(Tianjin Urban Planning and Design Institute, Tianjin, 30004 1)
From the demarcation of the project area, the construction of DEM to the suitability evaluation of the project area, on this basis, the land use zoning of the project area is carried out, the land use layout is arranged, and the comprehensive planning scheme is determined, thus forming a discussion on the comprehensive planning of small watershed land consolidation based on DEM.
Keywords: DEM；; Small watershed; Land consolidation; unified plan
The comprehensive planning and design of small watershed land consolidation based on DEM, the main technical routes of its research are divided into the delineation of land consolidation project area, DEM construction based on topographic map, suitability evaluation of project area, land use zoning, land use layout, project area planning and design scheme and major project planning and design, and the detailed flow is shown in Figure 1.
Figure 1 Technical Process of Land Consolidation Planning and Design in Small Watershed
1 Delineation of small watershed land consolidation project area
1. 1 data collection and field investigation
According to the land development and consolidation standards and land development and consolidation planning requirements issued by the Ministry of Land and Resources, the materials required for project planning generally include: ① basic information of the project area, including administrative jurisdiction, geographical location, land area and population; ② Natural conditions, including topography, landform, climate, soil, hydrology, geology, vegetation and natural disasters in the project area; (3) Socio-economic conditions, including industrial development, market conditions, income level, national habits, etc. ; (4) Present situation of land use, including quantity, ownership, land production capacity and main problems in land use; ⑤ Laws and policies of land development and consolidation, technical specifications and standards of agriculture, water conservancy, forestry, environmental protection and other related industries; ⑥ Land resources, water resources investigation report, overall land use planning, land development and consolidation planning, etc. ⑦ Topographic map of the project area, land use status map, land ownership boundary map, overall land use planning map of the county (city) and township (town) where the project is located, land development and consolidation planning map, etc.
At the same time, the technical team and relevant local leaders must go to the project area for field survey many times, mainly to understand the topography, planting system, water source and existing facilities of the project area, and specifically define the boundaries of the project area, solicit the opinions of local grassroots leaders and farmers on specific issues that may be involved in the project planning and design, and jointly determine the planning ideas with local agricultural and water conservancy experts.
1.2 principles for delimiting small watershed land consolidation project areas
Delineation of small watershed land consolidation project areas should follow the following three principles: ① the principle of project integrity. The boundary of the project area should not only consider the boundary of the administrative area, but also consider the distribution of natural landforms around the project area, such as river basins, roads, ditches and canals. (2) Based on the measured topographic map, the boundary of land consolidation project area is defined from the perspective of small watershed ecosystem, and then the current map of the project area is superimposed and matched with the measured topographic map to obtain an electronic map with information such as topography and land type. ③ Try to ensure that the ownership of the land is within the jurisdiction of the same township.
2 DEM construction based on topographic map
2. 1 data source acquisition and transformation
This paper mainly establishes DEM according to digital topographic map. Taking the land consolidation project of Liu Lihe River as an example, this paper briefly explains the mapping process.
Firstly, the field control survey is carried out in the project area, using GPS static survey, laying an E-class GPS network along the line, combining point connection with edge connection, and the starting point of plane coordinates is two GPS points provided by Beijing Surveying and Mapping Institute near Baicazi Village, Liulimiao Town, Huairou District, Beijing. Then the total station is used for field measurement. All-station theodolite can automatically record and display relevant data. After collecting data in the field, the total station is connected with the computer through the data communication line, and the field observation data is transmitted to the computer, and then the data collected in the field is preprocessed to check all possible errors. Code the data collected in the field and convert the measured data into the coding format required by the drawing system. Data calculation is aimed at the relationship between landforms. Input the measurement data into the computer, generate plane graphics, establish graphics files and draw contour lines. According to the field sketch, modify the newly generated topographic map, make up the shortcomings or mistakes in the survey or retest, and then supplement the elevation and notes. Conduct indoor and outdoor inspections; Finally, the topographic map is decorated inside and outside the frame.
The result of mapping is a file in AutoCAD format. This paper mainly uses MAPGIS software to establish digital elevation model, so it is necessary to convert the file. First, save contour lines and elevation points in AutoCAD format as. Dxf format. Then, the "file conversion" module is applied in MAPGIS software to convert the contour line and elevation point data into. Convert dxf format into point file and line file in MAPGIS software. Point file is data containing elevation point information; A line file is data containing contour information.
