A method for eliminating parallel valleys in the process of extracting valley networks

Abstract

The present invention discloses a method for eliminating parallel valleys in the process of extracting valley networks, and relates to the field of terrain analysis. The key points of the technical solution include the following steps: S1, inputting DEM data to extract valley network data; S2, calibrating the level of the extracted valleys; S3, determining parallel valleys according to the change in the distance from the node on the valley line to the valley of the previous level; S4, for the valley line with parallel valleys, first find the parallel valley modification point, then regenerate the valley line, and delete the parallel valleys; S5, sequentially perform the parallel valley determination and elimination process for all non-first-level valleys. The parallel valley elimination method proposed by the present invention is carried out on the basis of the current mainstream valley network extraction method, and does not need to modify the original data and the key steps of valley network extraction. It only further processes the valley network extraction to eliminate the parallel valleys in the valley network extraction result. The method is simple to operate and has strong applicability.

Description

Technical Field

[0001] The invention relates to the field of terrain analysis, in particular to a method for eliminating parallel valleys in the process of extracting a valley network. Background Art

[0002] A gully is a narrow depression formed by water erosion on the earth's surface. In areas with severe water erosion, gullies of different levels are inlaid and interconnected to form a gully network in the area. In areas with severe water erosion, such as the Loess Plateau in my country, accurate extraction of gully networks is of great significance for agricultural production planning, soil and water conservation, sand control, and other scientific research, such as landform development and evolution.

[0003] The main method of extracting valley networks at present is to simulate the flow of surface runoff on the surface based on DEM data using the overland flow model to generate a water system, namely, a valley network. The basic process is to first fill the DEM data to eliminate the local depressions in the DEM data, and then determine the direction of the water flow on each grid unit according to the principle of maximum elevation difference, so as to simulate the confluence route of the water flow on the surface of a region. From the perspective of geomorphology theory, there should be only one confluence route in a valley to obtain a valley network with correct confluence relationship. However, for some relatively wide and flat valleys, due to the data filling or the terrain itself, there may be multiple confluence paths. When the valley network is extracted using the overland flow model, two or more valleys may be distributed in parallel, which is called parallel valleys in the field of digital terrain analysis. As mentioned above, parallel confluence paths may exist in real watersheds, but the existence of parallel valleys has an adverse effect on the relevant analysis of the valley network morphology and quantity, so it is necessary to eliminate such parallel valleys.

[0004] The parallel valley phenomenon may occur in the valley network extraction of different landform types. It is a common problem and has gradually attracted the attention of scholars in related fields. A comprehensive analysis of the current research on the method of eliminating parallel river networks shows that the research ideas are basically carried out from the perspective of changing the direction of water flow, which can be further divided into two ideas: the first is to modify the flow direction algorithm without changing the DEM data, and the second is to modify the DEM data without changing the flow direction algorithm, that is, to fine-tune the elevation of the local area with the help of other information to change the incorrect flow direction. Although these studies have achieved relatively ideal results in the extraction of valley networks in their experimental areas, their defects are also obvious. For example, whether it is to modify the surface flow direction algorithm or to modify the DEM elevation information, the algorithms are relatively complex. Before these methods are integrated into existing professional analysis software, it is difficult for front-line engineering application personnel to implement them. On the other hand, existing studies are basically aimed at the parallel valley phenomenon caused by flat land, while the effect of eliminating parallel valleys caused by real surface elevation characteristics is not obvious. It can be seen from the implementation examples of existing studies that in some experimental areas, the methods only change the morphological characteristics of two or more parallel valleys. There are still multiple valley segments in larger valleys. This elimination is essentially invalid for applications such as valley network density calculation.

[0005] In view of the above problems, the method of the present invention is designed with the goal of simple operation and obvious effect to eliminate parallel valleys in the process of extracting valley networks. The method does not need to change the DEM data and the current surface flow direction algorithm, but on the basis of the existing method of extracting valley networks, the parallel valleys are determined by the spatial form and relative position relationship between different valley segments, and the valley segments are reasonably modified on this basis, so as to achieve the purpose of eliminating parallel valleys. The method of the present invention is easy to operate, and the valley network obtained is more consistent with the valley network in the actual surface, which has important application value in practical projects such as agricultural planning, soil and water conservation planning and geomorphology-related scientific research fields. Summary of the invention

[0006] In view of the above problems, the present invention designs a method for eliminating parallel valleys in the process of extracting valley networks. The method includes key technical links such as valley level calibration, parallel valley determination, and parallel valley elimination, which can effectively eliminate the parallel valley problem that occurs in the process of extracting valley networks based on DEM data and improve the accuracy of calculation of relevant geomorphic parameters.

