Method and system for processing depression terrain

Abstract

The invention discloses a depression terrain processing method and system, and relates to the technical field of hydrological analysis, and the method comprises the steps: carrying out the ponding analysis of a depression to obtain a downward excavation initial elevation and a depression submerged area, and selecting a downward excavation submerged area from the depression submerged area; the pouring point of each depression serves as a starting point, downstream tracking is conducted in the water flow direction of the drainage basin till the pouring point is lower than the downward digging starting elevation, and a downstream water flow line of the pouring point of each depression is formed; an optimized refining algorithm is adopted to extract the center line of each depression from the depression submerging edge map of each depression, the pouring point of each depression is tracked to the downward excavation submerging area along the upstream of the center line, and the upstream water flow line of the pouring point is obtained; connecting the upstream water flow line with the downstream water flow line to obtain a pouring line; and according to the elevation of the dumping line of each depression, performing terrain digging along the dumping line, so that water flow in each depression smoothly flows out from the dumping point to correct the depression. It is guaranteed that the upstream of the original depression forms the correct water flow direction according to the terrain.

Description

Technical Field

[0001] The present invention relates to the technical field of hydrological analysis, and particularly relates to a method and system for processing depression terrain. Background Art

[0002] The basic data source for hydrological analysis is a raster DEM, which is an elevation matrix composed of regular grid cells on the ground surface. Due to factors such as atmospheric conditions, ground reflection properties, measurement instrument accuracy, and data interpolation methods, there are certain errors in the raster DEM. These errors become obstacles to hydrological analysis. Therefore, various algorithms are proposed to reduce terrain errors. In some areas, due to abnormal elevations, a local depression is formed, and the water flow direction cannot be determined. In order to make all the raster water flows in the basin finally flow out from the basin outlet, a depression filling algorithm is proposed. However, after the depression is filled, its elevation is set to the same value, and the terrain appears as flat ground. Since the water flow direction cannot be accurately calculated, deformed channels appear in this area, and the upstream area where the flow direction can be correctly calculated no longer outputs the correct flow direction because of depression filling. This is one of the main problems faced by current hydrological analysis.

[0003] In the prior art, for the problems of the depression filling algorithm, Method 1 proposes a river burning method to deal with the channel terrain error problem. This method integrates the actual river trend, and supplements the actual flow path information of the DEM data through river burning, so as to reduce the difference between the river trend extracted from the DEM and the real river; Method 2, on the basis of river burning, also designs a program to remove local depressions in the DEM. Since the river burning method has a high dependence on the trend and elevation of the actual river data and cannot process depressions outside the river, it cannot fundamentally solve the algorithm problem of depression filling. Method 3 proposes an algorithm for extracting the flow direction by using a two-dimensional hydrodynamic model. This method solves the problem that the depression water flow cannot flow through confluence simulation without deleting the terrain depression from the DEM. It is a new idea for obtaining the flow direction from the error terrain. Although this method does not fill the depression in advance, it is still an algorithm that fills the depression first and then determines the flow direction in essence.

[0004] In summary, for the depressions caused by terrain errors, the mainstream method is still to fill the depressions. Filling the depressions will set the elevation of the depressions to the same elevation as the surrounding terrain, making the depression area become flat terrain, affecting the water flow direction upstream and downstream of the depressions, and unable to solve the problem of abnormal upstream flow direction caused by filling the depressions. Summary of the Invention

[0005] The embodiments of the present invention provide a method and system for processing depression terrain, which can solve the problem of abnormal upstream flow direction caused by filling the depressions in the prior art.

[0006] The embodiments of the present invention provide a method for processing depression terrain, including the following steps: Obtain the raster digital elevation model (DEM) of multiple depressions within the basin; Extract the pour points on the depression edges of each depression, and conduct ponding analysis on the depressions to obtain the starting elevation for excavation and the inundation areas of the depressions; within the inundation areas of depressions containing multiple sub-areas, select the sub-area with the largest area and the largest width as the excavation inundation area; Starting from the pour point of each depression, trace the water flow trajectory downstream along the water flow direction of the basin; during the tracing process, if a position lower than the starting elevation for excavation is encountered, stop tracing, and obtain the downstream water flow line of the pour point of each depression according to the traced water flow trajectory; Extract the center line of each depression from the depression inundation edge map of each depression, and trace the water flow trajectory upstream along the center line from the pour point of each depression; during the tracing process, if the excavation inundation area is encountered, stop tracing, and obtain the upstream water flow line of the pour point according to the traced water flow trajectory; Connect the tail of the upstream water flow line of the pour point of each depression to the head of the downstream water flow line to obtain the pour line of each depression; according to the elevation of the pour line of each depression, conduct terrain excavation along the pour line so that the water flow within each depression can smoothly flow out from the pour point to correct the depression.

