CN116385683B - A three-dimensional small watershed channel fractal dimension calculation method and system - Google Patents

Abstract

The invention discloses a method and a system for calculating fractal dimension of a three-dimensional small-drainage-basin channel, which relate to the technical field of rock mass engineering and comprise the following steps: acquiring a digital elevation model DEM of a region to be researched, and extracting a channel grid; establishing three-dimensional geographic coordinates of a small-river basin channel grid, and establishing a cube space; dividing the cube space into a plurality of cube grid cells; sequentially judging whether each cube grid unit comprises a channel or not; taking the logarithm of the number of the cube grid cells occupied by the small watershed channels as an ordinate, taking the logarithm of the side lengths of the cube grid cells as an abscissa, drawing a scatter diagram, and determining a non-scale interval in the scatter diagram; fitting a straight line to points within the scale-free interval, the opposite number of slopes of the straight line being the fractal dimension of the small-drainage-basin channel in the area to be studied. The invention greatly reduces errors in the calculation process, and ensures extremely high precision while having high efficiency.

Description

A three-dimensional small watershed channel fractal dimension calculation method and system

Technical field

The invention relates to the technical field of rock mass engineering, specifically a three-dimensional small watershed channel fractal dimension calculation method and system.

Background technique

Fractal dimension is a concept in geometric fractal theory. It can reflect the degree of space occupied by irregular shapes. In 1910, Hausdorff proposed the concept of fractal dimension. Because of its simple concept, easy programming and high applicability, it is widely used in small watershed ditches. It is widely used in the description and evaluation of Tao.

The development of channels in small watersheds in mountainous areas promotes the occurrence of geological disasters such as landslides and debris flows to varying degrees. Fractal dimension is increasingly used as a good tool to describe the complexity of irregular shapes in space. Determine the quantitative indicators of channel structures in small watersheds, thereby providing a basis for risk assessment of channel structures that may cause disasters.

When the existing technology calculates the fractal dimension of small watershed channels, it mainly uses manual calculation, which is inefficient and not high-precision. Moreover, when calculating through programming methods, it not only requires high requirements for programmers, but also the existing methods are basically It is calculated based on the relationship between the points on the line and the grid, so errors will occur when the grid is divided into a finer grid.

Contents of the invention

The present invention proposes a three-dimensional small watershed channel fractal dimension calculation method and system, which are used to solve the problems of traditional fractal dimension calculation methods that are time-consuming, have large errors, and the calculation results are sensitive to changes in subjective factors.

The invention provides a three-dimensional small watershed channel fractal dimension calculation method, which includes the following steps:

Obtain the digital elevation model DEM of the area to be studied and extract the channel grid;

Establish the three-dimensional geographical coordinates of the small watershed channel grid in the channel grid, and establish a cubic space based on the three-dimensional geographical coordinates;

Determine the number of division intervals and divide the cubic space into multiple cubic grid units;

Determine whether each cubic grid unit contains a channel in turn, and count the number of cubic grid units occupied by channels in the small watershed;

Take the logarithm of the number of cubic grid cells occupied by the small watershed channels as the ordinate, and the logarithm of the side length of the cube grid cells as the abscissa, draw a scatter plot, and make a difference on the ordinate, Scatter points in the coordinate interval where the difference result is always smaller than the singular value separation point are excluded, and the coordinate interval composed of the remaining scatter points is regarded as a scale-free interval;

A straight line is fitted to the points in the scale-free interval, and the inverse of the slope of the straight line is the fractal dimension of the small watershed channel in the area to be studied.

Furthermore, it also includes:

When the determined number of division intervals n does not reach the maximum number of division intervals, the number of division intervals n is gradually increased by 1, and the proportion of the small watershed channel occupied by the small watershed channel when the cubic space is divided by the incremented number of division intervals n is statistically calculated. The number of cubic grid cells.

Further, it also includes re-judging the special intersection situation between the channel and each cubic grid unit, as follows:

When the channel only contacts the vertices, edges, and faces of the outer surface of the cubic grid unit, or the channel is only located on a certain face of the cubic grid unit, the cubic grid unit is removed from the small watershed channel. It is eliminated from the cubic grid cells it occupies.