[0007] The technical solution adopted by the present invention is as follows:

[0008] The method for eliminating parallel valleys in the process of extracting valley networks comprises the following steps:

[0009] S1, input DEM data, extract the gully network data of the study area through the processes of depression filling, flow direction calculation, runoff accumulation calculation, and setting threshold parameters;

[0010] S2, valley level calibration, starting from the outlet of the study area, upstream, along the direction of the largest increase in runoff accumulation, determine the first-level valley in the study area, then for all the tributaries of the first-level valley, upstream from the point where they intersect with the first-level valley, determine all the second-level valleys, and so on, complete the level calibration for all valleys;

[0011] S3, for all second-level valleys, starting from the intersection of the second-level valley and the first-level valley, the distance from the node on the second-level valley line to the first-level valley line is calculated according to a certain step length, and the parallel valleys are detected according to the change of the distance;

[0012] S4, after detecting the parallel valley, based on the distance value from the node on the valley line to the primary valley, determine the threshold mutation point of the convergence accumulation amount, take the point as the starting point, take the vertical line from the point to the primary valley as the reference direction, and generate the modified valley section from the point along the reference direction according to the maximum slope drop principle without exceeding a certain distance on the left and right, and delete the point to the intersection of the original parallel valley and the primary valley;

[0013] S5, execute steps S3-S4 for all secondary valleys, then start from the secondary valley, regard each secondary valley as a primary valley, determine and eliminate the parallel valleys of its tributaries, until all valleys are processed, output data, and obtain valley network data with parallel valleys eliminated.

[0014] Furthermore, the specific process of step S2 is as follows:

[0015] S21, read the grid data of runoff accumulation, determine the grid unit with the largest runoff accumulation value as the watershed outlet of the study area, take it as the first grid unit of the main valley, and mark the grid;

[0016] S22, starting from the watershed outlet, select the grid cell with the largest cumulative flow value among its adjacent grid cells as the second grid cell of the main valley, and use this grid cell as the current processing grid cell, and mark this grid cell at the same time;

[0017] S23, repeating step S22 until the cumulative flow values ​​of all adjacent grids of the currently processed grid unit are lower than the threshold parameter set for extracting the valley, completing the calibration of the first-level valley;

[0018] S24, for the grid data of the accumulated flow, determine whether the accumulated flow value of the grid outside the mark is greater than the threshold parameter set for extracting the valley. If not, end step S2. If so, find the grid unit with the largest accumulated flow value and start marking the next level valley.

[0019] S25, taking the grid cell with the largest cumulative flow value determined in step S24 as the outlet of the second-level valley, that is, the first grid cell of one of the tributaries of the first-level valley, and marking the grid cell;

[0020] S26, according to steps S22-S23, complete the calibration of a tributary valley of the first-level valley;

[0021] S27, repeating steps S24-S26 to continuously mark the level of the valley, wherein the level calibration principle is to add 1 to the level of the marked valley adjacent to the first grid unit;

[0022] Furthermore, the specific process of step S3 is as follows:

[0023] S31, selecting a secondary valley, starting from the intersection of the valley and the primary valley, inserting nodes in sequence along the selected secondary valley according to a specified parameter step length;

[0024] S32, for the nodes inserted in step S31, calculate the vertical distance from each node to the first-level valley in turn;

[0025] S33, starting from the intersection point of the currently processed secondary valley and the primary valley, traverse all nodes on the secondary valley, and count the length of the longest continuous valley section in the secondary valley that is parallel to the primary valley, wherein the judgment basis for a valley section in the secondary valley (i.e., the valley between two nodes) being parallel to the primary valley is that the distances from the two nodes to the primary valley are equal or the difference is within a specified parameter range;

[0026] S34, if the length of the longest continuous valley segment parallel to the primary valley in the currently processed secondary valley obtained by statistics in step S33 is greater than 3 times the node step length, it can be considered that the secondary valley has a parallel valley phenomenon, and its ID is recorded;

[0027] S35, for all other secondary valleys, perform the operations of steps S31-S34 in sequence.