[0007] Furthermore, after conducting terrain excavation along the pour line, it further includes: Re-fill the depressions that have completed terrain excavation and obtain the water flow direction; Update the terrain after re-filling and the obtained water flow direction into the original terrain of the basin.

[0008] Furthermore, the specific steps for obtaining the starting elevation for excavation include: Conduct ponding analysis on each depression, and when the ponding water level gradually increases, determine the inundation area corresponding to the ponding water level; During the ponding analysis process, record the minimum number of pixels required for each inundated pixel in the depression to reach the non-inundated area, which is the pixel width of the depression, and construct a depression inundation edge map according to the pixel width of the depression; Set a threshold for the pixel width of the depression, and conduct excavation on depressions with a width greater than the threshold; Use the bisection search algorithm to obtain the starting elevation for excavation.

[0009] Furthermore, the specific steps for using the bisection search algorithm to obtain the starting elevation for excavation include: Statistical the pixel elevations of all pixels within each depression, and obtain the lowest elevation of each depression H min and the highest elevation H max ; Set the current search elevation range as Hf , H t , let H f = H min , H t = H max ; Let the current waterlogging level be h , and the historical waterlogging level and the starting elevation of excavation be hi and hr , let hi=hr=H min ; Let h = ( H f + H t ) / 2. If h = hi , the algorithm ends and outputs the starting elevation of excavation hr , otherwise, obtain the waterlogged area of the depression D h ; Among them, a is the pixel that makes up the waterlogged area of the depression, H a is the elevation of the pixel a on the original terrain, D is the set of depression pixels; Generate D h corresponding depression waterlogged edge map, and obtain the pixel width of the depression, set as V ; If V > 2, let H t = h ; If V <= 2, let H f = h ; For V = 2, update hr = h ; When hi = h , return to h = ( H f + H t ) / 2 to execute the algorithm; The algorithm ends and outputs the starting elevation of excavation hr。

[0010] Further, extracting the center line of each depression from the depression inundation edge map of each depression, the specific steps include: Extracting the center line of each depression from the depression inundation edge map of each depression by using an optimized thinning algorithm; The optimized thinning algorithm adds a pixel constraint condition on the basis of the traditional thinning algorithm, and the optimized thinning algorithm f ( x , y ), the formula is: Wherein, Q is the set of constraint pixels, p ( x , y ) represents the pixel of each depression, x represents the abscissa of the pixel, y represents the ordinate of the pixel, t ( x , y ) represents whether the pixel p(x,y) satisfies the thinning condition according to the rules of the traditional thinning algorithm, False represents not satisfying the thinning condition.

[0011] Further, according to the elevation of the pouring line of each depression, excavating the terrain along the pouring line, specifically including: According to the elevation of each pixel on the pouring line, starting from the starting point of the pouring line, update the elevation of each pixel one by one, ensuring that the elevation of each pixel is not higher than the elevation of the previous pixel, and the elevation of the starting point of the pouring line is the starting elevation of the excavation.

[0012] An embodiment of the present invention provides a processing system for depression terrain, including: A data acquisition module, configured to acquire the raster digital elevation model DEM of multiple depressions in the basin; A depression analysis module, configured to extract the pouring points on the depression edge of each depression, and perform ponding analysis on the depression to obtain the starting elevation of the excavation and the inundation area of the depression; in the inundation area of the depression containing multiple sub-areas, select the sub-area with the largest area and the largest width as the excavation inundation area; A downstream water flow line acquisition module, configured to start from the pouring point of each depression and trace the trajectory of the water flow downstream along the water flow direction of the basin; during the tracing process, if a position lower than the starting elevation of the excavation is encountered, stop tracing, and obtain the downstream water flow line of the pouring point of each depression according to the traced water flow trajectory; An upstream water flow line acquisition module, which is used to extract the center line of each depression from the depression inundation edge map of each depression, and trace the water flow trajectory upstream along the center line from the pour point of each depression; during the tracing process, if the tracing encounters a dug-out inundation area, the tracing stops, and the upstream water flow line of the pour point is obtained according to the traced water flow trajectory; A pour line acquisition module, which is used to connect the tail of the upstream water flow line of the pour point of each depression to the head of the downstream water flow line to obtain the pour line of each depression; according to the elevation of the pour line of each depression, the terrain is dug down along the pour line so that the water in each depression can flow out smoothly from the pour point to correct the depression.