Further, obtaining the digital elevation model DEM of the area to be studied and extracting the channel grid includes the following steps:

Obtain positive photographic image data of the area to be studied;

Model the orthographic image data in ContextCapture to obtain the digital surface model DSM of the area to be studied;

Convert the digital surface model DSM into point cloud data in GlobalMapper, and use point cloud filtering to remove vegetation and surface object point cloud data to obtain the digital elevation model DEM of the area to be studied;

Import the digital elevation model DEM into ArcGis, splice the digital elevation model DEM, and perform filling depressions, flow direction analysis, ditch network analysis, ditch network grading, raster ditch network vectorization and ditch network grading optimization display on the digital elevation model DEM. , then the channel grid is extracted.

Further, establishing the three-dimensional geographical coordinates of the small watershed channel grid in the channel grid, and establishing a cubic space based on the three-dimensional geographical coordinates, includes the following steps:

Vectorize the small watershed channel grid in the channel grid, and add multiple control points to the vectorized small watershed channel grid;

Extract the longitude, latitude, and elevation data of multiple control points in ArcGis and import them into the Excel table to convert the longitude, latitude, and elevation data of each control point to obtain the three-dimensional geographical coordinates of the small watershed channel grid;

The maximum value of the length, width, and height of the three-dimensional geographical coordinates of the small watershed grid is used as the side length to establish a cubic space.

Further, the calculation formula of the fractal dimension of the small watershed channel is as follows:

Among them, D _f is the fractal dimension, that is, lnN _(r) is relative to The slope value;

N _(r) is the number of cubic grid cells occupied by the small watershed channel, that is, the number of cubic grid cells that intersect the channel;

M is the side length of the cubic space; n is the number of division intervals;

r is the side length of the cubic grid unit after division.

The invention also provides a three-dimensional small watershed channel fractal dimension calculation system, which includes:

The channel grid acquisition module is used to obtain the digital elevation model DEM of the area to be studied and extract the channel grid;

The cube space building module is used to establish the three-dimensional geographical coordinates of the small watershed channel grid in the channel grid, and establish a cubic space based on the three-dimensional geographical coordinates;

The grid unit division module is used to determine the number of division intervals and divide the cubic space into multiple cubic grid units;

The intersection grid determination module is used to determine whether each cubic grid unit contains a channel in turn, and to count the number of cubic grid units occupied by channels in small watersheds;

The scale-free interval determination module is used to draw a scatter plot by using the logarithm of the number of cubic grid cells occupied by small watershed channels as the ordinate and the logarithm of the side length of the cubic grid unit as the abscissa; Make a difference on its ordinate, exclude the scattered points in the coordinate interval whose difference result is always smaller than the singular value separation point, and use the coordinate interval composed of the remaining scattered points as a scale-free interval;

The fractal dimension determination module is used to fit a straight line to points in the scale-free interval. The opposite of the slope of the straight line is the fractal dimension of the small watershed channel in the area to be studied.

Compared with the existing technology, the beneficial effects of the present invention are:

In the present invention, when calculating the fractal dimension of a channel in a small watershed, the coordinates of the channel are first obtained using UAV aerial images and ArcGis, and then the cubic space occupied by the channel is determined, and the cubic space is divided according to the determined number of division intervals. Perform multiple equal-spaced divisions to obtain multiple cubic grid units. After each grid division, count the number of cubic grid units occupied by the channels. After multiple grid divisions and counting, draw the coordinate system. The abscissa is the logarithm of the number of intervals for each division, and the ordinate is the logarithm of the side length of the cubic grid unit occupied by the channel counted each time. The opposite of its slope is the fractal dimension result of the channel. . The three-dimensional small watershed channel fractal dimension calculation method provided by the present invention only requires the coordinates of the channel flow direction in the geodetic coordinate system, and can obtain a higher-precision fractal dimension based on the set number of division intervals. result.

The present invention further judges and eliminates the special situation of the intersection of the channel and each divided cubic grid unit in the traditional fractal dimension calculation method, improves the calculation accuracy of the fractal dimension, and forms a complete fractal dimension calculation work plan. The fractal dimension algorithm of the present invention ensures that the user only needs to determine the number of cubic grid cells that are finally divided into equal parts to obtain a higher-precision fractal dimension result of the channel. The threshold for use is low and the calculation efficiency is high. The present invention utilizes The calculation is based on the relationship between lines and grids, and special relationships such as parallel intersections of lines and grids are also taken into account, which greatly reduces errors in the calculation process and ensures extremely high accuracy while being highly efficient.

