transform on the ocean surface signal point cloud in the deep water area to calculate the wave period;

[0011] Step 2.2: In the shallow water area, perform two-dimensional Fourier transform on the ocean surface signal point cloud within the moving planar box along a single wave ray to extract the wave wavelength and wave direction information at different locations on the sea surface. Then, use the dispersion relation to calculate the shallow water depth along the single wave ray.

[0012] Step 3: Use multiple wave rays to cover the entire sea area to be measured, repeat the calculation process of inverting the water depth on a single wave ray in Step 2, and invert the water depth on multiple wave rays to obtain the water depth result of the entire sea area to be measured.

[0013] Furthermore, in step 1.1, the minimum elevation Z of the water surface signal point cloud is initially set.min and maximum elevation Z max The calculated elevation range is in [Z]. min Z max The mean μ and standard deviation σ of the point cloud signal are used. Using the 3σ criterion in the normal distribution, the elevation range of the water surface signal point cloud is set to [μ-3σ, μ+3σ]. Noise point clouds outside this elevation range are filtered out to extract the three-dimensional signal point cloud of the ocean surface.

[0014] Furthermore, in step 1.2, the extracted ocean surface signal point cloud in the sea area to be measured is divided into spatial grids. The intervals of each spatial grid in the x and y directions are Δx and Δy, respectively. The positive x direction is defined from west to east and the positive y direction is defined from south to north. The average elevation of the ocean surface signal point cloud in each grid is calculated and the obtained average value is recorded as the elevation value of the grid point.

[0015] Furthermore, in step 2.1, a two-dimensional Fourier transform is performed on the ocean surface signal point cloud in the deep-water area to be measured, obtaining the wavenumber spectrum distribution of the deep-water ocean surface signal point cloud, and extracting the wavenumber spectrum peak value k of the deep-water area. deep According to the relationship between wavelength and wavenumber L deep ×k deep =2π, the wavelength value L in the deep water region is calculated. deep .

[0016] For ocean surface gravity waves, the dispersion relation between wave wavelength L and period T at water depth D satisfies the following equation:

[0017]

[0018] In the formula, g is the gravitational acceleration constant, tanh is the hyperbolic tangent function, and π is the constant of pi.

[0019] In the deep water region, i.e. when D / L>1 / 2, tanh(2πD / L)≈1, and formula (1) simplifies to:

[0020]

[0021] According to formula (2), the wavelength value L of the deep water region is obtained. deep It can calculate the wave period T.

[0022] Define the positive x-direction as west to east and the positive y-direction as south to north. Starting from any point on the shore within the sea area to be measured, draw a straight line parallel to either the x- or y-direction towards the open sea as the wave ray. Starting from the nearshore sea surface, select a planar rectangular region of size a×a. Perform a two-dimensional Fourier transform on the three-dimensional signal point cloud of the ocean surface within the planar rectangular region to obtain the two-dimensional wavenumber spectrum distribution of the signal point cloud within the region. Extract the wavenumber k in the x-direction corresponding to the peak value. x and the wave number k in the y direction y The wave value at the spectral peak was calculated. Based on the relationship between wavelength and wave value L×k max =2π, calculate the wavelength L within the box, and simultaneously obtain the wave direction within the box. The wave direction is the angle between the wave propagation direction and the y-direction.

[0023] The formula for calculating water depth D is obtained by transforming equation (1):

[0024]

[0025] In the formula, arctanh is the inverse hyperbolic tangent function, and ω is the wave frequency, which is calculated from the wave period T according to the relationship T×ω=2π.

[0026] According to formula (3), the water depth D within the square area can be calculated using the wavelength L and wave period T of the square area.

[0027] Furthermore, in step 2.2, the planar frame is moved along a single wave ray, with the frame moving Δl each time. The wavelength and wave direction calculation process described above is repeated within the planar frame after each movement, and the wavelength L and wave direction corresponding to the peak of the sea surface signal wavenumber spectrum within each frame are recorded. To select an accurate wavenumber spectral peak, the wavelength to be tracked is restricted in the wavenumber domain: 1) Set the maximum value L of the tracked wavelength. max =gT 2 / (2π), 2) Limit the range of wave direction variation between adjacent planar boxes, allowing a maximum wave direction variation of . Based on the above constraints, when selecting a spectral peak in the wavenumber spectrum, the wavelength L and wave direction corresponding to the selected peak must first be calculated. Changes in wave direction compared to the previous box Let L be the wave direction value of the sea surface signal within the previous box of the current planar box. If L>L max or Then it is necessary to reselect the next peak in the wavenumber spectrum until the selected peak meets all the constraints. Using the wave period T and the wavelength L at each position on the tracked wave ray, the water depth D at each position on a single wave ray is calculated according to formula (3). Finally, the spatial resolution of the water depth along the wave ray direction is obtained by inversion as Δl.