[0028] Furthermore, the specific process of step S4 is as follows:

[0029] S41, read the valley marked as a parallel valley, start from the intersection point of the valley and the valley of the previous level, traverse all nodes in sequence, determine the parallel valley segment between the valley and the valley of the previous level according to the method designed in step S33, and record the end node position of the parallel valley segment;

[0030] S42, after step S41 is completed, among all nodes marked as the end of the parallel valley segment, the node farthest from the intersection of the current processing valley and the previous valley is selected as the parallel valley modification node;

[0031] S43, taking the parallel valley modification node determined in step S42 as the starting point, drawing a vertical line to the upper valley as the baseline, and then starting from the parallel valley modification node, generating a new valley line segment along the maximum slope direction along the baseline without exceeding a certain distance on the left and right, and deleting the part of the original valley from the parallel valley modification node to the intersection with the upper valley, and marking the parallel valley as a non-parallel valley;

[0032] S44, executing the operations of steps S41-S43 in sequence for all marked parallel valleys in the valley of the current level.

[0033] Technical features and beneficial effects of the present invention:

[0034] (1) The parallel valley elimination method proposed in the present invention is based on the current mainstream valley network extraction method. It does not require modification of the original data and the key steps of valley network extraction. It only further processes the valley network extraction to eliminate the parallel valleys in the valley network extraction results. The method is simple to operate and has strong applicability.

[0035] (2) The parallel valley elimination method proposed in the present invention is designed based on the relevant definitions and analysis specifications of valley networks in geomorphology. During its specific implementation, this method can not only effectively eliminate the parallel valley phenomenon, but also eliminate unreasonable valleys at the same time, thus avoiding the phenomenon of multiple valley lines appearing in the bottom of a valley. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a schematic diagram of the process of the present invention.

[0037] Figure 2 It is a schematic diagram of the spatial positions of nodes on the secondary valley line and the primary valley line in the method of the present invention.

[0038] Figure 3 Schematic diagram of the positions of parallel valley modification points determined in the method of the present invention.

[0039] Figure 4 Schematic diagram of parallel valley modification in the method of the present invention.

[0040] Figure 5 It is a schematic diagram of the valley processing method that intersects with the parallel valley deletion part in the method of the present invention.

[0041] Figure 6It is a schematic diagram of the result of parallel valley processing in the method of the present invention.

[0042] Figure 7 Schematic diagram of the initial valley extracted in the embodiment of the present invention.

[0043] Figure 8 Schematic diagram of valley classification in an embodiment of the present invention.

[0044] Fig. 9 Schematic diagram of the change in the distance from a node on a parallel valley to its upper valley line in an embodiment of the present invention.

[0045] Fig.10 It is a schematic diagram after the parallel valleys are eliminated in an embodiment of the present invention. DETAILED DESCRIPTION

[0046] The principles and features of the present invention are described below in conjunction with the accompanying drawings. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention.

[0047] like Figure 1 As shown, the specific steps of the method for eliminating parallel valleys in the process of extracting valley networks of the present invention are as follows:

[0048] Step 1: Read the DEM data of the study area, implement the processes of depression filling, flow direction calculation and runoff accumulation calculation in sequence, and then extract the valley network data of the study area based on a certain runoff accumulation threshold parameter;

[0049] Step 2: Determine the first-level valley line in the valley network. The specific method is to use the watershed outlet as the first grid unit of the first-level valley, then search for the grid with the largest runoff accumulation value in its adjacent grids, and use it as the second grid unit of the first-level valley. Search in sequence until no grid with a runoff accumulation value greater than the threshold parameter can be found. At this time, the first-level valley marking is completed.