[0013] The embodiment of the present invention provides a method and system for processing depression terrain. Compared with the prior art, the beneficial effects are as follows: Extract the pour point on the depression edge of each depression, and perform water accumulation analysis on the depression to obtain the starting elevation for digging and the inundation area of the depression; in the inundation area of the depression containing multiple sub-areas, select the sub-area with the largest area and the largest width as the dug-out inundation area; starting from the pour point of each depression, trace the water flow trajectory downstream along the water flow direction of the basin; during the tracing process, if a position lower than the starting elevation for digging is encountered, the tracing stops, and the downstream water flow line of the pour point of each depression is obtained according to the traced water flow trajectory; extract the center line of each depression from the depression inundation edge map of each depression, and trace the water flow trajectory upstream along the center line from the pour point of each depression; during the tracing process, if the tracing encounters a dug-out inundation area, the tracing stops, and the upstream water flow line of the pour point is obtained according to the traced water flow trajectory; connect the tail of the upstream water flow line of the pour point of each depression to the head of the downstream water flow line to obtain the pour line of each depression; according to the elevation of the pour line of each depression, the terrain is dug down along the pour line so that the water in each depression can flow out smoothly from the pour point to correct the depression.

[0014] Among them, the downstream water flow line of the pour point traces the water flow trajectory downstream along the water flow direction of the basin from the pour point of each depression, the upstream water flow line of the pour point traces the water flow trajectory upstream along the center line from the pour point of each depression, the downstream water flow line of the pour point is connected to the upstream water flow line of the pour point to form a pour line, and the terrain is dug down along the pour line so that the water in each depression can flow out smoothly from the pour point. The formation of the pour line takes into account the water flow directions upstream and downstream of the depression, and the depression can be corrected by digging down along the pour line. At this time, the water flow directions upstream and downstream of the depression are the correct water flow directions. Description of the Drawings

[0015] Figure 1 It is the terrain processing algorithm flow based on the pour line for a method of processing depression terrain provided by the embodiment of the present invention; Figure 2The depression discharge point and the downstream water flow line of a method for processing a depression terrain provided by an embodiment of the present invention; Figure 3 The channel lines generated by depressions of different widths in a method for processing a depression terrain provided by an embodiment of the present invention; Figure 4 The depression inundation area and the excavated inundation area of a method for processing a depression terrain provided by an embodiment of the present invention; Figure 5 The depression center line extracted by the refinement algorithm of a method for processing a depression terrain provided by an embodiment of the present invention, where (a) is the extraction result of the traditional algorithm, (b) is the first refinement result of the improved algorithm, and (c) is the second refinement result of the improved algorithm; Figure 6 The depression tracking network and the shortest path analysis of a method for processing a depression terrain provided by an embodiment of the present invention; Figure 7 The discharge line of a depression in a method for processing a depression terrain provided by an embodiment of the present invention; Figure 8 The topographic profile line and the correction profile line of the discharge line of a method for processing a depression terrain provided by an embodiment of the present invention; Figure 9 The comparison of the discharge line extraction effects of a method for processing a depression terrain provided by an embodiment of the present invention, where (a) is the extraction result of treating each depression in isolation, and (b) is the extraction result of treating the depressions in the order from upstream to downstream and performing depression re-filling and water flow direction calculation; Figure 10 The comparison of the channel extraction before and after terrain excavation of a method for processing a depression terrain provided by an embodiment of the present invention, where (a) is the original terrain and channels, and (b) is the excavated terrain and channels; Figure 11 The comparison of the channel extraction before and after terrain excavation at a certain place of a method for processing a depression terrain provided by an embodiment of the present invention, where (a) is the channels extracted from the original terrain, and (b) is the channels extracted from the excavated terrain. Detailed implementation manners

[0016] To make the above objects, features, and advantages of the present invention more obvious and understandable, the following will describe the detailed implementation manners of the present invention with reference to the accompanying drawings. Many specific details are set forth in the following description to fully understand the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0017] An embodiment of the present invention provides a method for processing a depression terrain, including the following steps: Step 1: Obtain the raster digital elevation model (DEM) of multiple depressions in the basin.

[0018] Step 2: Extract the pour points on the edge of each depression, and conduct ponding analysis on the depression to obtain the starting elevation for excavation and the flooded area of the depression; in the flooded area of the depression containing multiple sub-areas, select the sub-area with the largest area and the largest width as the excavation flooded area.

[0019] Step 3: Starting from the pour point of each depression, trace the water flow trajectory downstream along the water flow direction of the basin; during the tracing process, if a position lower than the starting elevation for excavation is encountered, stop tracing, and obtain the downstream water flow line of the pour point of each depression according to the traced water flow trajectory.

[0020] Step 4: Use an optimized thinning algorithm to extract the center line of each depression from the depression flooded edge map of each depression, and trace the water flow trajectory upstream from the pour point of each depression along the center line; during the tracing process, if the excavation flooded area is encountered, stop tracing, and obtain the upstream water flow line of the pour point according to the traced water flow trajectory; among them, the depression flooded edge map reflects the relationship between the flooded area of the depression and the extracted channel shape during the ponding analysis process.

[0021] Step 5: Connect the tail of the upstream water flow line of the pour point of each depression to the head of the downstream water flow line to obtain the pour line of each depression; according to the elevation of the pour line of each depression, conduct terrain excavation along the pour line so that the water flow in each depression can flow out smoothly from the pour point to correct the depression.

