A loess plateau gully area extraction and calculation method

Abstract

The application discloses a method for extracting and calculating gully area of the loess plateau, comprising the following steps: obtaining a digital elevation model of a sample area; determining a first gully area and a second gully area of the sample area according to the digital elevation model; determining a reference ratio according to the first gully area and the second gully area; obtaining a first gully area of a general survey area and a research area; determining a second gully area of the general survey area according to the reference ratio and the first gully area of the general survey area; and determining a second gully area of the research area according to the second gully area of the general survey area and the research area. The method can accurately extract the gully area of the loess plateau in a large range of research area, without a large amount of manual labor, and provides accurate and reliable data support for the research of the loess plateau.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of land resource investigation, in particular to a loess plateau gully area extraction and calculation method. BACKGROUND

[0002] The loess plateau belongs to a typical loess landform, which can be divided into two categories of inter-gully and gully according to the shape and position. The gully, also known as gully or gully, is composed of gully slope and gully bottom. The gully slope refers to the part below the gully edge line and above the gully bottom slope angle line, and the gully bottom refers to the part below the gully bottom slope angle line.

[0003] At present, the national and local census agencies can conduct a census of the loess plateau gully area within a certain range, but the census object is mainly the gully area with the upper edge boundary line of the slope as the gully edge line, which is called the first-level gully. However, in actual research, the gully part with the slope foot line as the gully edge line of the gully may be needed, which is called the second-level gully. The area of the first-level gully is much larger than that of the second-level gully. Although the area of the second-level gully in a small range can be extracted by manual interpretation, when the research area is large, the workload of manual interpretation will be very large, and the accuracy cannot be guaranteed. Therefore, how to accurately extract the area of the second-level gully in a large range is an important topic in the field of land resource investigation extraction. SUMMARY

[0004] The loess plateau gully area extraction and calculation method provided by the embodiments of the present application can solve the problem that there is no method for accurately determining the area of the second-level gully in the prior art.

[0005] In one aspect, the loess plateau gully area extraction and calculation method provided by the embodiments of the present application comprises:

[0006] Obtaining a digital elevation model of a sample area;

[0007] Determining the first-level gully area and the second-level gully area of the sample area according to the digital elevation model;

[0008] Determining a reference ratio according to the first-level gully area and the second-level gully area;

[0009] Obtaining the first-level gully area of a census area and the area of a research area;

[0010] Determining the second-level gully area of the census area according to the reference ratio and the first-level gully area of the census area;

[0011] Determining the second-level gully area of the research area according to the second-level gully area of the census area and the area of the research area.

[0012] The loess plateau gully area extraction and calculation method provided by the embodiments of the present application has the following advantages:

[0013] A method for accurately extracting the gully area of the loess plateau in a large-scale research area is provided, without a large amount of manual labor, and accurate and reliable data support is provided for the research of the loess plateau. BRIEF DESCRIPTION OF DRAWINGS

[0014] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced. Obviously, the drawings in the following description only constitute some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0015] Figure 1 A flowchart of a loess plateau gully area extraction and calculation method provided by the embodiments of the present application;

[0016] Figure 2 A flowchart of a method for extracting the area of primary and secondary gullies in samples provided by the embodiments of the present application;

[0017] Figure 3 An extraction schematic diagram of primary and secondary gullies provided by the embodiments of the present application;

[0018] Figure 4 A general survey process schematic diagram of a soil and water conservation office provided by the embodiments of the present application;

[0019] Figure 5 A gully area schematic diagram drawn by a soil and water conservation office provided by the embodiments of the present application. DETAILED DESCRIPTION

[0020] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments only constitute some embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0021] Figure 1 A flowchart of a loess plateau gully area extraction and calculation method provided by the embodiments of the present application. The embodiments of the present application provide a loess plateau gully area extraction and calculation method, which comprises:

[0022] S100, acquiring a digital elevation model (DEM) of a sample area.

