Analysis of several common topographic mapping methods

Chen Yibao (Shanghai Tunnel Engineering Quality Inspection Co., Ltd., Shanghai 20 140 1) China Library CategoryNo.: TB2 Document IDNo.: A ItemNo.:1003-2738 (2012) 05-0327-02 Abstract: This paper mainly introduces two mapping methods, total station mapping and GPS-RTK technology, and summarizes their working principles, characteristics and application scope, which can be used as reference for relevant staff. Keywords: total station; GPS-RTK； Surveying and mapping methods. Introduction The rapid development of modern science and technology has greatly promoted the updating of topographic mapping technology. The mapping method of large-scale topographic map has replaced the original theodolite mapping and plane white paper mapping by the existing total station with mapping guide and total station with sketch, and the new mapping method has made a qualitative leap in technology and accuracy. In addition, the GPS-RTK technology used in the survey greatly improves the efficiency and accuracy of surveying and mapping large-scale topographic maps. Based on the author's many years of topographic mapping experience, this paper briefly analyzes the total station mapping and GPS-RTK technology, which can be used as a reference for technicians engaged in surveying and mapping operations. Second, the use of total station mapping topographic map (a) the main characteristics of total station. Total station has the characteristics of durability, precision, portability, simple operation, large storage capacity and high measurement accuracy, and can realize the correction of multiplication constant and constant, automatic compensation correction, horizontal distance conversion and so on. With the development of technology, the total station is developing in the direction of standardization, intelligence, openness and full automation, which prevents its application in construction survey, topographic mapping and other fields. As a new measuring instrument, total station has the following characteristics: ① topographic survey and control survey can be carried out at the same time in topographic mapping. (2) When the total station is used for project construction lofting, the relevant points in the design drawings can be quickly lofting on the ground. ③ When total station is used for deformation monitoring, it can realize real-time monitoring of geological disasters and building deformation. (4) When using control survey, the total station has the functions of rear intersection, front intersection and traverse survey, with high measurement accuracy and simple instrument operation, which can greatly improve the measurement speed. ⑤ All measurement contents can be completed in one station, mainly including height difference measurement, distance measurement and angle measurement, and the measurement data can be stored and transmitted. ⑥ Total station can be connected with plotter and computer by means of transmission equipment to form an integrated surveying and mapping system, which greatly improves the efficiency and quality of topographic surveying and mapping. (2) Introduction of main workflow. Topographic mapping with total station mainly includes the following processes: collecting data, processing data, editing graphics and outputting graphics. Data collection is to obtain the data information needed for topographic mapping, mainly the attributes and spatial positions of mapping entities. There are two different working methods for field collection. One is to use the total station to obtain data in the field and store the obtained data in the memory of the total station, or transmit the data to PDA, and then import the transmitted data into the computer for post-processing; The other is to directly connect the computer with the total station in the field, and the data measured by the total station are transmitted to the connected computer in real time, and the geographical attributes are added on the spot to directly map. (3) Analysis of surveying and mapping methods. 1. Establish the plane control coordinate system of topographic mapping. Before formal topographic mapping, the plane control coordinate system should be established first. In practice, the geodetic coordinate system is generally chosen as the plane control coordinate system, so that the existing national triangulation points can be conveniently used. If there is no definite control point in the survey area, you can establish your own rectangular coordinate system in the survey area, with the positive magnetic north of the earth as the starting azimuth of the rectangular coordinate system. 2. Collect data. Choose a place with a wide line of sight in the survey area, and you can observe most of the points in the survey area as the site of the total station, and set up signs at this point. Set up a total station on site, connect PDA and other equipment to the total station, and start the mapping wizard to collect data. When surveying and mapping landforms and features, the required measuring points are different, including multi-point features, strip features and single-point features, which need to be treated separately according to the site conditions. For example, when surveying and mapping buildings, you need to collect data from at least three points, while when surveying and mapping single-point objects, you only need to collect data from one point. When surveying and mapping special features, the position and number of sampling points should be reasonably determined according to the changes of features, so as to control the changes of their shapes and reduce the measurement errors. In addition, the following points should be paid attention to when collecting data: ① If multiple prisms are used at the same time, the height of each prism should be as equal as possible; If you need to change the prism height at a certain point, pay attention to update the prism height. (2) Surveying and mapping operations, surveying and mapping personnel should be equipped with walkie-talkies, so that surveying and mapping personnel can communicate in time to avoid mistakes due to communication problems. (3) When setting the measuring points of the total station, it is necessary to specify the number of points where the machine is located and the number of points of the rear viewpoint. If the dot number is confused, all the measurement data will be discarded. 3. Data processing. According to the coordinates of measuring points and the requirements of surveying and mapping, the topographic map is drawn. At present, total station mapping is mainly realized by CASS mapping software, which is a digital mapping data acquisition system based on Auto CAD. Through its "electronic tablet" operation mode, all data interfaces of the total station can be connected, so that the collected data can be automatically input and recorded, and topographic maps can be drawn under the condition of field work. Or download the mapping guide data through PDA, and then combine the data to complete the mapping on CASS. Draw topographic map with reference to the sketch drawn during field survey, connect all measuring points with standard symbols, and draw contour lines in combination with the actual topographic situation of the survey area after completing the feature map for repair. Because the deformation of different papers is different and the projection of topographic map and satellite image is different, it is impossible for topographic map and satellite image to overlap completely when they are put together. At this time, it is necessary to draw ground objects by comparing