Advantages and disadvantages of GPS positioning technology in engineering surveying

In the field of surveying and mapping, with the popularization of total stations, traditional theodolites and rangefinders have gradually been replaced. In recent years, with the development of GPS measurement technology, the operation methods of engineering surveying have undergone a historic change. GPS measurement determines the spatial position of the measurement point by receiving signals transmitted by satellites and processing data. It has omnipotent, global, all-weather, continuous and real-time precision three-dimensional navigation and positioning functions, and has good anti-interference and confidentiality. It has been successfully applied to many fields such as engineering surveying, aerial photogrammetry, engineering deformation measurement, resource investigation, etc.

GPS is mainly composed of three parts: space satellite constellation, ground monitoring station and user equipment.

1. The GPS space satellite constellation consists of 21 working satellites and 3 spare satellites in orbit. The 24 satellites are evenly distributed in 6 orbital planes with an inclination of 55°. The average altitude of the satellites is 20,200 km and the operation cycle is 11 hours 58 minutes. The satellites use two radio carriers in the L band to continuously send navigation and positioning signals to the majority of users. The navigation and positioning signals contain the location information of the satellite, making the satellite a dynamic known point. At any place and at any time on the earth, at an altitude angle of more than 15°, an average of 6 satellites can be observed at the same time, and up to 9 satellites can be observed.

2. GPS ground monitoring stations are mainly composed of a master control station, three injection stations and five monitoring stations distributed around the world. The master control station calculates the orbit parameters and clock error parameters of each satellite based on the observation data of GPS satellites from each monitoring station, compiles these data into navigation messages, and transmits them to the injection stations, which then inject the navigation messages sent by the master control station into the storage of the corresponding satellite.

3. GPS user equipment consists of GPS receivers, data processing software and terminal equipment (such as computers). GPS receivers can capture the signals of the satellites to be measured selected according to a certain satellite altitude cutoff angle, track the movement of the satellites, and exchange, amplify and process the signals. Then, through computers and corresponding software, the three-dimensional coordinates of the center of the GPS receiver (the measuring station) are obtained through baseline solution and network adjustment.

From the implementation and application of engineering surveying, we can fully see the superiority of GPS measurement, which fully demonstrates the high precision and high efficiency of this satellite positioning technology.

1. Using GPS technology to survey and lay out grids is more adaptable than conventional methods. The grid structure is simple, and the density of points and the length of sides can be flexibly selected. Even if they are far away from known control points, they can be connected and the control network can be positioned and oriented. In addition, it solves the problem of lack of line of sight between points, and the point selection is flexible, no high mark is required, and it can also ensure that field surveys are not affected by weather. Its superiority is particularly evident when surveying large (long-side) grids and when line of sight conditions are particularly difficult. Although GPS itself is not restricted by line of sight conditions when measuring, engineering surveys are generally small-scale surveys and are restricted by engineering costs. Therefore, in actual engineering surveys, it is still necessary to consider using commonly used and less expensive instruments such as total stations, theodolites, and levels. These commonly used instruments generally require mutual line of sight between points, especially when laying out control networks. The lack of line of sight between points will bring more trouble and difficulties to the survey work. Especially in the control network of large bridges, if there is no line of sight between points, it will inevitably affect the strength and accuracy of the network, and then affect the accuracy of the bridge itself. Therefore, when laying out the GPS control network in engineering surveying, as many points as possible should be able to see each other if necessary.

2. The GPS grid points have high accuracy and uniform error distribution, which not only meets the requirements of the specifications but also has a large accuracy reserve.

3. It is feasible to use the mean error of point position as the accuracy index of grid measurement, which is more reasonable than using relative mean error to express the accuracy index.

4. Using GPS method to lay out the geodetic control network can effectively improve the point approach speed due to its high graphic intensity coefficient. The network shape optimization is relatively convenient.

5. Compared with conventional surveying methods, the efficiency of surveying and setting up building grids by using GPS-RTK can be more than doubled, and the labor intensity of operators can be greatly reduced. A reference station can have multiple mobile stations operating, and the mobile stations do not need the command of the base station, and a single person can operate independently. [page]

GPS technology, with its unique and powerful functions and advantages, fully demonstrates its superiority in the field and its greater and broader development space. However, some shortcomings have also been exposed in the actual construction process and the construction and monitoring of subsequent projects in this field.