[0023] Specifically, S100 specifically comprises: coordinate conversion and projection conversion on the original digital elevation model; and splicing on the converted digital elevation model to obtain a digital elevation model of the sample area. Specifically, the original digital elevation model can adopt a DEM collected at 30m, and a remote sensing image with a resolution of 2.5m can also be used as auxiliary data. In actual application, the original digital elevation model and the remote sensing image can be combined to extract the gully bottom line and the catchment range line by using a combination of automatic software analysis and manual post-processing, and the final accuracy can reach more than 90%. After theoretical calculation, field investigation is also needed to verify the length, width and area of the gully in the sample area.

[0024] S110, determining a first gully area and a second gully area of the sample area according to the digital elevation model.

[0025] Specifically, as shown in Figure 2 S110 specifically comprises: determining a water flow direction according to the digital elevation model of the sample area, and then determining a flow accumulation; determining a corresponding flow threshold according to the river network density and the flow accumulation of the sample area; rasterizing the sample area according to the flow threshold to obtain a raster river network; determining a small watershed outlet data according to the raster river network; determining a catchment basin according to the small watershed outlet data, and performing vectorization processing on the area of the catchment basin; performing vectorization processing on the raster river network, and determining a first gully of the sample area according to the vectorized raster river network and the vectorized catchment basin area; determining a positive and negative terrain distribution of the sample area according to the original digital elevation model and the digital elevation model of the sample area; extracting a positive terrain raster area in the positive and negative terrain distribution of the sample area; selecting the first gully of the sample area to obtain a gully bottom landform; merging the positive terrain raster area and the gully bottom landform to obtain a second gully of the sample area; and determining the area of the first gully and the second gully of the sample area.

[0026] In the embodiment of the present application, before determining the water flow direction of the sample area according to the digital elevation model of the sample area, it further comprises: determining whether there is a depression in the digital elevation model of the sample area, if there is a depression, first performing depression filling processing, and then determining the water flow direction of the sample area according to the digital elevation model of the sample area. If there is no depression in the digital elevation model, the water flow direction of the sample area can be directly determined.

[0027] The selection of the first gully of the sample area to obtain the gully bottom landform comprises: generating a corresponding digital slope layer according to the digital elevation model of the sample area; extracting the slope in the digital slope layer; and taking the area with a slope less than or equal to a slope threshold as a gully bottom land to form a gully bottom landform.

[0028] In the embodiments of the present application, the slope threshold is 25 degrees. Moreover, the slope direction variation of the error-free DEM can be determined according to the digital elevation model of the sample area, and then the original digital elevation model is subtracted from the slope direction variation grid of the error-free DEM to determine the positive and negative terrain distribution of the sample area. When extracting the positive terrain grid area in the positive and negative terrain distribution of the sample area, due to the complex terrain of the study area, the directly extracted positive terrain has local grid discontinuity. In order to minimize the influence of the positive terrain grid empty area lip line on the judgment of the gully bottom area, and to obtain a gully bottom ground that is consistent with the actual situation, the present application uses the extracted primary gully and slope analysis to select, and obtains a gully bottom ground sketch that meets the distribution characteristics. After determining the gully bottom ground sketch, it is merged with the directly extracted positive terrain grid area, that is, the gully bottom ground grid area that meets the slope requirement and belongs to the relatively complete positive terrain area. The vectorization of the merged grid can obtain the complete gully bottom ground area, so as to facilitate the calculation of the area.

[0029] After determining the primary gully and the secondary gully of the sample area, the shape of the primary gully and the secondary gully is obtained by combining automatic analysis of software and manual post-processing. Then, the length and width of the primary gully and the secondary gully are measured, and the linear data layer is topologically converted into a surface, so as to obtain the area of the gully.

[0030] In S120, the reference ratio is determined according to the area of the primary gully and the area of the secondary gully.

[0031] For example, S120 specifically includes: taking the quotient of the area of the secondary gully and the area of the primary gully of each sample area as a sample ratio; determining the average value of the plurality of sample ratios to obtain the reference ratio.