[0028] Moreover, in step 3, the wave rays are set to be parallel to each other, the distance between adjacent wave rays is Δl, and the number of wave rays is n. By performing two-dimensional Fourier transform and wavelength tracking on the ocean surface signal point cloud in each plane box on the n wave rays, the wavelength of each box is recorded, and then the water depth of each box is calculated by formula (3), thereby obtaining the water depth of the entire sea area to be measured.

[0029] Compared with the prior art, the present invention has the following advantages:

[0030] This invention, based on linear wave theory, establishes the relationship between sea surface wave period, wavelength, and water depth. It uses airborne lidar to acquire wave information from the ocean surface, thereby retrieving nearshore water depth. The method proposed in this invention is unaffected by water quality parameters and can effectively fill the gaps in underwater topographic data for turbid water areas, providing a new approach to obtaining water depth information in shallow nearshore areas. Attached Figure Description

[0031] Figure 1 This is a technical flowchart of an embodiment of the present invention.

[0032] Figure 2 This is a schematic diagram of the movement of the planar box when inverting the water depth on a single wave ray. The gray bars correspond to the distance between the bottom of the water and the surface of the plane.

[0033] Figure 3 It is the result of wave wavelength and water depth inversion for a single wave rays.

[0034] Figure 4 This is the result of underwater 3D topographic inversion; the gray bars correspond to water depth values. Detailed Implementation

[0035] This invention provides a method for inverting water depth using airborne lidar point cloud data. The following example uses airborne lidar point cloud data obtained from the waters surrounding Ganquan Island in my country as an illustration, and the technical solution of this invention will be further explained in conjunction with the accompanying drawings.

[0036] like Figure 1 As shown, this invention provides a method for inverting water depth using airborne lidar point clouds over the sea surface, comprising the following steps:

[0037] Step 1: Extract the three-dimensional signal point cloud of the ocean surface from the three-dimensional point cloud data acquired by the airborne lidar, and resample it.

[0038] Step 1.1: Preprocess the raw point cloud data acquired by the airborne lidar to extract the ocean surface signal point cloud.

[0039] By observing the three-dimensional point cloud signals acquired by the airborne lidar, the minimum elevation Z of the water surface signal point cloud was initially determined. min and maximum elevation Z max In this embodiment Z min The value is -2.5m, Z max The value is 2.5m, and the calculated elevation range is [Z]. min Z max The mean μ and standard deviation σ of the point cloud signal are used. Using the 3σ criterion in the normal distribution, the elevation range of the water surface signal point cloud is set to [μ-3σ, μ+3σ]. Noise point clouds outside this elevation range are filtered out to extract the three-dimensional signal point cloud of the ocean surface.

[0040] Step 1.2: Resample the ocean surface signal point cloud extracted in Step 1.1.

[0041] To more intuitively observe the wave characteristics of the ocean surface, the extracted ocean surface signal point cloud needs to be resampled. The extracted ocean surface signal point cloud in the sea area to be measured is divided into spatial grids. The intervals of each spatial grid in the x and y directions are Δx and Δy, respectively. In this embodiment, the positive x direction is defined as from west to east, and the positive y direction is defined as from south to north. The values ​​of Δx and Δy are both set to 5m. The average elevation of the ocean surface signal point cloud in each grid is calculated, and the obtained average value is re-recorded as the elevation value of that grid point.

[0042] Step 2: Perform a two-dimensional Fourier transform on the ocean surface signal point cloud to extract wave wavelength and wave direction information at different locations on the sea surface. Based on linear wave theory, establish the relationship between sea surface wave wavelength, period and water depth. Estimate the wave period through the wave characteristics of deep water areas. Utilize the tracked nearshore wave wavelength and wave direction information to calculate the nearshore water depth through dispersion relation.

[0043] Step 2.1: Perform a two-dimensional Fourier transform on the ocean surface signal point cloud in the deep water area to calculate the wave period.