2.2 Editing and Processing of Vector Data
In the process of contour digital production, due to the complex terrain, large amount of data, human-computer interaction and the defects of image processing technology, geometric information and attribute information errors of contour data are inevitable. Geometric information errors mainly include point or line errors of contour lines, data redundancy and uneven contour lines caused by intersection or selfing of contour lines, and uneven data acquisition. Attribute information error mainly includes a small amount of elevation error and system elevation error. Contour vector data is the basic data for establishing digital elevation model, and its quality directly affects its use effect, so it is necessary to check and modify contour data.
In the processing of contour data, contour checking tools in DTM analysis application module of MAPGIS software can be used for processing. Firstly, the contour data are checked for errors, including line self-intersection, line closure, line intersection, height jump value and height assignment. Then find out the specific reasons for the error and edit the contour file, mainly including the addition, deletion, movement, continuation, closure, division and modification of line data attributes, as well as the addition, deletion and movement of elevation point data.
2.2. 1 contour line is endowed with the attribute of elevation value.
The contour file converted by AutoCAD format has no attribute about elevation value, so it is necessary to specify elevation value for each contour. Firstly, the attribute structure of contour file is established. According to the elevation value, the field type is double precision, the field length is 8, and the number of decimal places is 2. Secondly, find out the difference of elevation lines, then start the tool of automatically assigning elevation values, draw an extension line from the known elevation points, and click it to appear a dialog box. Enter Current Elevation and Elevation Increment in the dialog box, where the current elevation is the elevation value of the starting point and the elevation increment is the difference between contour lines, which can be positive or negative. Infer whether the elevation value is decreased or increased from the map, and the decreased value is positive, and vice versa. Finally, all row attributes are entered in the same way. Save as a line file with the name of "Elevation Line".
2.2.2 The elevation point is given the attribute of elevation value.
The elevation point file converted by AutoCAD format also has no elevation value attribute, so each elevation point must specify an elevation value. Unlike the above process of assigning elevation values to contour lines, the annotation of elevation points is assigned as an attribute. It should be noted that after assigning attributes, the attribute structure of elevation points must be changed, that is, the "field type" should be changed from string to double precision, otherwise DEM cannot be generated.
2.3 DEM generation
2.3. 1 TIN model generation
Generation of TIN model of (1) line file. First, start the "DTM analysis" function module in MAPGIS software and load the contour file. Extract elevation points from line data. According to the resolution requirement, the sparse coefficient is selected, and then the triangulation is generated. Save the generated triangulation file as a TIN file.
(2) The generation of TIN model of point file. At the same time, start the "DTM analysis" function module in MAPGIS software and load the elevation point file. Extract elevation points from point data, select the attribute items of elevation points, and then generate triangulation. Fig. 2 is a DEM model of Liu Lihe land consolidation project area based on elevation point data.
Figure 2 DEM model of Liu Lihe land consolidation project area
Figure 3 DEM model of typical plots in Liu Lihe land consolidation project area
2.3.2 Grid model generation
Different from generating TIN model, the generation of grid model mainly rasterizes contour lines and elevation points, and saves the results of rasterization in a grid format file. Figure 3 shows the grid model generated by typical blocks of Liu Lihe River. Three-dimensional terrain information such as ridges, valleys and watersheds can be clearly seen from the map, so as to intuitively understand the overall situation of the project area and have an overall grasp and understanding of the planning of the whole project area.
3 Land suitability evaluation of small watershed based on DEM
Land suitability evaluation is the basis of small watershed land consolidation planning, which provides scientific basis for land consolidation planning, reduces the blindness of planning, and gives full play to the four basic functions of small watershed eco-economic system: material circulation, energy flow, information transmission and value appreciation. There are many factors that affect land suitability. For small watershed areas, slope and aspect are important factors for land suitability evaluation. Slope and aspect are important factors for the ground to be eroded by sunlight and wind and rain.
In order to realize quantitative and scientific evaluation methods and processes, modern GIS analysis technology is needed. Using DEM data to extract and express the surface morphological structure, its technical means have matured. DEM can be used to extract high-quality slope and aspect information quickly and simply, and GIS overlay analysis method is used to overlay, group and statistically analyze the current land use information. This paper studies how to establish an evaluation model from the perspective of topography, and takes the land suitability evaluation of Liu Lihe River Basin in Huairou District of Beijing as an example to realize quantitative evaluation in GIS environment.