[0032] In the embodiments of the present application, 300 pairs of gullies are selected as the data of the sample area, and the data such as the gully area, the gully length and the gully width of the gully sample in the sample area is obtained, so as to establish the relationship model of the geomorphic type (gully length, gully width) and the gully area. For example:

[0033] (1) A certain number of gully samples are selected. First, the range of the primary gully is drawn with the upper boundary line of the slope as the boundary line, which is called A. The range of the secondary gully is drawn with the gully edge line as the gully edge line within the range, which is called B. As shown in FIG. 1, Figure 3 Figure 3 has two closed areas, and the larger closed area is the primary gully, and the smaller closed area inside the primary gully is the secondary gully.

[0034] (2) The areas of A and B are calculated respectively, and are recorded as X and Y respectively.

[0035] (3) The reference ratio Y / X is calculated.

[0036] ​Through analysis of the sample area, the first channel area of multiple channel samples can be obtained: X1, X2, X3…, and the average of the sample ratios S1=S1 / X1, S2=Y2 / X2, S3=Y3 / X3… can be used as the reference ratio S=(S1+S2+S3+…+Sn) / n, where n is the number of samples.

[0037] In the embodiments of the present application, the reference sample is determined by using the average value, which is simple, but ignores the differences between different samples. Therefore, after obtaining the reference ratio, the present application further includes: determining the aspect ratio of the channel in each sample area and the corresponding reference ratio; clustering the reference ratios according to the aspect ratios; and correcting the reference ratios according to the clustering results.

[0038] Specifically, the correction of the reference ratio according to the clustering results includes: establishing a correction function in the form of a segmented function according to the clustering results; and correcting the reference ratio by using the correction function.

[0039] The present application uses the channel aspect ratio data to cluster and analyze the sample ratios corresponding to 300 groups of samples, to form a correction function in the form of a segmented function determined by multiple correction coefficients, which is as follows:

[0040]

[0041] It should be understood that the correction function described above containing three correction coefficients is only an example, and more and more complex correction functions in different forms can be set according to the situation and needs in actual application.

[0042] S130, obtaining the first channel area of the survey area and the area of the study area.

[0043] Exemplarily, as shown in Figure 4 , the survey area is an erosion channel with a channel length (main channel length) of 500 m or more and a catchment area of not more than 50 km 2 . The survey area covers the high tableland gully region and the hilly gully region of the Loess Plateau, and involves Qinghai, Gansu, Ningxia, Inner Mongolia, Shaanxi, Shanxi and Henan, 33 cities and 182 counties (cities, districts), with a total area of about 248,000 km 2 . During the survey, 2.5 m resolution remote sensing images and 1:50,000 topographic maps (DLG) are used as the main information source, and the channel length, area and longitudinal gradient are extracted by using the GIS software and the man-machine interaction mode on the computer, so as to statistically analyze the number, characteristics and distribution of the erosion channels on the Loess Plateau. The channel area of the 2.5 m resolution remote sensing image based on the survey result is as shown in Figure 5The result of the survey of the erosion gully in the loess plateau by the office of soil and water conservation shows that the total area of the erosion gully in the loess plateau is 187200km 2 .

[0044] S140, the area of the secondary gully in the survey area is determined according to the benchmark ratio and the area of the primary gully in the survey area.

[0045] Exemplarily, the survey result of the office of soil and water conservation shows that the area of the primary gully in the range of 248000km 2 is 187200km 2 , i.e. the ratio of X in the total area is P=75.48%, and the area of the secondary gully in the survey area, i.e. Y, can be represented as 18720*S.

[0046] S150, the area of the secondary gully in the research area is determined according to the area of the secondary gully in the survey area and the area of the research area.

[0047] Exemplarily, the area of the survey area is 248000km 2 , which cannot meet the research requirement, and therefore the research area is set as 640000km 2 in the present application. However, the above only determines the area of the secondary gully in the range of 240000km 2 in the loess plateau, and the survey area accounts for 38.75% of the total area of the loess plateau, i.e. the research area in the present application, and the area of the secondary gully in the range of 640000km 2 in the loess plateau is Y / 38.75%.