[0022] 1. Algorithm description.

[0023] First, perform terrain depression filling - water flow direction calculation - stream accumulation calculation on the original terrain, extract the depression list based on the filled terrain, and determine the order of depression processing according to the stream accumulation from downstream to downstream; then perform terrain processing on each filled depression, including extraction of pour points, extraction of starting elevation for excavation and excavation flooded area, extraction of pour lines, and terrain processing based on pour lines. The terrain processing algorithm based on pour lines is as Figure 1 shown.

[0024] 1.1 Extraction of pour points.

[0025] After terrain depression filling, the elevation of the depression area is set to the same elevation value. The depression list can be obtained by tracing the connected areas with the same elevation. For each depression, let the set of pixels forming the depression be D . According to the terrain depression filling algorithm, there must be a pixel a on its edge whose flow direction points to the outside of the depression b . The pour point of the depression is expressed as:

[0026] (1) In the formula: represents the water flow direction; for the depressions inside the terrain, b is a pixel; for the depressions at the terrain edge, b represents the terrain edge.

[0027] Through the water flow direction, it is not only easy to determine the pour point of each depression, but also to obtain the downstream water flow direction of the pour point, such as Figure 2 the blue arrows in

[0028] 1.2 Extraction of the starting elevation of excavation and the inundation area of excavation.

[0029] The pour point is located at the edge of the filled depression, and the terrain elevation is higher than the internal area of the depression. To enable the normal outflow of the water in the depression, it is necessary to find the starting position of excavation inside the depression and connect it outward to the pour point to form an internal pour line. Obtaining the starting position of excavation is the key to generating the pour line.

[0030] Finding the lowest position of the original terrain in the depression as the starting position of excavation is a simple and feasible method. However, the lowest point of the terrain may just be a local small depression inside the depression, and directly excavating with the minimum value may lead to an excessive excavation depth. Here, the ponding analysis of the depression is adopted to reasonably determine the starting elevation of excavation.

[0031] (1)Ponding analysis of the depression and extraction of the inundation area.

[0032] If there is no error in the terrain elevation, then the area obtained by filling the depression is the lake in reality. Among them, the elevation after filling the depression represents the maximum ponding elevation of the lake, and the elevation of the original terrain represents the bottom elevation of the lake. When the water level gradually increases from the lowest elevation of the depression, the inundation area of the lake continuously increases. In particular, when the ponding water level is h , the corresponding inundation area of the depression D h is defined as:

[0033] (2) In the formula, a is the pixel that composes the inundation area of the depression, H a is the pixel a 's elevation on the original terrain.

[0034] When the ponding water level reaches the elevation after filling the depression, all the pixels in the pixel set D of the depression are inundated. Let the elevation after filling the depression of the depression be h max , then there is .

[0035] (2) Generation of the depression flooding edge map.

[0036] To study the relationship between the depression flooding area and the shape of the extracted channels, the concept of the depression flooding edge map is introduced. The depression flooding edge map refers to the minimum number of pixels that each flooded pixel passes through along the horizontal and vertical directions to reach the non-flooded area in the flooded area composed of pixels in.

[0037] For the convenience of subsequent elaboration, when h = h max , the depression flooding edge map is also called the depression edge map, such as Figure 2 the pixel text annotation of.

[0038] The depression flooding edge map is generated by the erosion algorithm in the mathematical morphology method. Let B, T ≡ (T1, T2) be the given structuring element, X be the image to be studied, and X, B, T EN, then the erosion operator is defined as:

[0039] X B = (3) For each pixel in, if there is at least one non-flooded pixel among the adjacent pixels in its four directions of up, down, left, and right, then the pixel is an edge pixel. Setting all the edge pixels in the flooded pixels to non-flooded pixels performs 1 erosion on the depression flooding edge map. To obtain the depression flooding edge map, it is necessary to continuously erode the depression flooding edge map until all the depression pixels are set to non-flooded area pixels. During the erosion process, record the number of times each pixel is removed in which erosion, and this number is the pixel value of the depression flooding edge map.

[0040] For the convenience of subsequent elaboration, the definition of the depression pixel width is introduced. The maximum pixel value of the depression flooding edge map is statistically calculated and called the depression pixel width.

[0041] (3) Determination of the starting elevation of excavation.

[0042] Not all depressions after filling will cause serious abnormal water flow directions. Whether there will be a large abnormal water flow direction can be roughly determined through the depression edge map. Figure 3 The blue pixels in are the channels extracted based on the filled terrain, and the pink pixels are the filled pixels. Figure 3 (a) In, the depression pixel width is 2. For such a depression with a small width, the water flow direction generally extends along the real channel, and the extracted channel basically does not show extreme straight lines; while in Figure 3In (b), the width of the depression pixel reaches 6. In a local area with a relatively large width, the water flow direction cannot be accurately judged, resulting in a linear deformed channel.

