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An optimum Global Geopotential Model (GGM) is required either to represent the long wavelength of the Earth's gravitational field in gravimetric geoid modelling or to act as a stand-alone national geoid in some developing countries. Thus, the current research aims to investigate the performance of seven recent high-order GGMs over Egypt utilizing the most recent precise Global Navigation Satellite Systems(GNSS)/Levelling datasets. Attained results showed that the investigated GGMs perform differently over Egypt with an accuracy level varying between ±0.249m for the SGG-UGM-1 model and ±0.300m for the GECO model. Removing outliers in the terrestrial dataset reveal significant improvements, in terms of standard deviations. Consequently, the performance of the investigated GGMs has been modified where the EIGEN-6C4 model became the best model with a standard deviation equals ±0.172m. Furthermore, a 3D spatial correction surface has been constructed and has been added to the original EIGEN-6C4 model to get an enhanced version of the global EIGEN-6C4. Over checkpoints, the average error of the enhanced model equals -0.018m with a standard deviation equal ±0.011m. That means that incorporating terrestrial geodetic data into the global EIGEN-6C4 model has increased its accuracy in Egypt by almost 36 %. Accordingly, such developed enhanced GGM represent the optimum stand-alone geoid model in Egypt. Nevertheless, such accuracy levels of GGMs do not fulfill the requirements of high-accuracy surveying and civil engineering applications. That concludes that a precise national geoid model is still crucial. It is recommended that all available geodetic datasets should be collected from all governmental and private organizations to construct a national geodetic database that would be implemented in modelling an Egyptian local geoid. [ABSTRACT FROM AUTHOR]

Geodetic observations in any country or region require a precise local geoid model. Hence, this study has improved the geoid modelling using simulated terrestrial gravity data. However, the sparse and limited number of terrestrial gravity data is the primary reason for the inability to develop an accurate gravimetric geoid model in Iraq, including within the Sulaymaniyah Province selected as the case study in this research work. The ability to use the global navigation satellite system (GNSS) to determine orthometric height has been restricted due to the lack of precise geoid models within the region. Hence, 3327 gravity points from several international and local datasets were applied, 160 of which were collected via gravity survey, to simulate and model the gravity in the Sulaymaniyah Province. A stepwise multiple linear regression with a correlation coefficient (r) of 0.997 and a determination coefficient of (R2) of 0.993 (both very close to 1) was deployed to extract the geographical coordinates and the orthometric height of the points to formulate a cutting-edge gravity model. Next, 120 local gravimetric models were generated using software from KTH (a university in Sweden) with two conditions: (1) the simulated gravity data were composed of a variety of grid and cap sizes, and (2) both the interpolated gravity data and the terrestrial data were combined with the downloaded World Gravity Map 2012 (WGM2012) data. Next, ITU_GRACE16 and IGGT_R1 global geoid models (GGMs) were used to support the cap size area. As the quadratic model fit the 11 available global positioning system (GPS)-levelling points, the simulated gravity data revealed the lowest root mean square error (RMSE) result of ± 17 cm when using IGGT_R1 GGM, in comparison to the other two datasets. Meanwhile, EGM2008 scored an RMSE of ± 31 cm. In conclusion, this new data entry method improves the accuracy of local geoid models by mathematically simulating the gravity data instead of interpolating them. [ABSTRACT FROM AUTHOR]

Geoid-quasigeoid separation term is a crucial parameter that allows the conversion between different national height systems based on geoid and quasigeoid surfaces. The consideration of free-air Vertical Gravity Gradient Anomalies (VGGA) in the separation term formula is generally ignored and is not sufficiently investigated. However, this may be important, particularly for the mountainous areas. In this study, the impact of considering modelled VGGA in the special formulation of separation term is investigated. The study area is in mountainous western Turkey where modelled VGGA are previously validated with the homogenous and dense VGGA measurements (in total: 159). The separation term is calculated with and without considering the VGGA. Thus, gravimetric geoid models have been obtained with two different strategies. These gravimetric geoid models are compared with geometric geoid model calculated from recently measured GPS/levelling measurements. The results demonstrate that the consideration of modelled VGGA in the special formula of separation term changes the gravimetric geoid model with the statistics of min: −10.6 cm, max: 17 cm, mean: 0.2 cm, std.: 1.5 cm. This is quite significant for a goal of cm accurate geoid modelling. Especially at elevations higher than 1000 m, the use of modelled VGGA is found to be more effective on the gravimetric geoid. • This study makes use of successfully modelled Vertical Gravity Gradients Anomaly (VGGA) values. • The impact of modelled VGGA in the special formulation of separation term is investigated. • The use of modelled VGGA contributes to the accuracy of the gravimetric geoid model. • VGGA reduces the gravimetric geoid model calculations errors up to 17 cm. [ABSTRACT FROM AUTHOR]

