Hierarchical coding method for boundary line of catchment area

Abstract

The invention discloses a catchment area boundary hierarchical coding method, and relates to the technical field of terrain analysis based on a digital elevation model.The method comprises the steps that firstly, a water system network and a catchment area of an analysis area are extracted through depression filling and a flow direction calculation process based on DEM data, and catchment area boundary line data are obtained from the catchment area through data conversion; performing level coding on the water system network by adopting a Strahler hierarchical coding method, and determining the maximum level of the water system line segment of the to-be-analyzed area and a corresponding water collection area; according to the water collecting area boundary line hierarchical coding method, the boundary lines shared by the water collecting areas of multiple levels are subjected to level coding according to the principle of high-level priority, the related theory of geomorphology is met, and the boundary coding result of the whole water collecting area is accurate.

Description

Technical Field

[0001] The present invention relates to the technical field of terrain analysis based on digital elevation models, and particularly relates to a method for hierarchical coding of catchment boundary lines. Background Art

[0002] In the field of digital terrain analysis, a catchment is a topographic unit where surface water converges and is discharged, and the boundary between adjacent catchments in space is called the catchment boundary line. A catchment is a basic unit for surface hydrological analysis and is also a basic geomorphic division unit in mountainous areas. As the linear generalization information of a catchment, the catchment boundary line has important research and application values in fields such as the extraction of typical topographic features like mountaintop points and ridge lines.

[0003] The technology for dividing and extracting catchments is currently very mature and can be realized based on the surface runoff simulation process. The basic process is to simulate the surface runoff process of water, first generate the surface water system network, and then determine the water system line segments into which each point on the surface flows. The set of surfaces that flow into the same water system line segment is the catchment of that water system line segment. As is well known, there are typical level differences in the water system network. Therefore, experts and scholars in the fields of hydrology, geomorphology, etc. have designed different water system network hierarchical methods very early, among which the more famous ones are the Strahler hierarchical method, the Shreve hierarchical method, etc. According to the relationship between the above-mentioned catchment and the water system line segment, when the hierarchy of the water system network is determined, the hierarchical method of the catchment corresponding to each water system line segment can also be determined accordingly; however, the hierarchical coding information of the catchment cannot be directly associated with the catchment boundary line because when a region is divided by catchments of different levels, the catchment boundary line is also divided into different line segments, and a catchment boundary line may be the common boundary of multiple catchments of different levels, which results in the inability to directly assign the level coding of the catchment to the catchment boundary line.

[0004] Experts and scholars in the field of digital analysis have also realized the complexity of the problem of assigning levels to catchment boundary lines. The current common practice is to set different runoff accumulation thresholds during the extraction process of the water system network, extract the water system network and catchments multiple times, and use the level coding of the catchment to assign level coding values to the catchment boundary after each extraction, and then superimpose the different results and complete the final assignment of the catchment boundary level coding according to certain rules. However, the defects of this operation method are also very obvious. First, the operation of simulating the runoff process multiple times makes the process of assigning level coding to the catchment boundary line very complex; second, and more importantly, for the vast majority of regions, there is no strict consistency relationship between the level of the catchment and its area. Therefore, this method essentially cannot avoid the problems discussed above, resulting in the possibility that the final level coding of the catchment boundary line may still be incorrect.

[0005] Based on the above analysis, although it is necessary to classify the catchment boundary lines, there are uncertainties in the existing methods. Currently, there is a lack of a method with a clear operation process and correct level coding. Therefore, although the current mainstream GIS spatial analysis software basically provides tools for grading and coding of river network, there is no tool for grading and coding of catchment boundary lines, which undoubtedly brings inconvenience to the application of catchment boundary lines. To solve this problem, an invention designs a method for grading and coding catchment boundary lines. This method draws on the basic process of geomorphic evolution, clarifies the level coding rules when a catchment boundary line serves as the boundary line of multiple catchments at different levels simultaneously, and then determines the level coding value of each catchment boundary segment according to the confluence paths of the catchments on both sides. This method is simple to operate and the results are accurate. The technical operation process can be easily integrated into the existing GIS spatial analysis software, which can bring convenience to relevant analyses in the field of digital analysis. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for grading and coding catchment boundary lines to solve the problems existing in the prior art as described in the above background art.

