| The rapid development of GNSS measurement technology provides a new efficient method for the establishment of vertical control network.The adjustment result of GNSS survey is the geodetic height based on the reference ellipsoid,while the normal height based on the quasi geoid is used in the elevation system of our country,which makes the conversion between the geodetic height and the normal height become the hotspot and difficulty in the field of geodesy.At present,the main construction method of the quasi geoid is to use the classic Stokes theory of physical geodesy for calculation.In this paper,the regional quasi geoid is obtained by fitting the leveling results,then the coordinate information of the fixed point is obtained by GNSS measurement technology,and the normal height of the fixed point is obtained by interpolation.Before data interpolation,the fitting accuracy can be effectively improved by dividing the measuring area according to certain principles.The main work of this paper is as follows:1.The common methods of constructing quasi geoid are analyzed systematically.The advantages and disadvantages of various methods are discussed in detail.The combination method(remove restore method)which can meet the accuracy requirements and is cost-effective is selected.The egm2008 global gravity model based on interpolation resolution of 1’× 1’is used to extract the middle and long wave components of elevation anomaly,and the data of the remaining part is interpolated to improve the fitting accuracy.2.Describe the calculation process of polyhedral function fitting method,introduce the selection method of core points,common kernel function forms,the characteristics and selection range of smoothing factors,the calculation process of undetermined coefficients of the model,and provide the evaluation method of internal and external coincidence accuracy according to the fitting and interpolation data.3.The principle of zone fitting and the scheme of zone division are described,including the methods of zone division by terrain,zone division by elevation abnormal value and zone division by mean error of fitting.The calculation process of polynomial fitting method for calculating mean error of fitting is introduced.In order to improve the fitting accuracy of boundary coincidence point,bicubic spline function is used to smoothly connect the boundary of each zone.4.Taking Jining City of Shandong Province as an example,the GNSS data of 110 points and the second and third level data are collected.The experimental area is fitted in different zones according to the scheme of zones.According to the characteristics of different zones,a certain number of core points are set,the smoothing factor with the highest fitting accuracy is selected,and the parameters of multi-faceted function fitting are determined.Combined with the egm2008 global gravity model,the interpolation calculation is carried out,and the residual analysis is carried out one by one for three zoning schemes.The maximum and minimum residual values of each zoning are calculated,and the internal and external coincidence accuracy is also calculated.The boundary coincidence points are connected smoothly,and the fitting residuals of coincidence points are calculated to evaluate the effect of surface smoothing.5.Compare the fitting results of three zoning schemes,and analyze the advantages and disadvantages of various zoning methods in this area.The results show that the accuracy of out of the terrain zoning is 0.0251m,the accuracy of out of the elevation anomaly zoning is 0.0285m,and the accuracy of out of the mean error zoning is 0.0197m,so we can get the conclusion that the fitting accuracy is the highest according to the mean error of the fitting.This paper introduces the parameters of the optimal quasi geoid model,including the distribution of the core points,the information of the number and proportion of the core points in the small area,the optimal smoothing factor of each area,the undetermined coefficient of each area,etc. |
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