Research On Control Surveying Of GNSS And Height Fitting In Linshui County

In recent 20 years, GNSS technology which was once used for military purposes only has been gradually expanded to the civil fields and fast applied in various industries by means of quick and efficient positioning. In the surveying field, this technology has been used as the most common geodetic surveying means for its advantages of large surveying scope, no need on intervisibility, high accuracy, low cost, etc. During control surveying, its main task is to accurately measure the coordinates(X, Y, H) of the control points. GNSS can measure the 3D coordinates of the ground space, convert the coordinates into geodetic coordinates and perform gauss projection to obtain plane coordinates(x, y). GNSS belongs to the technical means in the field of normal system, can obtain the geodetic height only, and cannot obtain the normal height. If the normal height is obtained by using level survey or trigonometric leveling is utilized, cost will be multiplied, which fails to reflect the priorities of the superiorities of GNSS.Actually, many researches have been made to measure the normal height by using GNSS. To further improve the surveying accuracy and reduce cost, some improvements are made on the basis of the previous researches and some new methods for directly measuring the normal height by using GNSS in this text. In accordance with a traditional solution, GNSS height points are required to be uniformly distributed in the surveyed area. The geodetic height is obtained at those points through GNSS, while the normal height h is measured through level surveying, and the difference between the former two is the height abnormality. By performing quadratic fit, the height abnormality at any point in the surveyed area can be obtained, thus getting the normal height. This solution requires uniform arrangement of the GNSS height points. The more the points are, the higher the accuracy is. However, a huge amount of level surveying work is needed. In order to improve efficiency and reduce cost, the following improvement solution is put forward. A certain appropriate number of GNSS height points are arranged in the a surveyed area to obtain height abnormalities with full spectrum information; a gravity model is used to directly calculate the height abnormalities without ultrashort wave information; quadratic fitting is carried out on the reminders of the height abnormalities with ultrashort wave information; in order to improve the accuracy, tryding to make some improvements to the quadratic fitting--weighted reminder fitting is carried out on the fitting residual error; and finally, the height abnormalities at any point in the surveyed are can be obtained by adding the final fitting result and the gravity model calculation result.In order to verify the feasibility of this solution, GNSS controlled network data of a certain place in Sichuan Province were used to make an experiment. It is worth noting that, except EGM2008, the GECO model issued in 2015 was also adopted so as to ensure that the gravity model also keeps the currency. The calculation result shows that, the accuracy is about 10 mm if the quadratic fitting of the gravity model is taken into consideration and less than 1mm if the weighted reminder fitting with the gravity model is taken into consideration. From the aspect of precision, it is more than pure geometrical quadric surface fitting theory.The results of a great deal of experiments and calculations prove the correctness and feasibility of the improvement method put forward in this text.