[0007] To achieve the above purpose, the present invention provides the following technical solutions:

[0008] A method for grading and coding catchment boundary lines includes the following steps:

[0009] S1: Input DEM data, and after steps of depression filling, flow direction calculation, and flow accumulation calculation, then set a flow accumulation threshold to extract the river network data of the analysis area;

[0010] S2: Based on the river network data and flow direction data, extract the catchments of the analysis area, convert the obtained catchment data into vector polygons, and then obtain the catchment boundary lines by the method of polygon to surface;

[0011] S3: Assign level coding values to the river network segments according to the Strahler coding rule for the obtained river network;

[0012] S4: Determine the river network segments at the highest level that need to perform the grading operation and the corresponding catchments according to the river network extraction results. The determined catchments are the areas to be processed, and the areas outside the catchments are the non-processing areas;

[0013] S5: For each catchment boundary segment, select a method for determining the level code according to its position: If one side of the segment is the area to be processed and the other side is the non - processed area, then assign the level code of the segment as the maximum level code value of the water system segments determined in step S4; If both sides of the segment are areas to be processed, then execute step S6 to determine its level code value; If the level code values of all catchment boundary segments to be processed have been determined, then execute step S7;

[0014] S6: For the currently processed catchment boundary segment, determine the level code value of the catchment boundary segment according to the intersection position of the paths in the catchment areas on both sides and the level codes of the relevant water system segments;

[0015] S7: Add a level code field to the attributes of the catchment boundary segment data, assign the determined level code to the level code field of the corresponding segment, and save the output to complete the hierarchical coding operation of the catchment boundary segment.

[0016] Preferably, the specific process of S6 is as follows:

[0017] S61: Take the mid - point of the catchment boundary segment whose level code is to be determined, and then take a grid adjacent to the grid space where the mid - point is located on each side of the catchment boundary segment, and mark them as P and Q respectively;

[0018] S62: According to the grid flow direction information calculated in step S1, generate confluence paths starting from grids P and Q respectively. If the generated confluence paths enter the water system in grid form, record the level code values of the water systems passed by the confluence paths;

[0019] S63: During the process of generating the confluence paths, every time the confluence paths extend by one grid, it is necessary to judge whether the two confluence paths intersect, that is, the newly extended grids of the two confluence paths are the same grid; When the two confluence paths intersect, mark the level code value of the water system segment where the intersecting grid is located as R;

[0020] S64: Determine the level code values of the water system segments passed by the two confluence paths before entering the water system segment with level R according to the level code information of the water system segments passed by the confluence paths recorded in step S62, and mark them as S and T respectively, corresponding to the two confluence paths starting from P and Q; If the confluence paths do not pass through other water system segments before entering the water system segment with level R, then assign 0 to S or T;

[0021] S65: Assign a level code value to the current catchment boundary segment according to the size relationship between S and T. The specific method is: If S is greater than or equal to T, then the level code value of the current catchment boundary segment is S; If S is less than T, then the level code value of the current catchment boundary segment is T.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] 1. There is a good correspondence between the hierarchical coding of the catchment boundary segments obtained by the method designed based on this research and the hierarchical coding of the gully network, which is specifically manifested in two aspects: First, the levels correspond to each other. The first-level gully corresponds to the first-level catchment boundary line, and the fourth-level gully also corresponds to the fourth-level catchment boundary line. Second, the spatial position relationship corresponds. The boundary of a catchment at a certain level completely contains the gully at the same level.

[0024] 2. The method of the present invention is simple and easy to operate. The main operation processes can all be completed using the spatial analysis tools of current mainstream GIS software, and it is easy to be quickly applied in practice. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a flowchart of the method of the present invention.

[0026] Figure 2 is a distribution map of the water system network extracted in the embodiment.

[0027] Figure 3 is a distribution map of the catchment boundary lines extracted in the embodiment.

[0028] Figure 4 is a schematic diagram of the hierarchical classification of the water system network in the embodiment.

[0029] Figure 5 is a schematic diagram of the catchment area range and water system classification for which hierarchical coding processing is to be carried out in the embodiment.

[0030] Figure 6 is a schematic diagram of the process for determining the level coding of the catchment boundary segments in the embodiment.