[0044] A two-dimensional Fourier transform is performed on the ocean surface signal point cloud in the deep-water area within the measurement area. In this embodiment, the deep-water area is defined as the sea area with a straight-line distance from the land greater than 2.5 km. The wavenumber spectrum distribution of the ocean surface signal point cloud in the deep-water area is obtained, and the peak value k of the wavenumber spectrum in the deep-water area is extracted. deep According to the relationship between wavelength and wavenumber L deep ×k deep =2π, the wavelength value L in the deep water region is calculated.deep .

[0045] For gravity waves at sea surface, according to linear wave theory, the dispersion relation between wave wavelength L and period T at water depth D satisfies the following equation:

[0046]

[0047] In the formula, g is the gravitational acceleration constant, which is taken as 9.8 m / s² in this embodiment. 2 tanh is the hyperbolic tangent function, and π is the constant of pi, which is taken as 3.1416 in this embodiment.

[0048] Furthermore, in the deep water region, i.e. when D / L>1 / 2, tanh(2πD / L)≈1, then formula (1) can be simplified to:

[0049]

[0050] According to formula (2), the wavelength value L of the deep water region can be obtained. deep The wave period T is calculated. In this embodiment, the calculated wave period is T = 9.8.

[0051] Step 2.2: In the shallow water area, perform two-dimensional Fourier transform on the ocean surface signal point cloud within the moving planar box along a single wave ray to extract the wave wavelength and wave direction information at different locations on the sea surface. Then, use the dispersion relation to calculate the shallow water depth along the single wave ray.

[0052] From any point on the shore within the sea area to be measured, draw a ray parallel to either the x-direction or the y-direction towards the open sea. In this embodiment, the positive x-direction is defined as moving from west to east, and the positive y-direction as moving from south to north. For example... Figure 2 As shown by the dashed line with arrows, the wave rays in this embodiment are parallel to the y-direction. Starting from the nearshore sea surface, a planar rectangle of size a×a is set. In this embodiment, the planar rectangle size is set to 320m×320m. A two-dimensional Fourier transform is performed on the three-dimensional signal point cloud of the ocean surface within a planar rectangle to obtain the two-dimensional wavenumber spectrum distribution of the signal point cloud within the rectangle. To obtain the wave wavelength and wave direction information within the rectangle, it is necessary to identify the wavenumber spectrum peaks and extract the wavenumber k in the x-direction corresponding to the peak. x and the wave number k in the y direction y Then the wave value at the spectral peak can be calculated. Based on the relationship between wavelength and wave value, 2π = k max ×L can be used to calculate the wavelength value L within the box, and also to obtain the wave direction within the box. The wave direction is the angle between the wave propagation direction and the y-direction.

[0053] The formula for calculating water depth D is obtained by transforming equation (1):

[0054]

[0055] In the formula, arctanh is the inverse hyperbolic tangent function, and ω is the wave frequency, which can be calculated from the wave period T according to the relationship 2π=T×ω.

[0056] According to formula (3), the water depth D within the square can be calculated using the wavelength L and wave period T of the square region.

[0057] The movement of the planar box on a single wave ray is as follows: Figure 2 As shown, along a single wave ray ( Figure 2 (Dashed line with arrow) Move the planar box ( Figure 2 The dashed box in the diagram moves Δl each time, which is 20m in this embodiment. The wavelength and wave direction calculation process is repeated within the planar box after each movement, and the wavelength L and wave direction corresponding to the peak of the sea surface signal wavenumber spectrum within each box are recorded. In this process, the local estimation and tracking of wavelength by the theoretical algorithm may be unstable. Therefore, in order to obtain more accurate inversion results, this invention further processes and optimizes the theoretical algorithm, mainly by proposing specific restrictions on the wavenumber spectrum analysis results. In order to select the accurate wavenumber spectrum peak, the wavelength value to be tracked is restricted in the wavenumber domain. Specifically: (1) Set the maximum value L of the tracked wavelength. max =gT 2 / (2π), in this embodiment L max The value is set to 149m; (2) the range of wave direction variation of adjacent planar squares is limited, and the maximum allowable wave direction variation is In this embodiment Set to 0.18 rad.

[0058] Based on the above constraints, when selecting a spectral peak in the wavenumber spectrum, the wavelength L and wave direction corresponding to the selected peak must first be calculated. Changes in wave direction compared to the previous box This represents the wave direction of the sea surface signal within the previous box of the current planar box. If L>L max or Then it is necessary to reselect the next peak in the wavenumber spectrum until the selected peak meets all the constraints.