[0050] Step 3: Find the grid with the largest cumulative flow value in the grids other than the first-level valley as the grid cell of the first secondary valley, and then use this grid cell as the starting point and refer to step 2 to determine the first secondary valley by searching the surrounding grid cells;

[0051] Step 4: Referring to step 3, mark all valleys in the study area by grade;

[0052] Step 5: Select a secondary valley line, start from its intersection with the primary valley, insert nodes in sequence according to a certain step length, and then calculate the shortest distance values ​​from these nodes to the primary valley in sequence, as shown in the figure Figure 2 As shown;

[0053] Step 6: Starting from the node adjacent to the intersection of the secondary valley and the primary valley, determine whether the valley line segment between adjacent nodes is parallel to the primary valley line by comparing the shortest distance between adjacent nodes to the primary valley line, traverse all nodes in the secondary valley line, and count the length of the longest continuous valley segment in the secondary valley line that is parallel to the primary valley line. If the length is greater than three times the node step length, it can be considered that the secondary valley line has a parallel valley phenomenon and its ID is marked;

[0054] Step 7: Perform parallel valley judgment on other secondary valleys that have not yet been judged, and mark the ID of the secondary valleys with parallel valley phenomenon;

[0055] Step 8: Starting from the secondary valley line, refer to the operation method of step 6 to determine whether there is a parallel valley phenomenon in the valley one level lower than it. If so, mark its ID;

[0056] Step 9: Select a valley line marked as a parallel valley, starting from the intersection of the valley line and the valley of the previous level, determine the parallel valley section along the valley line toward the starting point of the valley line, and record the nodes farthest from the intersection in the parallel valley section. When the parallel valley section of the valley line is determined, select the node farthest from the intersection as the parallel valley modification point, as shown in the figure. Figure 3 As shown;

[0057] Step 10: Draw a vertical line from the parallel valley modification node to the upper valley to modify the parallel valley, as shown in the figure. Figure 4 As shown;

[0058] Step 11: Determine whether there is another valley line joining the line segment between the intersection of the current parallel valley and its previous valley and the parallel valley modification node. If so, draw a vertical line from the intersection point with other valley lines to its previous valley, as shown in the figure. Figure 5 As shown;

[0059] Step 12: Delete the line segment between the current parallel valley and its previous valley intersection and the parallel valley modification node, and complete the elimination of the current parallel valley line. The result is as shown in the figure. Figure 6 As shown;

[0060] Step 13: Refer to Step 9 to Step 12 to complete the processing of other parallel valley lines;

[0061] Step 14: Output the results to obtain the valley network data with parallel valleys eliminated.

[0062] Example

[0063] In order to verify the feasibility of the method of the present invention in extracting valley networks, the embodiment selects a small watershed in northern Shaanxi where the loess is higher than the ground as the experimental area. The experimental area is a typical water erosion terrain with a very well-developed valley network system. The bottom of the main valley in the basin is flat and wide, and parallel valleys are very likely to occur when extracting the valley network based on DEM data.

[0064] Step 1: For the sample area, after the processes of depression filling, flow direction calculation, and runoff accumulation calculation, the mean change point analysis method is used to determine the optimal runoff accumulation threshold parameter for the valley network extraction in the basin as 1000. Based on this parameter, the valley network of the basin is extracted. The results are as follows: Figure 7 As shown in Figure 1, typical parallel valleys appearing in the sample area are shown in Figure 1. Figure 7 As shown in the dashed box;

[0065] Step 2: The valleys in the watershed are graded one by one. The results are as follows: Figure 8 As shown;

[0066] Step 3: Determine parallel valleys. Fig. 9 To select the shortest distance from a node calculated in one of the parallel valleys to its upper valley, the black solid circle in the figure indicates the position of the modified node in the parallel valley;

[0067] Step 4: Eliminate the marked parallel valleys one by one. The result is as follows: Fig.10 shown.

[0068] It can be seen from the operation steps of this example that the parallel valley elimination method designed by the method of the present invention is carried out on the current mainstream valley network extraction results. The operation process is simple and easy to implement, and does not involve the modification of the original data and the change of the valley network extraction algorithm. Fig.10 ) shows that this method has a good effect on eliminating parallel valleys and can meet the needs of applications such as landform parameter calculation.