[0043] Set the depression pixel width threshold to 2, and only dig down the depressions with a depression pixel width greater than 2. The starting elevation for digging down is obtained according to the binary search algorithm, and the algorithm description is as follows:

[0044] 1) Count the pixel elevations of the current depression to obtain the lowest elevation H min and the highest elevation H max .

[0045] 2) Set the current search elevation range as H f , H t , let H f =H min , H t =H max ; Set the current water accumulation level as h , the historical water accumulation level and the starting elevation for digging down are respectively hi and hr , let hi = hr = H min .

[0046] 3) Let h= ( H f + H t ) / 2. If h=hi , go to 7); otherwise, obtain according to Equation (2).

[0047] 4) Generate the depression flooding edge map corresponding to , and obtain the depression pixel width, set as V .

[0048] 5) If V > 2, let H t =h ; If V <= 2, let H f =h ; For V = 2, update hr = h .

[0049] 6) hi=h , go to 3).

[0050] 7) The algorithm ends, output the starting elevation of excavation hr .

[0051] (4) Extraction of the excavated inundation area

[0052] The corresponding depression inundation area can be determined through the starting elevation of excavation. If the depression inundation areas are spatially connected, trace upstream along the pouring point to the nearest edge of the inundation area as the starting point of the pouring point, and the upstream flat ground generated by filling depressions can be eliminated without serious abnormal water flow directions

[0053] However, the depression inundation area is obtained through a simple elevation comparison method without considering the spatial connectivity between pixels. That is to say, the depression inundation area may consist of one to multiple spatially independent sub - regions, which are located at different positions in the upstream and downstream of the watershed Figure 4 Taking a typical example, the blue and purple pixels together constitute the depression inundation area of the starting elevation of excavation, where the purple part is the inundation sub - region with the largest width. According to the above - mentioned upstream extraction algorithm of the pouring line, the pouring line cannot reach the purple area

[0054] To solve this problem, the concept of the excavated inundation area is introduced. The so - called excavated inundation area refers to the inundation sub - region within the depression inundation area corresponding to the starting elevation of excavation that is spatially connected and has the largest width and the largest area

[0055] For the depression inundation area hr corresponding to the starting elevation of excavation D hr , the excavated inundation area is D dig . Obviously D dig belongs to D hr a part of, and is composed of multiple inundated pixels that are spatially connected. When extracting D dig , it must first meet the condition that among all sub - regions in D hr , D dig has the largest width. If there are multiple sub - regions with the same maximum width, they are sorted according to area, and the sub - region with the largest area is selected as the excavated inundation area D dig .

[0056] 1.3 Pouring line extraction

[0057] The extraction of the pour line is divided into two parts. One is the downstream water flow line outside the depression, and the other is the upstream water flow line inside the depression. The combination of the two constitutes the pour line.

[0058] (1)Extraction of the downstream water flow line.

[0059] The downstream water flow line can be extracted through the water flow direction. Starting from the pour point and tracing downstream according to the water flow direction, a water flow line can be obtained. The key to the extraction of the downstream water flow line lies in the definition of the end point. To allow the water to flow normally downstream, the pour line must satisfy that the elevation of the upstream starting point is greater than the elevation of the downstream end point, and the elevation of the upstream starting point is the starting elevation of excavation. hr . Therefore, for the pour point P , when tracing downstream, simultaneously judge the elevation of the pixel where the current tracing position is located on the original terrain. If the elevation is less than hr , then stop tracing downstream.

[0060] (2)Extraction of the upstream water flow line.

[0061] In the upstream area of the pour point, the original terrain is a depression, and the filled depression terrain is flat. There is no available correct water flow direction for upstream tracing. Therefore, a thinning algorithm is used to extract the center line of the depression from the depression edge map, and the upstream water flow line of the pour point is obtained by tracing along the center line.

[0062] Thinning is a mathematical morphological transformation. It finds the central axis or skeleton of a graph or stroke and replaces the graph or stroke with its skeleton. After thinning, the pixel width of the graph is 1. The traditional thinning algorithm only considers the shape of the primitive. After thinning, the center line not only cannot be connected to the pour point, as shown in Figure 5 (a), but also may have a large deviation from the excavated inundation area. In view of this, the present invention adds a pixel constraint condition on the basis of the traditional thinning algorithm. Let the set of constraint pixels be Q , then the pixel thinning judgment function is:

[0063] (4) In the formula, represents whether the pixel p(x,y) meets the thinning condition according to the rules of the traditional thinning algorithm.

[0064] The thinning is carried out in two steps. In the first step, the pour point P and the depression inundation area D hr are combined into Q , and the first thinning is carried out. After thinning, except for the depression inundation area, the rest of the depression edge map becomes a center line, as shown in Figure 5 (b). On this basis, the second thinning is carried out. At this time, the set QOnly the pour point is retained. After two refinements, not only is the centerline connected to the pour point, as shown in Figure 5 Figure (b), but also its orientation is consistent with the depression inundation area, as shown in Figure 5 Figure (c).