[0031] Figure 7 is the result of the hierarchical coding of the catchment boundary lines in the processing area in the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0032] In order to make the technical means, creative features, achieved purposes and functions of the present invention easy to understand, the present invention will be further described below in conjunction with specific embodiments.

[0033] Please refer to Figures 1-7 , the present invention provides the following technical solutions:

[0034] A method for hierarchical coding of catchment boundary lines, characterized by comprising the following steps:

[0035] S1: Input DEM data, and after steps of depression filling, flow direction calculation, and flow accumulation calculation, then set a flow accumulation threshold to extract the water system network data of the analysis area;

[0036] S2: Based on the water system network data and flow direction data, extract the catchment areas of the analysis area, convert the obtained catchment area data into vector polygons, and then obtain the catchment boundary lines by the method of polygon to surface conversion;

[0037] S3: Code and assign levels to the water system line segments according to the Strahler coding rule for the obtained water system network;

[0038] S4: Determine the water system line segments of the maximum level that need to perform the grading operation and the corresponding catchment areas according to the water system network extraction results. The determined catchment areas are the areas to be processed, and the areas outside the catchment areas are the non-processing areas;

[0039] S5: For each catchment boundary line segment, select the method for determining the level code according to its position: If one side of the line segment is the area to be processed and the other side is the non-processing area, then assign the level code of the line segment as the maximum level code value of the water system line segments determined in step S4; If both sides of the line segment are the areas to be processed, then execute step S6 to determine its level code value; If the level code values of all catchment boundary line segments to be processed have been determined, then execute step S7;

[0040] S6: For the currently processed catchment boundary line segment, determine the level code value of the catchment boundary line segment according to the intersection position of the paths in the catchment areas on both sides and the level codes of the relevant water system line segments; The specific process of S6 is:

[0041] S61: Take the midpoint of the catchment boundary line segment whose level code is to be determined, and then take a grid adjacent to the grid space where the midpoint is located on each side of the catchment boundary line segment, and mark them as P and Q respectively;

[0042] S62: According to the grid flow direction information calculated in step S1, generate catchment paths starting from grids P and Q respectively. If the generated catchment paths enter the water system in grid form, record the level code values of the water systems passed by the catchment paths;

[0043] S63: During the process of generating the catchment paths, every time the catchment paths extend by one grid, it is necessary to judge whether the two catchment paths intersect, that is, the newly extended grids of the two catchment paths are the same grid; When the two catchment paths intersect, mark the level code value of the water system line segment where the intersecting position grid is located as R;

[0044] S64: Determine the level coding values of the water system line segments passed by the two catchment paths before entering the water system line segment of level R according to the level coding information of the water system line segments passed by the catchment paths recorded in step S62, and mark them as S and T respectively, corresponding to the two catchment paths starting from P and Q; if there are no other water system line segments passed by the catchment path before entering the water system line segment of level R, then assign 0 to S or T.

[0045] S65: Assign a level coding value to the current catchment boundary line segment according to the size relationship between S and T. The specific method is: if S is greater than or equal to T, the level coding value of the current catchment boundary line segment is S; if S is less than T, the level coding value of the current catchment boundary line segment is T.

[0046] S7: Add a level coding field to the attributes of the catchment boundary line segment data, assign the determined level coding to the level coding field of the corresponding line segment, and save the output to complete the hierarchical coding operation of the catchment boundary line segment.

[0047] Embodiment 1

[0048] In order to verify the usability of the method of the present invention in the hierarchical coding of the catchment boundary line, in Embodiment 1, a certain loess hilly and gully area in northern Shaanxi is selected as the experimental area. This area is a typical terrain of fluvial erosion, and the gully system is very well developed. Therefore, the corresponding catchment boundary is clear and can be used as an ideal area for the verification experiment of the hierarchical coding of the catchment boundary line.

[0049] Step 1: For the example area, through processes such as depression filling, flow direction calculation, and flow accumulation calculation, and then set the flow accumulation threshold to 1000, and extract the water system network of the example area ( Figure 2 ). The base map in the figure is a hill shade map generated from the DEM, which can enhance the display effect of data such as the water system visually.

[0050] Step 2: Based on the flow direction data and water system network data of the example area, divide the catchments of different water system line segments, and then obtain the catchment boundary line through data format conversion ( Figure 3 ).