Description of drawings

The drawings are used to provide a further understanding of the present invention and constitute a part of the specification. They are used to explain the present invention together with the embodiments of the present invention and do not constitute a limitation of the present invention. In the attached picture:

Figure 1 is a schematic flow chart of a three-dimensional small watershed channel fractal dimension calculation method extracted by the present invention;

Figure 2 is a schematic diagram of some drainage channels in the area to be studied in an embodiment of the present invention;

Figure 3 is a schematic diagram of a special intersection situation between a channel and a cubic grid unit in an embodiment of the present invention;

Figure 4 is a schematic diagram of the calculation results of the fractal dimension applied to a channel in the Heihe River Basin in the embodiment of the present invention.

Detailed ways

The technical solutions in the embodiments of the present invention will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present invention. However, it should be understood that the protection scope of the present invention is not limited by the specific embodiments.

Example 1

As shown in Figure 1, the present invention provides a three-dimensional small watershed channel fractal dimension calculation method, which includes the following steps:

Step S1: Obtain the digital elevation model DEM of the area to be studied and extract the channel grid;

Step S2: Establish the three-dimensional geographical coordinates of the small watershed channel grid in the channel grid, and establish a cubic space based on the three-dimensional geographical coordinates;

Step S3: Determine the number of division intervals and divide the cubic space into multiple cubic grid units;

Step S4: Determine whether each cubic grid unit contains a channel in turn, and count the number of cubic grid units occupied by channels in the small watershed;

Step S5: When the determined number n of division intervals does not reach the maximum number of division intervals, the number of division intervals n is gradually increased by 1, and the cubic space is divided by the incremented number n of division intervals by repeating steps S3 and S4 respectively. When , the number of cubic grid cells occupied by small watershed channels;

Step S6: Use the logarithm of the number of cubic grid cells occupied by the small watershed channel as the ordinate, and the logarithm of the side length of the cubic grid unit as the abscissa, draw a scatter plot, and do Difference, exclude the scattered points in the coordinate interval whose difference result is always smaller than the singular value separation point, and use the coordinate interval composed of the remaining scattered points as a scale-free interval;

Step S7: Fit a straight line to the points in the scale-free interval. The inverse of the slope of the straight line is the fractal dimension of the small watershed channel in the area to be studied.

In step S1, obtain the digital elevation model DEM of the area to be studied and extract the channel grid, including the following steps:

Obtain positive photographic image data of the area to be studied, which can be obtained using drone aerial photography;

Model the orthographic image data in ContextCapture to obtain the digital surface model DSM of the area to be studied;

Convert the digital surface model DSM into point cloud data in GlobalMapper, and use point cloud filtering to remove vegetation and surface object point cloud data to obtain the digital elevation model DEM of the area to be studied;

Import the digital elevation model DEM into ArcGis, specifically use the DataManagement Tools.tbx in the toolbox in ArcGis to splice the digital elevation model DEM, and then use Spatial Analyst Tools.tbx to sequentially fill in depressions, flow direction analysis, and ditch the digital elevation model DEM. Network analysis, channel network grading, raster channel network vectorization and channel network grading optimization display, then the channel grid is extracted.

In step S2, establish the three-dimensional geographical coordinates of the small watershed channel grid in the channel grid, and establish a cubic space based on the three-dimensional geographical coordinates, including the following steps:

Step S2.1: Vectorize the small watershed channel grid in the channel grid, and add multiple control points to the vectorized small watershed channel grid. Specifically, use the toolbox in ArcGis Data ManagementTools.tbx—Sampling—Generate points along the line and add control points by setting the density of the points;

Step S2.1: Extract the longitude, latitude, and elevation data of multiple control points in ArcGis, and import them into the Excel table respectively to convert the longitude, latitude, and elevation data of each control point to obtain the three-dimensional channel grid of the small watershed. geographical coordinates;

Specifically, import the longitude and latitude of multiple control points into an Excel table, enter formula (1) and formula (2) in the Excel table to perform coordinate conversion, and obtain the x-axis and y-axis coordinates in the two-dimensional coordinate system;

MID(A2,1,3)+MID(A2,6,2)/60+MID(A2,10,5)/3600 (1)

MID(B2,1,2)+MID(B2,5,2)/60+MID(B2,9,5)/3600 (2)

Among them, MID(A2,1,3) means taking three numbers from the first one in the data in cell A2; the remaining MID(A2,6,2), MID(A2,10,5), MID(B2, 1,2), MID(B2,5,2), MID(B2,9,5) are the same. Through this formula, the longitude and latitude coordinates can be converted into the required numerical form.