1. The key to accurate positioning of the GPS system lies in the accurate calculation of the distance between the satellite and the receiver, according to a fixed pattern: distance = speed × time. After the time is determined, the speed is determined by the propagation speed of the electromagnetic wave. It is well known that electromagnetic waves propagate very fast in a vacuum, but the atmosphere is not a vacuum state, and the signal is subject to heavy interference from the ionosphere and troposphere. The GPS system can only perform an average calculation, and there will definitely be errors in certain specific areas; in large cities or mountainous areas, the influence of high-rise buildings and trees on the signal will also cause non-linear propagation of the signal, which will also introduce certain errors in the calculation;

2. As with the control measurement performed by conventional instruments, the accuracy of the starting reference point should be verified first when using GPS-RTK technology. The starting point should be a high-level control point, and there should be a good position distribution between the starting reference point and the observation point. When using dynamic GPS-RTK for observation, the accuracy of the reference station should be measured and verified by 3-5 high-level control points to ensure that the coordinates of the reference station have consistent accuracy in all azimuth observations.

3. A large number of engineering examples have proved that although GPS elevation measurement can achieve a certain degree of accuracy, the municipal engineering measurement control points measured by GPS should be further leveled with conventional instruments to ensure that the elevation accuracy meets the needs of municipal engineering construction.

4. The difference in the position of the control points selected in GPS measurement directly affects the accuracy of the observation points. Since GPS measurement obtains the point coordinates (including elevation) by receiving the signal transmitted by the satellite and processing the data, any factors that may affect the signal reception may cause errors in the measured point coordinates. Therefore, the following points should be noted when selecting the measurement point: (1) The point has a wide field of view, 15° upward, and obstacles should be avoided as much as possible within the field of view. (2) Keep away from high-power radio transmitters as much as possible, with a spacing of no less than 400 m, and keep away from high-voltage transmission lines, with a spacing of no less than 200 m. (3) Keep away from objects that strongly interfere with satellite signal reception, and try to avoid large areas of water.

5. GPS measurement is more suitable for the construction of new areas with wide fields of vision and fewer obstacles, field exploration and positioning, etc. In the construction of old urban areas, when using GPS measurement, either no signal is received, or even if the signal is received, it is always in a floating state, resulting in false fixation or inability to fix. Therefore, the data obtained often has large errors, which is neither efficient nor accurate, and cannot show the superiority of GPS measurement.

6. There are differences between GPS measurement results and conventional measurement results, and between different models of GPS measurement results, and sometimes the differences are quite large. When the GPS network is performing adjustment calculations, the side length generally needs to be corrected in two ways: (1) Correction to the geoid; (2) Correction to the Gaussian projection surface. The two-dimensional joint adjustment model cannot solve the problem of unifying the plane position and the elevation position, while the three-dimensional joint adjustment model is a multifunctional advanced adjustment system that can realize the conversion of adjustment models. The result of the adjustment is the three-dimensional spatial position of the point and its accuracy, which is extremely beneficial for the comprehensive analysis and research of the point position and its components. However, in the three-dimensional joint adjustment, the ground points need to have geodetic height observation values ​​with corresponding accuracy requirements, which is difficult to achieve in some cases.

7. GPS and its related technologies are new technologies, and their application standards are not yet perfect. At present, my country has not promulgated a unified geographic information standard. Most navigation product manufacturers use electronic maps developed and produced by themselves, which are generally incompatible with each other. In addition, there is no unified standard specification for products, and the product market has not formed a standard, especially for software products. This needs further research and formulation by relevant departments.

In summary, in the field of engineering surveying, due to its unique and powerful functions, GPS positioning technology has fully demonstrated its greater superiority and adaptability than conventional control measurement in actual surveying work in this field. At the same time, there are also some shortcomings, which need further research and improvement to adapt to actual surveying work. With the rapid development and popularization of this technology, as well as the application of related technologies, GPS positioning technology will be more widely used in urban construction and engineering surveying.