[0065] Figure 5 The red pixels shown in Figure (c) are the refined depression centerline. As can be seen from the figure, the pixel width of the refined depression centerline is 1. To obtain a path from the pour point to the nearest excavated inundation area, first, starting from the pour point, trace upward along the centerline. During the tracing, simultaneously determine whether the current pixel is a pixel of the excavated inundation area. If the condition is met, the current path tracing is completed. Combine all the traced paths into a network, and obtain the upstream streamline of the pour point through the shortest path algorithm of the network. Figure 6 The numbers on the centerline form a path from the pour point as the starting point to the starting point upstream of the pour line in sequence.

[0066] 1.4 Terrain processing based on the pour line.

[0067] The terrain processing based on the pour line is carried out in two steps. The first step is to correct the terrain pixel by pixel along the pour line; the second step is to re-fill the depression area and calculate the water flow direction.

[0068] (1) Terrain correction pixel by pixel along the pour line.

[0069] Combine the upstream streamline and the downstream streamline of the pour point to form the pour line, as shown in Figure 7 Figure. The elevation starting point of the depression pour line is the starting elevation of excavation, and extract the elevation on the original terrain pixel by pixel along the pour line to form a terrain profile line, as shown in Figure 8 Figure.

[0070] As can be seen from the figure, the abnormal bulge of the channel is the fundamental reason for the formation of the depression. Under normal circumstances, along the channel downward, the terrain elevation gradually decreases, and the water flow can smoothly flow out of the channel. If the abnormal elevation of a certain pixel P in the channel is h , and no pixel with an elevation smaller than h can be found in the upstream area, then a depression filling area with P as the pour point and the water level of h will be formed. To avoid the occurrence of the depression filling area, it is necessary to excavate the terrain along the depression pour line to ensure that the water flow can smoothly flow out of the pour point P without the need for full depression filling.

[0071] The terrain processing flow based on the pour line is as follows: 1) Set the elevation of the upstream starting point of the pour line as the starting elevation of excavation hr, the elevation of the downstream end point of the pour line is the elevation of the corresponding position of the original terrain he ; If he is greater than hr , then he=hr .

[0072] 2) Read the elevation of the original terrain pixel by pixel along the pour line from upstream to downstream to form an elevation array T . The number of elements of T is m , and the first element and the last element are the elevations of the starting point and the ending point of the pour line respectively, that is T 0 = hr , T m-1 = he .

[0073] 3) Initialize the current elevation h=hr .

[0074] 4) Starting from the first elevation point of the elevation array, traverse each elevation point of T in turn T i (0 ≤ i < m ). If T i > h , then T i = h ; Otherwise h = T i .

[0075] 5) Traverse each element in T T i , and set the elevation at the corresponding position of the original terrain to T i .

[0076] Figure 7 The red line of

[0077] (2) Depression re-filling and water flow direction calculation after depression terrain processing.

[0078] In the actual state, the spatial distribution of terrain errors is uneven, and there may be another depression near the downstream of the depression. If the spatial relationship between depressions is not considered and each depression is processed in isolation, when extracting the water flow line downstream of the pour line, it may enter the filled depression downstream. Due to the abnormal water flow direction in the filled depression downstream, the pour line of the upstream depression cannot find the downstream pour point according to the correct water flow direction, resulting in an abnormal shape of the pour line. Figure 9In (a), A and B respectively represent two filled depressions in the downstream and upstream. The red line is the pour line of B obtained by isolating and processing the depression. Using the abnormal pour line for depression terrain processing will cause abnormal gullies to appear in the new terrain.

[0079] Therefore, it is necessary to count the maximum accumulated water volume in each depression, sort them in descending order according to the accumulated water volume, and process each depression from downstream to upstream. For each depression, after using the pour line to dig the terrain, it is necessary to re-fill the depression and calculate the water flow direction for the dug terrain, and update the re-filled and dug terrain and water flow direction to the global original terrain and water flow direction arrays, so that the upstream depressions can correctly find the downstream end point of the pour point when extracting the pour line. Figure 9 (b) shows the final pour line of depression B extracted by the algorithm of the present invention.

[0080] The embodiment of the present invention provides a processing system for depression terrain, including: A data acquisition module for acquiring the raster digital elevation model DEM of multiple depressions in the basin.

[0081] A depression analysis module for extracting pour points on the edge of each depression and performing ponding analysis on the depression to obtain the starting elevation for excavation and the flooded area of the depression; in the flooded area of a depression containing multiple sub-areas, select the sub-area with the largest area and the largest width as the excavation flooded area.

[0082] A downstream pipeline acquisition module for starting from the pour point of each depression and tracing the water flow trajectory downstream along the water flow direction of the basin; during the tracing process, if a position lower than the starting elevation for excavation is encountered, stop tracing, and obtain the downstream water flow line of the pour point of each depression according to the traced water flow trajectory.