Add a z-axis field to the attribute list of the Excel table, and import the elevation data of multiple control points into the Excel table as z-axis coordinates;

Based on the x-axis, y-axis, and z-axis coordinates obtained in the Excel table, the three-dimensional geographical coordinates of the small watershed channel grid are obtained. During the operation of step S2.1, the coordinate systems of multiple control points need to be consistent. In this embodiment, the WGS_1984 coordinate system is used.

Step S2.3: Use the maximum value of the length, width, and height of the three-dimensional geographical coordinates of the small watershed grid as the side length to establish a cubic space.

In step S3, starting from 2 equal divisions by default, the three-dimensional cube space will be divided into 8 new cubes, then 3 equal divisions, 4 equal divisions... and so on, until the artificially determined n equal divisions are reached. At this time, the three-dimensional space is divided into n ³ new cubic grid cells. For different channels, the appropriate number of equal intervals is determined based on the actual situation.

In step S4, n equal fractals of the cubic space under study are generated, n ³ cubic grid units of equal volume are generated, and each cubic grid unit is judged in turn whether it contains part of the channel. If so, the small watershed channel occupies The number s of cubic grid cells is increased by one. If it is not included, the next cubic grid unit will be judged until all grids are judged. For each cubic space studied equally, the cube occupied by the small watershed channel The number of grid cells is s _min =1, s _max =n ³ .

As shown in Figure 3, under normal circumstances, the intersection between the channel in the three-dimensional space and the cubic grid unit is obvious. However, when the channel only contacts the vertices, edges, and faces of the outer surface of the cubic grid unit, or the channel is only located When looking at a certain face of a cubic grid unit, the computer usually determines that the channel belongs to the cubic grid unit. However, in actual situations, these situations should not logically belong to the cubic space. Therefore, in order to take a closer look at the results obtained in step 4 For optimization, the cubic grid unit is eliminated from the cubic grid unit occupied by the small watershed channel. The cubic grid unit in the above situation is not counted, and a new result N _(r) is obtained, N _(r) ≤ s.

In step S6, only the fractal dimension within the scale-free interval can effectively reflect the fractal characteristics of the channel. Combining the existing methods of determining the scale-free interval and taking into account the characteristics of the watershed channel, the present invention A new algorithm for determining scale-free intervals was developed and achieved good results in multiple calculations, as follows:

As shown in Figure 4, for the channel fractal dimension, calculate the coordinates of the scatter plot, make a difference on the y-axis data of the scatter plot, and take 0.05 as the singular value separation point. The interval where the difference result is always less than the singular value is unsatisfactory. For the interval required by the fractal, the scattered points in the interval that do not meet the requirements are eliminated, and the interval composed of the remaining scatter point coordinates is the scale-free interval.

In step S7, the calculation formula of the fractal dimension of the small watershed channel is as follows:

Among them, D _f is the fractal dimension, that is, ln N _(r) relative to The slope value;

N _(r) is the number of cubic grid cells occupied by the small watershed channel, that is, the number of cubic grid cells that intersect the channel;

M is the side length of the cubic space; n is the number of division intervals;

r is the side length of the cubic grid unit after division.