Claims

1. A method for eliminating parallel valleys in the process of extracting valley networks, characterized in that: The steps include: S1, input DEM data, extract valley network data of the study area through the process of depression filling, flow direction calculation, runoff accumulation calculation, and setting threshold parameters; S2, valley level calibration, starting from the outlet of the study area, upstream, along the direction of the largest increase in runoff accumulation, determine the first-level valley in the study area, then for all the tributaries of the first-level valley, upstream from the point where they intersect with the first-level valley, determine all the second-level valleys, and so on, complete the level calibration for all valleys; S3, for all second-level valleys, starting from the intersection of the second-level valley and the first-level valley, the distance from the node on the second-level valley line to the first-level valley line is calculated according to a certain step length, and the parallel valleys are detected according to the change of the distance; S4, after detecting the parallel valley, based on the distance value from the node on the valley line to the primary valley, determine the threshold mutation point of the convergence accumulation amount, take the point as the starting point, take the vertical line from the point to the primary valley as the reference direction, and generate the modified valley section from the point along the reference direction according to the maximum slope drop principle without exceeding a certain distance on the left and right, and delete the point to the intersection of the original parallel valley and the primary valley; S5, execute steps S3-S4 for all secondary valleys, and then start from the secondary valleys, regard each secondary valley as a primary valley, judge and eliminate the parallel valleys of its tributaries, until all valleys are processed, output data, and obtain valley network data with parallel valleys eliminated.

2. The method for eliminating parallel valleys in the process of extracting valley networks according to claim 1, characterized in that: The specific process of step S2 is: S21, read the grid data of runoff accumulation, determine the grid unit with the largest runoff accumulation value as the watershed outlet of the study area, take it as the first grid unit of the main valley, and mark the grid; S22, starting from the watershed outlet, select the grid cell with the largest cumulative flow value among its adjacent grid cells as the second grid cell of the main valley, and use this grid cell as the current processing grid cell, and mark this grid cell at the same time; S23, repeating step S22 until the cumulative flow values ​​of all adjacent grids of the currently processed grid unit are lower than the threshold parameter set for extracting the valley, completing the calibration of the first-level valley; S24, for the grid data of the accumulated flow, determine whether the accumulated flow value of the grid outside the mark is greater than the threshold parameter set for extracting the valley. If not, end step S2. If so, find the grid unit with the largest accumulated flow value and start marking the next level valley. S25, taking the grid cell with the largest cumulative flow value determined in step S24 as the outlet of the second-level valley, that is, the first grid cell of one of the tributaries of the first-level valley, and marking the grid cell; S26, according to steps S22-S23, complete the calibration of a tributary valley of the first-level valley; S27, repeating steps S24-S26 to continuously mark the level of the valley, wherein the level calibration principle is to add 1 to the level of the marked valley adjacent to the first grid unit.

3. A method for eliminating parallel valleys in the process of extracting valley networks according to claim 1 or 2, characterized in that: The specific process of step S3 is: S31, selecting a secondary valley, starting from the intersection of the valley and the primary valley, inserting nodes in sequence along the selected secondary valley according to a specified parameter step length; S32, for the nodes inserted in step S31, calculate the vertical distance from each node to the first-level valley in turn; S33, starting from the intersection point of the currently processed secondary valley and the primary valley, traverse all nodes on the secondary valley, and count the length of the longest continuous valley section in the secondary valley that is parallel to the primary valley, wherein a valley section in the secondary valley, that is, the valley between two nodes, is parallel to the primary valley based on the fact that the distances from the two nodes to the primary valley are equal or the difference is within a specified parameter range; S34, if the length of the longest continuous valley segment parallel to the primary valley in the currently processed secondary valley obtained by statistics in step S33 is greater than 3 times the node step length, it is considered that the secondary valley has a parallel valley phenomenon, and its ID is recorded; S35, for all other secondary valleys, perform the operations of steps S31-S34 in sequence.

4. The method for eliminating parallel valleys in the process of extracting valley networks according to claim 3, characterized in that: The specific process of step S4 is as follows: S41, read the valley marked as a parallel valley, start from the intersection point of the valley and the valley of the previous level, traverse all nodes in sequence, determine the parallel valley segment between the valley and the valley of the previous level according to the method designed in step S33, and record the end node position of the parallel valley segment; S42, after step S41 is completed, among all nodes marked as the end of the parallel valley segment, the node farthest from the intersection of the current processing valley and the previous valley is selected as the parallel valley modification node; S43, taking the parallel valley modification node determined in step S42 as the starting point, drawing a vertical line to the upper valley as the baseline, and then starting from the parallel valley modification node, generating a new valley line segment along the maximum slope direction along the baseline without exceeding a certain distance on the left and right, and deleting the part of the original valley from the parallel valley modification node to the intersection with the upper valley, and marking the parallel valley as a non-parallel valley; S44, executing the operations of steps S41-S43 in sequence for all marked parallel valleys in the valley of the current level.

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