[0083] An upstream pipeline acquisition module for extracting the center line of each depression from the depression flooded edge map of each depression, and tracing the water flow trajectory upstream from the pour point of each depression along the center line; during the tracing process, if the excavation flooded area is encountered, stop tracing, and obtain the upstream water flow line of the pour point according to the traced water flow trajectory.

[0084] A pour line acquisition module for connecting the tail of the upstream water flow line of the pour point of each depression to the head of the downstream water flow line to obtain the pour line of each depression; according to the elevation of the pour line of each depression, dig the terrain along the pour line so that the water in each depression can flow out smoothly from the pour point to correct the depression.

[0085] A specific embodiment is as follows: This embodiment discloses a method for processing depression terrain, and the specific steps include: S1. Obtain the basin raster digital elevation model (DEM). Perform depression filling, calculation of water flow direction, and calculation of accumulated water flow volume on the DEM; extract the list of depressions of the filled terrain with equal elevation as the condition; sort the depressions in reverse order according to the accumulated water flow volume to ensure that each filled depression is processed from downstream to upstream.

[0086] S2. Within each depression, extract the pour points on the depression edge according to the water flow direction, and perform ponding analysis on the depression to obtain the starting elevation for excavation.

[0087] S3. Within each depression, select the sub-region with the largest area and the largest width as the excavation inundation area from the inundation area of the depression containing multiple sub-regions where the starting elevation for excavation is located.

[0088] S4. Starting from the pour point of each depression, trace the water flow trajectory downstream along the water flow direction of the basin; during the tracing process, if a position lower than the starting elevation for excavation is encountered, stop tracing, and obtain the downstream water flow line of the pour point of each depression according to the traced water flow trajectory.

[0089] S5. Use an optimized thinning algorithm to extract the center line of each depression from the depression inundation edge map of each depression, and trace the water flow trajectory upstream along the center line from the pour point of each depression; during the tracing process, if the excavation inundation area is encountered, stop tracing, and obtain the upstream water flow line of the pour point according to the traced water flow trajectory.

[0090] S6. Connect the tail of the upstream water flow line of the pour point of each depression to the head of the downstream water flow line to obtain the pour line of each depression; according to the elevation of the pour line of each depression, perform terrain excavation along the pour line so that the water flow in each depression can flow out smoothly from the pour point to correct the depression.

[0091] Experimental verification and analysis results: First, select the largest depression area in experimental area 1 for the experiment. The pour line of this depression is as Figure 7 shown. Use the algorithm of the present invention to excavate this depression, and then use ArcGIS to perform depression filling - water flow direction calculation - accumulated water flow volume calculation on the processed terrain, extract the upstream channel of the depression, and conduct a comparative analysis with the channel of the original terrain, as Figure 10 (a) and Figure 10 (b) shown.

[0092] It can be seen from the figure that due to the abnormal elevation of the channel, pseudo-depressions are generated upstream of it. Due to the existence of the pseudo-depressions, the correct water flow direction that could have been calculated upstream cannot output the correct flow direction because of depression filling. After the original pseudo-depressions upstream disappear through terrain excavation along the pour line, the correct flow direction will be generated along the original terrain upstream.

[0093] Figure 11 It is a comparison chart for algorithm verification of the topographic data of a certain place with a resolution of 100 meters. Since the scope is too large, the effectiveness of the algorithm of the present invention is viewed by zooming in on a local area. Figure 11 (a) shows the channels extracted directly using ArcGIS hydrological analysis; Figure 11 (b) shows the effect of first processing the topographic depressions through pour lines and then extracting the channels. As can be seen from the figure, the algorithm of the present invention can effectively handle the abnormal problems of channels in the filled depression areas caused by topographic data errors, improve the quality of watershed generation for error topographies, and is a new idea for raster DEM quality control, with important application value.

[0094] The above-described embodiments merely represent several implementation manners of the present invention. Their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the appended claims.

Claims

1. A method for processing depression terrain, characterized in that: The following steps are involved: Obtain the raster digital elevation model DEM of multiple depressions in the watershed; Extract the pouring point on the edge of each depression, and perform water accumulation analysis on the depression to obtain the starting elevation of the excavation and the flooded area of ​​the depression; in the flooded area of ​​the depression containing multiple sub-areas, select the sub-area with the largest area and the largest width as the flooded area for the excavation; Taking the pouring point of each depression as the starting point, the trajectory of water flow is traced downstream along the water flow direction of the basin; During the tracking process, if a position lower than the starting elevation of the digging is encountered, the tracking is stopped, and the downstream water flow line of the pouring point of each depression is obtained according to the tracked water flow trajectory; Extract the center line of each depression from the depression inundation edge map of each depression, and trace the flow trajectory upstream along the center line from the pour point of each depression; During the tracking process, if the tracking encounters a submerged area, the tracking is stopped, and the upstream flow line of the pouring point is obtained according to the tracked water flow trajectory; The tail of the upstream flow line of each depression's pouring point is connected with the head of the downstream flow line to obtain the pouring line of each depression; according to the elevation of the pouring line of each depression, the terrain is dug down along the pouring line so that the water in each depression can flow out smoothly from the pouring point to correct the depression.