Example 2

The invention also provides a three-dimensional small watershed channel fractal dimension calculation system, which includes:

The channel grid acquisition module is used to obtain the digital elevation model DEM of the area to be studied and extract the channel grid;

The cube space building module is used to establish the three-dimensional geographical coordinates of the small watershed channel grid in the channel grid, and establish a cubic space based on the three-dimensional geographical coordinates;

The grid unit division module is used to determine the number of division intervals and divide the cubic space into multiple cubic grid units;

The intersection grid determination module is used to determine whether each cubic grid unit contains a channel in turn, and to count the number of cubic grid units occupied by channels in small watersheds;

The scale-free interval determination module is used to draw a scatter plot by using the logarithm of the number of cubic grid cells occupied by small watershed channels as the ordinate and the logarithm of the side length of the cubic grid unit as the abscissa; Make a difference on its ordinate, exclude the scattered points in the coordinate interval whose difference result is always smaller than the singular value separation point, and use the coordinate interval composed of the remaining scattered points as a scale-free interval;

The fractal dimension determination module is used to fit a straight line to points in the scale-free interval. The opposite of the slope of the straight line is the fractal dimension of the small watershed channel in the area to be studied.

The technical solutions in the present invention will be described below in conjunction with specific examples.

Taking some channels in the Heihe River Basin as an example to specifically illustrate the calculation method of the channel fractal dimension of the present invention, the specific implementation steps are as follows:

Step 1. Obtain the DEM of the Heihe River Basin area.

The specific steps are: use drone aerial photography to obtain orthophotos of the Heihe River Basin area, model in ContextCapture to obtain a digital surface model DSM, convert the digital surface model DSM to point cloud data in GlobalMapper, and use point cloud filtering to remove vegetation, Features, and then generate a digital elevation model DEM.

Step 2. Extract the channel network of the study area.

The specific steps are: import the digital elevation model DEM into ArcGis, use the DataManagement Tools.tbx in its toolbox to splice the DEM data, and then use the Spatial Analyst Tools.tbx to sequentially perform filling of depressions, flow direction analysis, ditch network analysis, ditch network classification, Raster ditch network vectorization and ditch network hierarchical optimization display complete the simple extraction of the channel network. Among them, the ditch network classification defaults to the STRAHLER method. In this example, the river network is divided into 6 levels, as shown in Figure 2.

Step 3. Vectorize the studied channel network.

After the ditch network is extracted, manual vectorization is performed. Only the small watershed ditches in the study area are vectorized. Points are generated along the line in Data ManagementTools.tbx—Sampling—and the density of the points is set to add control points for coordinate extraction.

Step 4. Extract the geographical coordinates of the channel network.

The specific steps are: first extract the longitude and latitude of the control points obtained in step 3 in ArcGis and import them into the Excel table, enter the formula (1) and formula (2) in the above content into the Excel table to perform coordinate conversion, and then convert the x-axis, Import the y-axis coordinates into ArcGis, convert the geographical coordinates into projection (note that the coordinates must be unified) and generate a sheet file. After adding the x-axis and y-axis, convert it directly to the plane coordinate system, then add the z-axis field in the attribute list, load the digital elevation model DEM to generate the elevation, and finally export the x-axis, y-axis, and z-axis in the attribute list to obtain the result. Geographical coordinates of the three-dimensional channel network.

It should be noted that the coordinate system needs to be consistent during the operation. In this embodiment, the WGS_1984 coordinate system is used for extraction.

Step 5. Determine the side length M of the research space generated by the channel.

According to the coordinates of the channel, the maximum value of the length, width and height determined by the three-dimensional coordinates of the channel is the length of the space occupied by the channel, 63850. Therefore, the three-dimensional cube space side length M=63850 is selected.

Step 6. Determine the number of intervals to be divided.

According to the M obtained in step 1, it is determined to divide the three-dimensional cubic space equally to 100. By default, starting from 2 equal parts, the three-dimensional space will be divided into 8 new cubes, and then divided into 3 equal parts, 4 equal parts... and so on. And so on, up to 100 equal parts.

Step 7: Count the number s of grids occupied by the channels after equally dividing the grids.

Divide the research space into n equal parts, and generate n ³ cubic grid units of equal volume for the three-dimensional case. Based on this, each grid is judged to determine whether the cubic grid unit contains part of the channel. If so, what is the channel? The number s of the cubic grid cells occupied is increased by one. If it is not included, the next cubic grid unit will be judged until all cubic grid cells are judged. For each equal division of the research space, s _min = 1, s _max = n ³ .

Step 8: Determine the special intersection situation between the channel and the grid to improve the accuracy of the statistical result N _(r) .