3. 1 Determination of land suitability evaluation index and weight of small watershed
By studying the relationship between the slope and slope topography of Liu Lihe River Basin in Huairou District and the agricultural production conditions in this area, the weight of each factor is determined, as shown in Table 1 and Table 2.
Table 1 Weight Table of Liu Lihe River Basin Classification in Huairou District
Table 2 Weight Table of Slope Classification of Liu Lihe River Basin in Huairou District
3.2 Generation of Slope Map and Aspect Map
Slope is a commonly used parameter in terrain description, and it is a vector with direction and size. The slope of a spatial surface is a function of points. Unless the surface is flat, the slopes of different positions on the surface are not equal. The slope of a given point is the angle between the normal direction and the vertical direction of the point on the surface.
Based on the topographic map of 1∶ 1000 in Liu Lihe, Huairou District, topographic elements such as contour lines in AutoCAD format are converted into line files and point files in MAPGIS software, and then DEM for analysis is generated by MAPGIS software. According to the above gradient grading weight table, the gradient is divided into five grades in the "Slope Grading Map Output Settings" in MAPGIS software, and the upper and lower limits and area parameters are input respectively. Open the point, line and polygon files just generated in input editing, modify the attribute structure of polygon files, add two items of "slope" and "slope score", and set their field types and field lengths accordingly. The field type of "Slope" is string type, and the field length is10; The field type of "Slope Score" is double, the field length is 2, and the number of decimal places is 1. Then, using the "parameter attribution" function, according to the slope grading weight table, two attribute information of "slope" and "slope score" are added to the slope map, and finally the point, line and surface files are saved. Figs. 4 and 5 are the slope maps of the land consolidation project area and typical plots in Liu Lihe River Basin, respectively. The overall slope of the project area can be reflected on the map, and the slope information of the project area can be clearly and accurately obtained.
Fig. 4 Slope map of land consolidation project area in Liu Lihe River Basin.
Fig. 5 Slope map of typical plots in the land consolidation project area of Liu Lihe River Basin.
When calculating the slope direction based on DEM, it is usually defined as the angle between the projection of the normal direction of the tangent plane of a point on the surface fitted by grid elements on the plane and true north, that is, the azimuth of the projection vector of the normal direction in the horizontal direction.
Similar to the attribute assignment of the slope map described above, the attributes of "Slope Range" and "Slope Score" are assigned to the slope map. Figures 6 and 7 are the slope maps of the land consolidation project area and typical plots in Liu Lihe River Basin, respectively, which laid a good foundation for the next suitability evaluation.
Fig. 6 Aspect map of typical plots in the land consolidation project area of Liu Lihe River Basin.
Fig. 7 Slope map of land consolidation project area in Liu Lihe River Basin.
3.3 Small watershed land suitability evaluation results
Land suitability evaluation is carried out with the support of GIS software. The division of evaluation units is based on MAP-GIS software, and the "spatial analysis" module in MAPGIS software is used for spatial superposition. Firstly, add the area files of slope map and aspect map to the module, and then use the function of "area-to-area merging" to superimpose the slope map and aspect map. Finally, the comprehensive map can be saved, and the unit map of land suitability evaluation can be obtained, and it can be adjusted to make each unit achieve similar natural conditions, management methods and economic benefits, and finally form an evaluation unit.
In this study, the comprehensive index evaluation model is used to evaluate the land suitability, and the comprehensive grading parameters of each land are determined according to the following formula:
Comprehensive scoring parameter = slope score+aspect score
Using equidistant cutting method, the land suitability in this area is divided into four categories (see Table 3).
Table 3 Comprehensive Scoring Table for Land Suitability Evaluation
Using the "attribute library management" module of MAPGIS software, the regional attributes of the superimposed comprehensive map are output to a database file in dbf format, and then opened in Excel. According to the comprehensive scoring formula, the scores of each evaluation unit are calculated. Finally, the calculation result is saved as a database file in. Dbf format. Using the "attribute connection" of the "attribute library management" module of MAPGIS software, the calculation results are connected with the regional attributes of the comprehensive map to establish a database.
4 DEM-based land consolidation planning scheme for small watersheds
Through the evaluation of land suitability, the research on the spatial layout of land use has been strengthened. The development of digital elevation model provides a theoretical basis for the spatial layout of land use. Applying the theory and method of digital elevation model, the spatial layout of land use is determined, which makes the planning and design of land consolidation more reasonable. According to the evaluation results of land suitability in small watershed, planning and designing land use layout mainly includes the following aspects.