2. A method for processing depression terrain as claimed in claim 1, characterized in that: After digging down along the dip line, the following steps are also included: Refill each depression dug under the terrain and obtain the direction of water flow; The refilled terrain and the obtained water flow direction are updated to the original terrain of the watershed.

3. A method for processing depression terrain as claimed in claim 1, characterized in that: The acquisition of the starting elevation of the digging includes the following specific steps: Conduct water accumulation analysis on each depression, and determine the inundated area corresponding to the water level as it gradually increases; In the process of waterlogging analysis, the minimum number of pixels required for each flooded pixel in the depression to reach the non-flooded area is recorded, which is the pixel width of the depression, and the depression flooding edge map is constructed based on the pixel width of the depression; Set the depression pixel width threshold and dig down the depressions whose width is greater than the threshold; Use the binary search algorithm to obtain the starting elevation of the excavation.

4. A method for processing depression terrain as claimed in claim 3, characterized in that: The binary search algorithm is used to obtain the starting elevation of the digging, and the specific steps include: Count the pixel elevations of all pixels in each depression and obtain the lowest elevation of each depression H min and the highest elevation H max ; Suppose the elevation range currently being searched is [ H f , H t ],make H f = H min , H t = H max ; Assume the current water level is h The historical water level and the starting elevation of excavation are hi and hr ,make hi=hr=H min ; make h =( H f + H t ) / 2, if h = hi The algorithm ends and outputs the starting elevation of the digging hr , otherwise get the flooded area of ​​the depression D h ; in, a are the pixels that make up the inundated area of ​​the depression, H a For pixels a The elevation of the original terrain, D is a collection of depression pixels; generate D h The corresponding depression submerges the edge map and obtains the depression pixel width, set as V ; like V >2, let H t = h ;like V <=2, let H f = h ;for V =2, Update hr = h ; when hi = h When, return to h =( H f + H t ) / 2 to execute the algorithm; The algorithm ends and outputs the starting elevation of the digging hr .

5. A method for processing depression landform according to claim 1, characterized in that: The step of extracting the center line of each depression from the depression submerged edge map of each depression specifically comprises: The centerline of each depression is extracted from the depression inundation edge map of each depression using an optimized thinning algorithm; The optimized thinning algorithm is based on the traditional thinning algorithm, adding pixel constraints. f ( x , y ), the formula is: in, Q To constrain the pixel set, p ( x , y ) represents the pixel of each depression, x represents the horizontal coordinate of the pixel, y represents the vertical coordinate of the pixel, t ( x , y ) indicates whether the pixel p(x,y) satisfies the thinning condition according to the rules of the traditional thinning algorithm. False Indicates that the refinement conditions are not met.

6. A method for processing depression landform according to claim 1, characterized in that: According to the elevation of the dip line of each depression, digging down along the dip line specifically includes: According to the elevation of each pixel on the pour line, starting from the starting point of the pour line, the elevation of the pixels is updated one by one to ensure that the elevation of each pixel is not higher than the elevation of the previous pixel, and the elevation of the starting point of the pour line is the starting elevation of the downcut.

7. A system for processing depression terrain, characterized in that: include: The data acquisition module is used to obtain the raster digital elevation model DEM of multiple depressions in the watershed; The depression analysis module is used to extract the pouring point on the depression edge of each depression, and perform water accumulation analysis on the depression to obtain the starting elevation of the down-digging and the inundation area of ​​the depression; in the inundation area of ​​the depression containing multiple sub-areas, the sub-area with the largest area and the largest width is selected as the down-digging inundation area; The downstream flow line acquisition module is used to track the flow trajectory downstream along the flow direction of the basin, starting from the pouring point of each depression; During the tracking process, if a position lower than the starting elevation of the digging is encountered, the tracking is stopped, and the downstream water flow line of the pouring point of each depression is obtained according to the tracked water flow trajectory; An upstream flow line acquisition module is used to extract the center line of each depression from the depression inundation edge map of each depression, and trace the flow trajectory from the pouring point of each depression along the center line upstream; During the tracking process, if the tracking encounters a submerged area, the tracking is stopped, and the upstream flow line of the pouring point is obtained according to the tracked water flow trajectory; The pour line acquisition module is used to connect the tail of the upstream water flow line of the pour point of each depression with the head of the downstream water flow line to obtain the pour line of each depression; according to the elevation of the pour line of each depression, the terrain is dug down along the pour line so that the water in each depression can flow out smoothly from the pour point to correct the depression.

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