As shown in Figure 3, when the channel only touches the vertices, edges, faces, or on a certain surface of the outer surface of the cubic grid unit, the computer usually determines that the channel belongs to the cubic grid unit, but in actual situations, these The situation logically should not belong to the cubic grid unit, so the result obtained in step 7 is further optimized, the count of the cubic grid unit in the above situation is eliminated, and the new result N _(r) , N _(r) ≤ s is obtained.

Step 9: Repeat steps 7 and 8 to obtain the number of equal divisions in increments of 1, and the number of grids N _(r) occupied by the channels corresponding to each equal division of the cube. Finally, we get the number of equal parts increasing from 2 to 100 by 1, and 99 N _(r) .

Step 10. For the calculation formula of fractal dimension, see formula (3) in the above content.

Step 11. Determine the scale-free interval.

Only within the scale-free interval can the fractal dimension effectively reflect the fractal characteristics of the channel. As shown on the right side of Figure 4, applying the scale-free interval algorithm of the present invention, the scale-free abscissa interval is [8.1, 10.2].

Step 12. Fit a straight line to the points in the scale-free interval.

As shown in Figure 4, the opposite number of the slope of the straight line is the fractal dimension of the research channel. The linear correlation coefficient between the fitted straight line and the calculated value is 0.9931, which shows that the better the fitting effect.

The present invention uses UAV photogrammetry technology and three-dimensional modeling technology, combined with ArcGis, to quickly and accurately extract the three-dimensional geographical coordinates of the channel for the calculation of fractal dimension. It further judges and eliminates the channel and all other factors in the traditional fractal dimension calculation method. The special situation of the intersection of each divided cubic grid unit improves the calculation accuracy of fractal dimension and forms a complete work plan for calculating fractal dimension. The fractal dimension algorithm of the present invention ensures that users only need to determine the number of cubic grid cells that are finally divided into equal parts to obtain higher-precision fractal dimension results for the channel. The threshold for use is low and the calculation efficiency is high, solving the traditional problem of The fractal dimension calculation method is time-consuming, has large errors, and the results are sensitive to changes in subjective factors.

Finally, it should be noted that the above disclosure is only a specific embodiment of the present invention. However, the embodiment of the present invention is not limited thereto. Any changes that those skilled in the art can think of should fall within the protection scope of the present invention.

Claims (5)

1. The method for calculating the fractal dimension of the three-dimensional small-drainage-basin channel is characterized by comprising the following steps of:

acquiring a digital elevation model DEM of a region to be researched, and extracting a channel grid;

establishing three-dimensional geographic coordinates of small-river-basin channel grids in the channel grids, and establishing a cube space according to the three-dimensional geographic coordinates;

determining the number of dividing intervals, and dividing a cube space into a plurality of cube grid cells;

sequentially judging whether each cube grid cell comprises channels or not, and counting the number of the cube grid cells occupied by the channels of the small drainage basin;

taking the logarithm of the number of the cube grid units occupied by the small drainage basin channels as an ordinate, taking the logarithm of the side length of the cube grid units as an abscissa, drawing a scatter diagram, differentiating the ordinate, eliminating scattered points in a coordinate interval with a differential result always smaller than a singular value separation point, and taking the coordinate interval formed by the residual scattered points as a non-scale interval;

fitting a straight line to points in the scale-free interval, wherein the opposite number of the slope of the straight line is the fractal dimension of a small-drainage-basin channel in the area to be researched;

the method for acquiring the digital elevation model DEM of the area to be researched and extracting the channel grid comprises the following steps:

acquiring positive photographic image data of an area to be studied;

modeling the positive photographic image data in ContextCapture to obtain a digital surface model DSM of the region to be studied;

converting the digital surface model DSM into point cloud data in a GlobalMapper, and removing vegetation and ground point cloud data by using point cloud filtering to obtain a digital elevation model DEM of the area to be researched;

importing the digital elevation model DEM into ArcGis, splicing the digital elevation model DEM, sequentially filling the depressions, analyzing the flow direction, analyzing the ditch net, grading the ditch net, vectorizing the grid ditch net and grading and optimizing the ditch net, and extracting to obtain the channel grid;

the method for establishing the three-dimensional geographic coordinates of the small-river-basin channel grids in the channel grids comprises the following steps of:

vectorizing a small-drainage-basin channel grid in the channel grid, and adding a plurality of control points into the vectorized small-drainage-basin channel grid;

respectively extracting longitude, latitude and elevation data of a plurality of control points from ArcGis, respectively importing the longitude, latitude and elevation data into an Excel table, and converting the longitude, latitude and elevation data of each control point to obtain three-dimensional geographic coordinates of a small-drainage-basin channel grid;

and taking the maximum value of the length, width and height of the three-dimensional geographic coordinates of the small watershed grid as the side length, and establishing a cube space.