[0059] Using the wave period T and the wavelength of each position on the tracked wave ray, the water depth D at each position on a single wave ray can be calculated according to formula (3). Finally, the spatial resolution of the water depth along the direction of the wave ray is obtained by inversion as Δl.

[0060] Figure 3 (a) The wavelength value obtained by tracing along the trajectory of a single wave ray. Figure 3 (b) The solid line represents the water depth inversion result on a single wave ray, and the dashed line represents the airborne radar underwater signal point cloud data for the corresponding point. Figure 3 (b) It can be seen that the calculated water depth is very close to the measured water depth. The root mean square error (RMSE) between the calculated inversion result and the actual water depth is 0.944m, and the mean absolute percentage error (MAPE) is 5.23%.

[0061] Step 3: Use multiple wave rays to cover the entire sea area to be measured, repeat the calculation process of inverting the water depth on a single wave ray in Step 2, and invert the water depth on multiple wave rays to obtain the water depth result of the entire sea area to be measured.

[0062] The wave rays are set to be parallel to each other, the distance between adjacent wave rays is set to Δl, and the number of wave rays is n. In this embodiment, n is 16. By performing a two-dimensional Fourier transform on each plane box on the 16 wave rays, wavelength tracking is performed, the wavelength of each box is recorded, and then the water depth of each box is calculated by formula (3), thereby obtaining the water depth of the entire sea area to be measured.

[0063] In step 2.2, the spatial resolution along the direction of the wave rays is Δl, and in step 3, the distance between the wave rays is Δl. Therefore, the spatial resolution of the underwater three-dimensional terrain obtained by inversion is Δl×Δl, which is 20m×20m in this embodiment.

[0064] Figure 4 The water depth inversion results for the area to be measured in this embodiment are obtained using the method proposed in this invention. Underwater point clouds obtained directly from airborne laser echo sounders in the same area are used as verification data to evaluate the feasibility and accuracy of the proposed method. Comparative calculations show that the root mean square error (RMSE) between the underwater topography inversion results and the actual underwater topography in this embodiment is 0.97m, and the mean absolute percentage error (MAPE) is 7.8%. These results demonstrate the feasibility and accuracy of the proposed method. Therefore, this invention can use sea surface point cloud data obtained by airborne laser sounders flying over coastal areas and the observed sea surface wave parameters to calculate the local water depth and estimate the local three-dimensional underwater topography. This method has strong application prospects in turbid water areas that cannot be measured by traditional optical methods.

[0065] The specific embodiments described herein are merely illustrative of the spirit of the invention. Those skilled in the art to which this invention pertains may make various modifications or additions to the described specific embodiments or use similar methods to replace them, without departing from the spirit of the invention or exceeding the scope defined by the appended claims.