[0048] Although the preferred embodiments of the present application have been described, those skilled in the art can make additional changes and modifications to the embodiments once they know the basic inventive concept. Therefore, the appended claims are intended to be interpreted as including all the changes and modifications falling within the scope of the present application.

[0049] Obviously, various modifications and changes can be made to the present application by those skilled in the art without departing from the spirit and scope of the present application. Thus, it is intended that the present application cover the modifications and changes as falling within the scope of the claims of the present application and their equivalents.

Claims

1. A method for extracting and calculating gully area on the Loess Plateau, characterized in that, The method comprises the following steps: obtaining a digital elevation model of a sample area; determining a first-order channel area and a second-order channel area of the sample area according to the digital elevation model; determining a reference ratio according to the first-order channel area and the second-order channel area; obtaining a first-order channel area of a survey area and a study area area; determining a second-order channel area of the survey area according to the reference ratio and the first-order channel area of the survey area; determining a second-order channel area of the study area according to the second-order channel area of the survey area and the study area area; wherein the determining of the reference ratio according to the first-order channel area and the second-order channel area comprises: taking a quotient of the second-order channel area and the first-order channel area of each sample area as a sample ratio; determining an average value of a plurality of sample ratios to obtain the reference ratio; after obtaining the reference ratio, further comprising: determining an aspect ratio of a channel in each sample area and a corresponding reference ratio; clustering the reference ratios according to the aspect ratios; correcting the reference ratios according to the clustering results.

2. The loess plateau gully area extraction and calculation method according to claim 1, characterized in that, The obtaining of the digital elevation model of the sample area comprises: performing coordinate conversion and projection conversion on an original digital elevation model; performing splicing on the converted digital elevation model to obtain the digital elevation model of the sample area.

3. The loess plateau gully area extraction and calculation method according to claim 1, characterized in that, The determining of the first-order channel area and the second-order channel area of the sample area according to the digital elevation model comprises: determining a flow direction according to the digital elevation model of the sample area, and then determining a flow accumulation; determining a corresponding flow threshold according to a river network density of the sample area and the flow accumulation; performing rasterization on the sample area according to the flow threshold to obtain a raster river network; determining small watershed outlet data according to the raster river network; determining a catchment basin according to the small watershed outlet data, and performing vectorization processing on an area of the catchment basin; performing vectorization processing on the raster river network, and determining a first-order channel of the sample area according to the vectorized raster river network and the area of the vectorized catchment basin; determining a positive and negative terrain distribution of the sample area according to an original digital elevation model and the digital elevation model of the sample area; extracting a positive terrain raster area in the positive and negative terrain distribution of the sample area; selecting the first-order channel of the sample area to obtain a gully bottom land sketch; merging the positive terrain raster area and the gully bottom land sketch to obtain a second-order channel of the sample area; determining areas of the first-order channel and the second-order channel of the sample area.

4. The loess plateau gully area extraction and calculation method according to claim 3, characterized in that, Before the determining of the flow direction of the sample area according to the digital elevation model of the sample area, further comprising: determining whether there is a depression in the digital elevation model of the sample area, and if there is a depression, performing depression filling processing, and then determining the flow direction of the sample area according to the digital elevation model of the sample area.

5. The loess plateau gully area extraction and calculation method according to claim 3, characterized in that, The selecting of the first-order channel of the sample area to obtain the gully bottom land sketch comprises: generating a corresponding digital slope layer according to the digital elevation model of the sample area; extracting a slope in the digital slope layer; taking an area with a slope less than or equal to a slope threshold as a gully bottom land to form the gully bottom land sketch.

6. The loess plateau gully area extraction and calculation method according to claim 1, characterized in that, The correcting of the reference ratios according to the clustering results comprises: According to the clustering result, a correction function in the form of a segmented function is established; The correction function is used to correct the reference ratio.

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