[0051] Step 3: Perform hierarchical coding on the extracted water system network according to the Strahler method ( Figure 4 ). The value of the water system raster cell in the figure is the level coding value of the water system line segment where it is located.

[0052] Step 4: The hierarchical coding of the catchment boundary needs to be carried out in a complete catchment. Determine the catchment to be processed in this embodiment according to the hierarchical coding result of the water system network and the catchment division result. The maximum level of the internal water system line segments is 4 ( Figure 5 ).

[0053] Step 5: Select a catchment boundary line segment, take its midpoint, and in the catchments on both sides thereof, respectively take a grid adjacent to the grid where the midpoint of the catchment boundary line segment is located; generate catchment paths starting from these two grids respectively, and determine the level code of the water system line segment where the two catchment paths intersect and the code of the water system line segment before intersection( Figure 6 ), according to the assignment rule of the catchment boundary line level code, the level code value of the current catchment boundary line segment is 3;

[0054] Step 6: Assign level codes to all catchment boundary line segments in the operation mode of Step 5, and finally obtain the code assignment result of the catchment boundary lines in the example area( Figure 7 ).

[0055] Although the embodiments of the present invention have been shown and described, those of ordinary skill in the art can understand that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principle and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for hierarchical coding of catchment boundary lines, characterized in that, It includes the following steps: S1: Input DEM data, and through steps of depression filling, flow direction calculation, and flow accumulation calculation, then set a flow accumulation threshold to extract the water system network data of the analysis area; S2: Based on the water system network data and flow direction data, extract the catchment areas of the analysis area, convert the obtained catchment area data into vector polygons, and then obtain the catchment boundary lines by the method of polygon to surface conversion; S3: Code and assign levels to the water system line segments of the obtained water system network according to the Strahler coding rule; S4: Determine the water system line segments of the maximum level that need to perform the grading operation and the corresponding catchment areas according to the extraction result of the water system network. The determined catchment areas are the areas to be processed, and the areas outside the catchment areas are the non-processed areas; S5: For each catchment boundary line segment, select a method for determining the level coding according to its position: If one side of the line segment is the area to be processed and the other side is the non-processed area, then assign the level coding of the line segment as the maximum level coding value of the water system line segment determined in step S4; If both sides of the line segment are the areas to be processed, then execute step S6 to determine its level coding value; If the level coding values of all catchment boundary line segments to be processed are determined, then execute step S7; S6: For the currently processed catchment boundary line segment, determine the level coding value of the catchment boundary line segment according to the intersection position of the paths in the catchment areas on both sides and the level coding of the relevant water system line segments; S7: Add a level coding field to the attributes of the catchment boundary line segment data, assign the determined level coding to the level coding field of the corresponding line segment, and save and output to complete the grading and coding operation of the catchment boundary line segment.

2. The hierarchical coding method for catchment boundary lines according to claim 1, wherein: The specific process of S6 is as follows: S61: Take the midpoint of the catchment boundary line segment whose level coding is to be determined, and then take a grid adjacent to the grid space where the midpoint is located on each side of the catchment boundary line segment, and mark them as P and Q respectively; S62: According to the grid flow direction information calculated in step S1, generate catchment paths starting from grids P and Q respectively. If the generated catchment paths enter the water system in grid form, record the level coding value of the water system passed by the catchment path; S63: During the process of generating the catchment path, when the catchment path extends by one grid each time, judge whether the two catchment paths intersect, that is, the newly extended grids of the two catchment paths are the same grid; When the two catchment paths intersect, mark the level coding value of the water system line segment where the intersecting position grid is located as R; S64: Determine the level coding values of the water system line segments passed by the two catchment paths before entering the water system line segment of level R according to the level coding information of the water system line segments passed by the catchment paths recorded in step S62, and mark them as S and T respectively, corresponding to the two catchment paths starting from P and Q respectively; If the catchment path does not pass through other water system line segments before entering the water system line segment of level R, then assign S or T as 0; S65: Assign a level coding value to the current catchment boundary segment according to the size relationship between S and T. The specific method is as follows: If S is greater than or equal to T, the level coding value of the current catchment boundary segment is S; if S is less than T, the level coding value of the current catchment boundary segment is T.

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