Claims

1. A method for inverting water depth using airborne lidar point clouds over the sea surface, characterized in that, Includes the following steps: Step 1: Extract the three-dimensional signal point cloud of the ocean surface from the three-dimensional point cloud data acquired by the airborne lidar, and resample it; Step 1.1: Preprocess the raw point cloud data acquired by the airborne lidar to extract the ocean surface signal point cloud; Step 1.2: Resample the ocean surface signal point cloud extracted in Step 1.1; Step 2: Perform a two-dimensional Fourier transform on the ocean surface signal point cloud to extract wave wavelength and wave direction information at different locations on the sea surface. Based on linear wave theory, establish the relationship between sea surface wave wavelength, period and water depth. Estimate the wave period through the wave characteristics of deep water areas. Utilize the tracked nearshore wave wavelength and wave direction information to calculate the nearshore water depth through dispersion relation. Step 2.1: Perform a two-dimensional Fourier transform on the ocean surface signal point cloud in the deep water area to calculate the wave period; For gravity waves on the ocean surface, the wave wavelength is... L and cycle T At water depth D At this time, the dispersion relation satisfies the following equation: （1） In the formula, g π is the gravitational acceleration constant, tanh is the hyperbolic tangent function, and π is the constant of pi. Step 2.2: In the shallow water area, perform two-dimensional Fourier transform on the ocean surface signal point cloud within the moving planar box along a single wave ray to extract the wave wavelength and wave direction information at different locations on the sea surface. Then, use the dispersion relation to calculate the shallow water depth along the single wave ray. The definition is from west to east. x The positive direction is from south to north. y In the positive direction, starting from any point on the shore within the sea area to be measured, draw a line parallel to the ocean direction. x direction or y A straight line in the direction of the wave is taken as the wave ray; starting from the nearshore sea surface, a wave ray of size is selected. a × a A two-dimensional Fourier transform is performed on the three-dimensional signal point cloud of the ocean surface within a planar bounding box region to obtain the two-dimensional wavenumber spectrum distribution of the signal point cloud within the box, and the peak values ​​are extracted. x Wavenumber in direction and y Wavenumber in direction The wave value at the spectral peak was calculated. Based on the relationship between wavelength and wave value The wavelength value within the box is calculated. L At the same time, the wave direction within the box is obtained. The wave direction is the direction of wave propagation. y The angle between directions; The water depth is obtained by transforming equation (1). D The calculation formula is as follows: （3） In the formula, arctanh is the inverse hyperbolic tangent function. ω It is the wave frequency, through the wave cycle. T According to the relation Calculated; According to formula (3), the wavelength passing through the box region L and wave cycle T The calculation can obtain the corresponding water depth within the planar box. D ; The planar rectangle moves along a single wave ray, and the rectangle moves each time... Repeat the wavelength and wave direction calculation process in the planar box after each movement, and record the wavelength corresponding to the peak of the sea surface signal wavenumber spectrum in each box. L and wave direction To select an accurate wavenumber spectral peak, the wavelength to be tracked is restricted in the wavenumber domain: 1) Set the maximum value of the tracked wavelength. 2) Limit the range of wave direction variation between adjacent planar boxes, with the maximum allowable wave direction variation being [value missing]. Based on the above constraints, when selecting a spectral peak in the wavenumber spectrum, the wavelength corresponding to the selected peak must first be calculated. L and wave direction The change in wave direction compared to the previous box , This represents the wave direction value of the sea surface signal within the previous box of the current planar box. or Then it is necessary to reselect the next peak in the wavenumber spectrum until the selected peak meets all the constraints; utilize wave cycles. T and the wavelengths at various locations on the tracked wave rays L The water depth at each position on a single wave ray is calculated according to formula (3). D Finally, the spatial resolution of the water depth along the wave ray direction was obtained through inversion. ; Step 3: Use multiple wave rays to cover the entire sea area to be measured, repeat the calculation process of inverting the water depth on a single wave ray in Step 2, and invert the water depth on multiple wave rays to obtain the water depth result of the entire sea area to be measured.

2. The method for inverting water depth using airborne lidar point clouds as described in claim 1, characterized in that: In step 1.1, the minimum elevation of the water surface signal point cloud is initially set. and maximum elevation The calculated elevation range is within Mean value of point cloud signal and standard deviation Using the 3 in the normal distribution The guidelines define the elevation range of the water surface signal point cloud as follows: Noise point clouds outside this elevation range are filtered out, and the three-dimensional signal point cloud of the ocean surface is extracted.

3. The method for inverting water depth using airborne lidar point clouds of the sea surface as described in claim 1, characterized in that: In step 1.2, the extracted ocean surface signal point cloud within the area to be measured is divided into spatial grids, with each spatial grid on a plane. x direction and y The directional intervals are Δ x and Δ y The definition is from west to east. x The positive direction is from south to north. y In the positive direction, the average elevation of the ocean surface signal point cloud within each grid is calculated, and the obtained average value is re-recorded as the elevation value of that grid point.

4. The method for inverting water depth using airborne lidar point clouds as described in claim 1, characterized in that: In step 2.1, a two-dimensional Fourier transform is performed on the ocean surface signal point cloud in the deep-water area of ​​the sea area to be measured to obtain the wavenumber spectrum distribution of the ocean surface signal point cloud in the deep-water area, and the wavenumber spectrum peak value of the deep-water area is extracted. Based on the relationship between wavelength and wavenumber The wavelength value in the deep water region was calculated. ; In deep water areas, that is... D / L> At 1 / 2, tanh(2π) D / L )≈1, formula (1) simplifies to: （2） According to formula (2), the wavelength values ​​of the deep water region are obtained. Able to calculate wave cycles T .

5. The method for inverting water depth using airborne lidar point clouds of the sea surface as described in claim 1, characterized in that: In step 3, the wave rays are set to be parallel to each other, and the distance between adjacent wave rays is Δ. l The number of wave rays is n Through the n Two-dimensional Fourier transform and wavelength tracking are performed on the ocean surface signal point cloud within each plane box on the wave ray. The wavelength of each box is recorded, and then the water depth of each box is calculated by formula (3), thereby obtaining the water depth of the entire sea area to be measured.

Images