4. 1 land use zoning based on DEM
In the "Input Edit" module, open the comprehensive map, and use the "Attribute Assignment Parameter" function to divide the land use of the project area according to the parameter values of each land use division in Table 4. The land use zoning based on DEM is obtained. From the map, the patch is broken. Because there are no topographic elements such as river lines, residential areas, roads and land boundaries on the map, further graphic processing is needed.
Table 4 Parameter values of various categories in land use zoning
Figure 8 Land use zoning of typical plots in Liu Lihe land consolidation project area.
Fig. 9 Land use zoning of Liu Lihe land consolidation project area.
4.2 Add linear elements and residential areas
The land use zoning map generated above is mainly based on elevation points and contour lines, and lacks topographic elements such as river lines, residential areas, roads and land use boundaries. These terrain elements should be added to the map when planning and designing in the next step. Taking Liu Lihe project as an example, firstly, terrain elements such as river lines, residential areas, roads and land boundaries are transformed into point and line files that can be recognized by MAPGIS software; Then add these terrain elements to the already generated partition map. Finally, adjust the order of each layer so that all elements can be displayed.
4.3 Determine the layout of land use
Land use layout refers to the rational arrangement and implementation of various types of land on specific plots in land consolidation areas according to the natural suitability of land and the requirements of social and economic development. According to the field investigation, small patches are merged on the map. Take prominent linear features such as roads and river lines as the basic basis for demarcation, and adjust the land layout to make it more reasonable.
4.3. 1 soil and water conservation area
The area with comprehensive score parameter less than 0.4 is dominated by soil and water conservation, and natural patches of large grassland are formed through artificial management and natural restoration. Large natural vegetation patches have many important ecological functions and bring many benefits to the landscape, such as water conservation, soil and water conservation and biodiversity protection.
4.3.2 Economic Forest and Fruit Area
The areas with comprehensive grading parameters between 0.4 and 0.9 mainly play ecological and economic functions. The key points of landscape ecological construction in this area are: vigorously developing economic forest and fruit industry, appropriately developing animal husbandry, improving people's living standards, and giving consideration to ecological and environmental protection functions. The main reason is that the slope in this area is large, which is not suitable for agricultural planting and development. The landscape process is dominated by ecological degradation processes such as soil erosion and vegetation coverage reduction, but it is slightly lighter than the ecological environment protection functional area, which requires both ecological and economic functions and ecological environment protection functions.
4.3.3 Cultivated area of gentle slope farmland
The region with comprehensive grading parameters between 0.9 ~ 1.3 is dominated by agricultural cultivation function. The key points of landscape ecological construction in this area are: taking high yield, high quality and high efficiency agriculture as the leading direction, promoting the development of planting industry, meeting people's consumption demand and increasing economic income. Therefore, in the planning and construction, it is necessary to rationally adjust the structure of agricultural, forestry, animal husbandry and sideline crops, give priority to grain, appropriately increase oil crops and cash crops such as sesame, rape and cotton, introduce excellent varieties, rationally arrange rotation, gradually promote the interplanting area and improve the multiple cropping index. At the same time, due to the dislocation of supply and demand caused by insufficient precipitation, it is necessary to vigorously promote the application of dry farming techniques such as plastic film mulching and water-saving irrigation, actively develop water-saving and water-collecting agriculture, and effectively improve the efficiency and benefit of soil production. The basis is that the sloping farmland has thin soil layer and low soil fertility, and the conditions for developing intensive agriculture are not yet mature.
4.3.4 Flat beaches and residential areas
The expansion of residential areas and the development of mineral resources will lead to the impact on the ecological environment of areas with comprehensive grading parameters greater than 1.3. Therefore, we should limit the expansion of residential areas as much as possible, vigorously advocate afforestation, build green villages and pastoral villages, improve the amenity of the environment and promote the coordination between human beings and the natural environment.
4.3.5 Fisheries
Due to the shortage of rainwater and groundwater resources, the development of fishery in small watershed in mountainous areas is greatly restricted. The key points of land consolidation planning and construction in small watersheds are: in areas where conditions permit, make full use of the river water intercepted by small water conservancy projects, properly develop fisheries and increase farmers' income.
Based on DEM and prominent linear elements such as roads and river lines, the land use layout is determined. It may be inconsistent with the terrain on the spot, so it is necessary to check whether the land layout is in line with the reality on the spot and make corresponding adjustments according to the situation on the spot.
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