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How to give full play to the role of the computerized acre measurement instrument in the calculation of the map area and realize the automation and precision of the calculation of the land area on the map is a subject of great concern in recent years and has also achieved certain results. On the basis of summarizing the experience of previous generations, the author of this paper has established a set of practical and automatic computerized land area calculation systems through extensive analyses, experiments and comparisons. This article introduced the basic ideas and technical routes for the establishment of the system, using a large number of actual measurements. The data has analyzed the measurement accuracy and benefits of the system, and several of them have been discussed in depth.
Measure methods and steps The basic steps for the automatic calculation of the map area can be divided into five stages: base preparation, digitization of maps, human-computer interaction editing, adjustment processing, and result output according to the order of work. The main two phases are described below.
1 Preparation stage 1.1 Resolving and Correcting Map Distortion Parameters Any map will have a certain degree of distortion due to map projection and paper stretching. The conventional area calculation method is difficult to eliminate, but it can be solved satisfactorily in the calculation of machine-aided area.
For the correction of projection errors due to map projection, first of all, the analytical equation of the map projection is determined and can be converted to a corresponding error formula. In the projection error. It is a function of the plane rectangular coordinate on the graph. When using the digitizer to calculate the area of ​​the map, the above function formula can be used to directly calculate the error differential value of each measurement point. Finally, the accumulated error method is used to calculate the total error value and the result of the measurement map is corrected.
If the error correction of paper deformation is a non-twisting deformation, the deformation coefficient in x and y directions can be measured first, and then each digitized point can be corrected. If the map has obvious distortion, the affine transformation rule must be used to correct it.
1.2 Division and stitching of digital artwork Due to the limitation of digitizer table size, map digitization often needs to divide and digitize a digitized original image so that many original independent and complete map spots are divided into two parts. The above map spots need to appear as separate area data in the statistics. Therefore, after the digitization of the two maps is completed, the area data is merged and the graphic vector data file is stitched to ensure each figure. Spot data integrity.
1.3 The original icon is traced on the digitized artwork to perform color separation of various points and lines for digitization to reduce the error rate during digitization.
1.4 Setting the five-character code and function menu area to measure the area of ​​the multi-attribute map area generally requires opening the property code and system function menu area in the blank area of ​​the digitizing panel to facilitate the assignment of the property code and the function selection in the digitization.
2 Digital map spot digitization is the core work of automatic calculation of machine aid area. It has the following technical points.
The measurement process is generally performed in accordance with the sequence of the coded maps; the starting point for digitization is usually indicated first when each map spot begins to measure. For simpler polygon map spots, vertices are used as digitized input points; for irregularities For curved spots, you need to select feature points that have control significance for the boundary geometry as digitized points.
The digitization of the map spot must be performed strictly in a clockwise direction. For the case of a spot with other types of small spots in the spot, you can cut in from the spot boundary of the outer spot, digitize the spot in the opposite direction, and then enter the spot. Turn out and complete the digitization of the entire map. Since the internal small spot is digitized in the reverse direction, its area is negative, and after the total area is accumulated and added, it can be effectively deducted and the figure area can be calculated.
Digitizer access method can be used point method, time increment method and step increments three ways. The point method is generally applicable to a simple hemming polygon type graph; the step increment method is applicable to the graphic with gentle curved edges; the time increment method is applicable to the bending of the map point boundary with more than ten points of inflection points. The schematic diagram of the calculation method is divided into complex graphics. In the actual measurement of a map, the above three methods are used interchangeably to improve the measurement efficiency.
For the map area calculation with high precision (with an error value less than 0.5%), it is also possible to use the digitization of the segmentation line to establish the data structure of the first-line and third-level retrieval of the spot. The combination of line segment and point position is performed again on the spot of the area to be measured, and then the area value thereof is measured. This method adopts a unified line segment and node code, and each line segment only needs one-time digitization, which avoids the non-coincidence difference caused by the bidirectional repeat digitization of the common edge of the spot, and greatly improves the measurement accuracy. The formed graphic data file facilitates the establishment of a geographic information system graphics library and automatic mapping. However, this method requires a one-time unified coding of the nodes of each line segment on the digital map, thereby greatly increasing the amount of preprocessing work for area measurement.
At present, the resolution of general digitizers is higher than 0.olmm. As long as digitizers work properly, the systematic error of measurement can be ignored. Therefore, accidental errors and gross errors are the main sources of errors in measurement. From the experiment, it was found that the specific influencing factors are the graphic complexity of the spot and the digitized point density.
Different map spots have 9U2 boundary bends with different degrees of complexity, two spots with the same area and different shapes. The longer the boundary length is, the larger the complexity is, and the data required when using digitizers is calculated. The more points there are, the greater the chance of accidental errors and gross errors. Here, we use the spot complexity W to describe the complexity of the spot. W~S0/S. S in the formula. For the area value of the spot, S is the area of ​​the same circle as the perimeter of the spot. From the above formula, it can be seen that the complexity of the spot is inversely proportional to the perimeter of the spot.
As a calibration value, a statistical analysis chart for the determination of the area of ​​different complexities in a set of areas of 250m. The figure reflects the change in the accuracy of the digitization meter's measurement area by the complexity of the map spot.
Because the basic principle of the digitizer's measurement area is to accumulate and approximate the area with small polygons, the digitized point density also affects the measurement accuracy. In the experiment, multiple sets of area measurement accuracy tests were performed with different digitized densities. As shown in FIG. 4 , the area measurement error value is drastically reduced at the beginning as the number of digitized points increases and the density increases. When the number of points increases to a certain extent, the measurement error value will be stable within a certain range, but when the number of points is further increased, the relative error increases again. Taking an arbitrary rectangle as an example, there are only 4 feature points in the rectangle (4 corner points). If the number of points is more than 4 points, it is impossible to increase the measurement accuracy of the spot, but only to increase the measurement error value. From this we can see that for the general map spot, the number of digitized points must be limited to an appropriate range, and the input of non-characteristic points should be minimized.
In order to further analyze the effect of spot morphological characteristics on the accuracy of area measurement, and compare the efficiency and accuracy of different area-quantity algorithms, in the experiment, three groups of different quantification methods were used to conduct comparative experiments on a group of spots with the same area and different complexity. The test results are shown in the attached table.
From the above analysis, the following conclusions can be drawn: Compared with other methods, the auto-assisted area auto-algorithm algorithm has the advantages of complete system function, high measurement accuracy, and time-saving and labor-saving advantages; the obtained area measurement data is directly deposited into the area. At the same time, the database has established a graphic library of map boundary, which has created favorable conditions for later area adjustment processing and establishment of GIS. In addition, the hardware equipment of this system is cheap, and it has a wider application surface than area measurement. Promotion. In summary, we believe that the method of automatic calculation of machine-assisted area has broad application prospects.
Land is the source of all production and existence. Land resources are the living conditions and reproduction conditions that humans cannot surrender. Therefore, accurately grasping the status quo of China's land resources is the basis for understanding China's national conditions and developing production. Since the country began to carry out a detailed survey of land resources in 1985, a lot of manpower and material resources have been put into place and great achievements have been made. However, an extremely important link in the survey of land resources is the method of calculating the land area on the map. Very backward. The traditional grid method, planimeter method, and weighing method are low in measurement accuracy, and time-consuming and labor-consuming. It is difficult to process and summarize measurement result data, and some high-precision measurement methods such as electronic scanning Instrumentation also requires expensive equipment and deployment to promote difficulties.