Reaching Local Vertical Datum Using Global Geopotential Model And GPS/Leveling Data

With the advent of space-based technologies such as the Global Navigation Satellite Systems(GNSS),Very Long Baseline Interferometry(VLBI),Satellite Laser Ranging(SLR),Doppler Orbitography and Radiopositioning Integrated by Satellite(DORIS),and satellite radar altimetry,heights of any arbitrary point on the Earth's surface or above the Earth's surface can be easily obtained.The heights obtained utilizing spacebased techniques refer to a reference ellipsoid,which is an analytically defined geometric surface,and hence these heights,known as ellipsoidal heights,are geometric in nature.The emergence and popularization of positioning technology,subverting and replacing the traditional ground positioning observation technology,the horizontal position of the measurement,namely the earth latitude ? and the longitude ?,can be directly used for the Geographic Information System,engineering construction and national defense modernization,etc.,it can meet the needs of disciplines and industries such as surveying and mapping,earth science.The ellipsoidal heights that can distinguish the physically meaningful heights and their determination is dependent on the gravity potential of the Earth(i.e.,a combination of the gravitational potential due to the Earth's mass or density distribution and the centrifugal potential due to the Earth's rotation).The physically heights we used is orthogonal high or normal high,the physically heights are the basic information necessary for the construction of any water conservancy and hydropower project,including the South-to-North Water Diversion Project and the oil and gas pipeline network project,as well as various types of transportation infrastructure construction.Then unification the global vertical datum is of great significance for the international cooperation in the development of relevant transnational projects.Different approaches can be used for the uni cation of the local vertical datums.They can be connected by means of precise geodetic leveling combined with gravity measurements along the levelling lines.An example of this approach is the United European Levelling Network.Ocean leveling using oceanographic information is another approach that can be used to connect Local Vertical Datums without connection points on land.The Geodetic Boundary Value Problem(GBVP)solution is the rigorous method for LVD unification.GNSS-determined ellipsoidal heights,levelling heights from precise geodetic leveling,and geoid heights from lo-cal gravity data and global geopotential models(GGMs)are used in the GBVP method to unify the Local Vertical Datums.We used the method,based on adjusting the collocated GPS/Leveling and Global Geopotential Model data,for the estimation of the zero-height geopotential value or the vertical datum offset between the local vertical datum and the global vertical datum,the global vertical datum is defined by the value of 62636856.0 m2s-2 or 62636853.4 m2s-2.Based on the classical theory of the height system in physical geodesy,the definition of the height datum vertical offset is given,and the rigorous formulas for calculating the vertical offset are derived.The formulas can be used to deduce the three methods of the height datum vertical offset determination.On the basis of this,the influences of different reference system and reference ellipsoid parameters on the calculation of the vertical offset are analyzed.The accuracy of the geopotential value of the vertical datum is usually relies on the high-precision Global Geopotential Models,which are critical for high-precision uniformity of global and regional vertical datums or height systems.The advent of the GOCE and GRACE missions during the last decade have brought significant improvement in modelling of the low-frequency or rather medium-frequency part of the Earth's static gravity field.The European Space Agency's dedicated satellite gravity field mission the Gravity Field and Steady-state Ocean Circulation Explorer(GOCE)will at the end of its lifespan achieve 1-2 cm geoid accuracy at a spatial resolution of 100 km.We used the GPS/leveling data and the global geopotential models to estimation the vertical datum offset,the method is benefits from the significant improvements in the determination of the global gravity field by the recent satelite gravity misions such as GRACE and GOCE.The performance of GOCE-based or GRACE/GOCE-based Global Geopotential Models is asesed for estimation of geopotential of the regional vertical datum of China using the GPS/Leveling benchmarks from vertical control network.We should conver the normal height to orthometric height.From the evaluation with the GPS/Leveling BMs,it is concluded that the GOCE-based or GRACE/GOCE-based GGMs provide an absolute acuracy at of 40 cm,up to their maximum degre and order.Factors that affect the geopotential value of vertical datum include the GOCE commision and ommision errors.Among these factors,the efect of the GOCE omision eror is investigated by extending the models with the high resolution gravity field model EGM208.When extending the GOCE-based or GRACE/GOCE-based models with the high resolution gravity field model EGM208,we should provide the same parameters.At the same time we should unify the frame and tide system betwen the GGMs and GPS/Leveling dataset.The result shows that,the effect of the GOCE omission error is at the level of 1.6 m2s-2(about 16 cm),in China,is at decimetre-level when computing local vertical datum geopotential,the effect of the tide system is about 4-6 cm.The accuracy of the geopotential value of the vertical datum is usually relies on the high-precision Global Geopotential Models(GGMs),which are critical for highprecision uniformity of global and regional vertical datums or height systems.The GRACE/GOCE GGMs are extended from the degree and order nmax-1 to 2190 using EGM2008 in order to evaluated the effect of the omission error on the computed geopotential value of the China vertical datum of 1985,and the effect of the GRACE/GOCE GGMs omission error is at dm-level for China.The effect of the GRACE/GOCE GGMs omission error on the geopotential can be effectively reduced by extending the model with the high-resolution gravity field model EGM2008 and the optimal extend degree and order is difference based on the difference GRACE/GOCE GGMs,for example,the extend degree and order is 180 and 220 for DIR-1 and DIR-5.The GRACE/GOCE GGMs extended by the EGM2008 with the optimal extend degree and order provide a significant,compared to EGM2008,improvement in the comparisons with the GPS/Levelling data,by as much as 18 cm,in terms of the standard deviation.The WOLVD determined for the China local vertical datum was 62636853.47 m2s-2 and 62636853.49 m2s-2 for DIR-5 and TIM-5,and the accuracy both are 1.51 m2s-2.Lastly,the spatial tilts found in the levelling network can be modeled with a 2-parameter bias corrector model,the east-west tilt is about 9 cm,and the north-south tilt is about 1.4 cm.Based on the DIR-5 or TIM-5 models,the accuracy of the geopotential value of the 1985 is improved about 0.16 m2s-2.The results also show that there are still large differences among the accuracies of the current gravity field models on these GPS/Leveling points around Qingdao,which will lead to big differences in estimating the vertical datum offset between the China 1985 national height datum and the global height datum with respect to different selected data sets.Therefore,the accuracy and reliability of the current global gravity field models needs to be further improved if we use them for the height datum vertical offset determination.The theoretical and practical results of this paper could be used for the realization of the unification of the regional height datum and the global height datum.