2. The method for calculating the fractal dimension of a three-dimensional small-drainage-basin channel as recited in claim 1, further comprising:

when the determined dividing interval number n does not reach the maximum dividing interval number, the dividing interval number n is gradually increased by 1, and the number of the cube grid units occupied by the small watershed channels is counted when the cube space is divided by the increased dividing interval number n.

3. The method of calculating the fractal dimension of a three-dimensional small-basin channel according to claim 1, further comprising re-judging the special intersection of said channel with each of said cubic grid cells as follows:

when the channel only contacts the top point, side, face of the outer surface of the cube grid cell, or the channel is only located on one face of the cube grid cell, then the cube grid cell is eliminated from the cube grid cells occupied by the small basin channel.

4. The method for calculating the fractal dimension of a three-dimensional small-drainage-basin channel according to claim 1, wherein the calculation formula of the fractal dimension of the small-drainage-basin channel is as follows:

wherein D is _f For fractal dimension, i.e. lnN _(r) Relative toSlope values of (2);

N _(r) the number of cubic grid cells occupied by the channels of the small watershed, namely the number of cubic grid cells intersected with the channels;

m is the side length of the cube space; n is the number of dividing intervals;

r is the side length of the divided cube grid cell.

5. A three-dimensional small-drainage-basin channel fractal dimension calculation system, comprising:

the channel grid acquisition module is used for acquiring a digital elevation model DEM of the area to be researched and extracting a channel grid; the method specifically comprises the following steps: acquiring positive photographic image data of an area to be studied; modeling the positive photographic image data in ContextCapture to obtain a digital surface model DSM of the region to be studied; converting the digital surface model DSM into point cloud data in a GlobalMapper, and removing vegetation and ground point cloud data by using point cloud filtering to obtain a digital elevation model DEM of the area to be researched; importing the digital elevation model DEM into ArcGis, splicing the digital elevation model DEM, sequentially filling the depressions, analyzing the flow direction, analyzing the ditch net, grading the ditch net, vectorizing the grid ditch net and grading and optimizing the ditch net, and extracting to obtain the channel grid;

the cube space construction module is used for establishing three-dimensional geographic coordinates of the small-river basin channel grids in the channel grids and establishing a cube space according to the three-dimensional geographic coordinates; the method specifically comprises the following steps: vectorizing a small-drainage-basin channel grid in the channel grid, and adding a plurality of control points into the vectorized small-drainage-basin channel grid; respectively extracting longitude, latitude and elevation data of a plurality of control points from ArcGis, respectively importing the longitude, latitude and elevation data into an Excel table, and converting the longitude, latitude and elevation data of each control point to obtain three-dimensional geographic coordinates of a small-drainage-basin channel grid; taking the maximum value of the length, width and height of the three-dimensional geographic coordinates of the small watershed grid as the side length, and establishing a cube space;

the grid cell dividing module is used for determining the dividing interval number and dividing the cube space into a plurality of cube grid cells;

the intersecting grid determining module is used for sequentially judging whether each cube grid unit comprises channels or not and counting the number of the cube grid units occupied by the channels of the small drainage basin;

the scale-free interval determining module is used for drawing a scatter diagram by taking the logarithm of the number of the cube grid cells occupied by the small drainage basin channels as an ordinate and the logarithm of the side length of the cube grid cells as an abscissa; differentiating the ordinate of the coordinate interval, excluding scattered points in the coordinate interval of which the differential result is always smaller than the singular value separation point, and taking the coordinate interval formed by the rest scattered points as a scale-free interval;

the fractal dimension determining module is used for fitting a straight line to points in the scale-free interval, and the opposite number of the slope of the straight line is the fractal dimension of the small-drainage-basin channel in the area to be studied.