and partially overlapping two maps. The classification query of periodical articles is based on the fact that periodical libraries have been able to obtain maps, aerial photographs and other materials, and set up interpretation marks, such as place names, structures, building grades and other text symbols, so as to draw topographic maps and landforms. The basic working principle and technical characteristics of mapping topographic map with GPS-RTK (1). GPS-RTK is a real-time dynamic positioning technology, which can provide real-time three-dimensional positioning results of measuring points based on carrier phase observations. GPS-RTK usually consists of three parts: ① reference station, mainly dual-frequency GPS receiver; ② Mobile stations, mainly real-time differential software systems and dual-frequency GPS receivers; ③ Data link, mainly GSM mobile phone and data radio station. Using GPS-RTK technology to carry out topographic mapping has the following technical characteristics: ① the measurement results can be displayed dynamically in real time, and the working process is transparent and intuitive; It can check the positioning accuracy of coordinates in real time, and effectively solve the problem that the previous surveying and mapping technology can not quickly map and dynamically set out in real time. ② Short field operation time; Under suitable observation conditions, it only takes about 4s to obtain the three-dimensional coordinates of the measuring point. ③ The operation time is not limited; When surveying and mapping with GPS-RTK technology, as long as the signals of four satellites can be received at the same time at the measuring point, surveying and mapping operations can be carried out. ④ High automation level; GPS-RTK technology is relatively simple to operate. The surveyors only need to center and level the antenna, measure the height of the conductor, and then turn on the power supply to realize automatic measurement, which greatly reduces the workload of the surveyors and realizes intelligence. (2) Application of 2)GPS-RTK in engineering survey. GPS-RTK is widely used in the engineering survey of tunnels, bridges, roads, water conservancy and so on because of its high efficiency, automation, real-time and rapidity. ① General control measurement; Using GPS-RTK technology, the three-dimensional coordinates of encrypted control points can be continuously measured to meet the needs of local engineering survey with total station in local areas. ② Digital topographic map survey; Using GPS-RTK mapping can greatly reduce the number of control points needed for mapping, and change the previous surveying method of "controlling first and mapping later". Only one person needs to collect point coordinate data and then import it into digital software, so topographic maps of various scales can be generated, which greatly reduces the labor force and effectively improves the efficiency of surveying and mapping. ③ Cadastral survey; GPS-RTK technology can determine the location of the ownership boundary point of each land in real time, and import the obtained data into GPS system after processing, so as to get the cadastral map in time. (4) construction lofting measurement; RTK random software includes lofting function, which can be used for construction lofting measurement of points, lines and curves. Input the pre-designed points and line elements into the survey controller, and the corresponding lofting points can be automatically generated. The controller can guide the lofting work by displaying the mileage and offset distance of the measuring point in real time. (3) The workflow of measurement operation. The workflow of surveying operation using GPS-RTK technology is as follows: ① Collecting relevant data; Before the survey, the known high-grade control points in the survey area should be collected according to the actual situation, and the control point data should be checked to ensure its authenticity. ② Most of the high-grade control points collected in the early stage cannot be directly used, so it is necessary to encrypt the control points in the survey area, take the encrypted points and the original control points as the positions of reference stations, measure their actual elevation and coordinates, and then set a receiver on the reference stations for parameter configuration. ③ Put the GPS receiver on the mobile station and initialize the receiver; Dynamic measurement can be carried out after a certain measuring point is initialized, and the initialization of GPS receiver can also be completed under the condition of dynamic measurement. ③ The coordinate system used in ③GPS survey is WGS-84 coordinate system, but the local independent coordinate system is often used in practical engineering, so it is necessary to determine the parameters of coordinate transformation. If the survey area has been measured by static control network before, then the conversion relationship can be easily obtained; If static control survey is not carried out, control points should be selected in the survey area for on-site verification, and more than three points should be selected for RTK parameter correction. After defining the conversion parameters between the local independent coordinate system and WGS-84 coordinate system, the independent coordinates of the anchor point can be easily obtained by using the measurement controller in RTK. ⑤ After determining the correctness of the coordinate transformation parameters used, real-time measurement is carried out in the survey area in combination with the actual engineering situation. (4) Measures to improve the quality of surveying and mapping. When GPS-RTK technology is applied to topographic mapping, due to the limitation of satellite state and the influence of sky environment, the operation deformation is less than the nominal distance, and the data transmission is easily restricted and interfered by the external environment. Surveying and mapping in mountainous areas and urban high-rise areas is easy to lose signals, the initialization time is too long, and the accuracy and stability of individual equipment are not high, which directly affects the quality and accuracy of topographic surveying and mapping. Therefore, necessary measures should be taken to improve the quality of surveying and mapping, including: ① before surveying and mapping. ② Comparison with existing measuring points; By using the redundant control points of static GPS when establishing the control network, the measurement results are compared with those of RTK, and then the measurement results are checked. This method is easily limited by the number of control points, but it is better. ③ Test-retest comparison method; After each measurement initialization, the previously measured high-precision control points are retested and the results of the two measurements are compared. If the error is within the allowable range, it can be measured. This method is usually used where there are no control points. 4. Conclusion Using total station combined with drawing wizard program, objects and lines with different attributes can be collected and drawn ideally, and then rough graphics can be generated, which has the advantages of convenient operation and strong intuition. Under the condition of controlling the number of measuring points and poor visibility, it is difficult for ordinary measuring instruments to map smoothly in mountainous areas. Using GPS-RTK technology to do surveying and mapping work well, the effect is very good, which can save a lot of manpower and material resources. 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