This paper describes the development of a new geoid-based vertical datum from airborne gravity data, by the Department of Survey and Mapping Malaysia, on land and in the South China Sea out of the coast of East Malaysia region, covering an area of about 610,000 square kilometres. More than 107,000 km flight line of airborne gravity data over land and marine areas of East Malaysia has been combined to provide a seamless land-to-sea gravity field coverage; with an estimated accuracy of better than 2.0 mGal. The iMAR-IMU processed gravity anomaly data has been used during a 2014-2016 airborne survey to extend a composite gravity solution across a number of minor gaps on selected lines, using a draping technique. The geoid computations were all done with the GRAVSOFT suite of programs from DTU-Space. EGM2008 augmented with GOCE spherical harmonic model has been used to spherical harmonic degree N = 720. The gravimetric geoid first was tied at one tide-gauge (in Kota Kinabalu, KK2019) to produce a fitted geoid, my_geoid2017_fit_kk. The fitted geoid was offset from the gravimetric geoid by +0.852 m, based on the comparison at the tide-gauge benchmark KK2019. Consequently, orthometric height at the six other tide gauge stations was computed from HGPS Lev = hGPS - Nmy_geoid2017_.t_kk. Comparison of the conventional (HLev) and GPS-levelling heights (HGPS Lev) at the six tide gauge locations indicate RMS height difference of 2.6 cm. The final gravimetric geoidwas fitted to the seven tide gauge stations and is known as my_geoid2017_fit_east. The accuracy of the gravimetric geoid is estimated to be better than 5 cm across most of East Malaysia land and marine areas

Olga Bjelotomić defended her PhD thesis High Resolution Geoid Modelling of Croatia at the Faculty of Geodesy, University of Zagreb on May 15, 2015. The Defense Committee consisted of Assoc. Prof. Željko Hećimović from the Faculty of Civil Engineering, Architecture and Geodesy, University of Split, Prof. Dr. Tomislav Bašić (mentor) and Assist. Prof. Ramazan Alpay Abbak from Selcuk University in Konya, Turkey.

On recent times the federal agencies in charge of administrating the basic geographic information have received from the scientific community a clear recommendation to migrate the way vertical control coordinates are determined, moving from the local reference of mean sea level to a reference in terms of potential of the gravity field, like the geoid. The main advantage of such change is the ability to make the vertical datum compatible to that of any other country in the world and with the GNSS technology for surveying. In preparation for substituting the classic system, based on observations to the local sea level, a large amount of geographic agencies face the challenge of implementing the new concepts with new methodology and data. In this work we introduce the first results from an effort in international collaboration from 8 countries in the region of Mexico, Central America and the Caribbean to increase the technical capability in this subject and diminish the breach between the scientific advancement and the practical implementation for the geodetic data infrastructure in official use. The gravimetric geoid for Mexico, Central America and the Caribbean, GGM-CA-2015 is introduced as an original digital model in coverage and resolution. It represents an equipotential surface of the gravity field, parameterized by the constant value W0=62636856.0m²/s². This realization allows to refer vertical coordinates for geo-positioning among a large amount of countries and islands on a consistent datum through national borders and across the oceans. The processing of the GGM-CA-2015 was carried out by the Stokes-Helmert technique applied on the current coverage of gravimetric databases and takes advantage of the satellite information contained in the model GOCO05s. It is estimated that the accuracy of the geoidal heights obtained is on the level of decimeters, varying for zones according to the available observations of GNSS on benchmarks. Additional to this, the document contains a discussion on a few identified ways to improve the geoid modeling in the region, regarded as a note for future work. At the same time it is underlined the advantage of the strategy implemented for a coordinated work which was chosen among representatives of the geographic agencies involved. [ABSTRACT FROM AUTHOR]

Developing a gravimetric geoid model requires gravity data covering the whole surface of the earth. In practice, terrestrial data within a spherical cap is used, causing a truncation error, which may be minimised if the terrestrial data is combined with a Global Geopotential Model (GGM). The choice of a GGM that fits the observed terrestrial data best, determines the accuracy of a gravimetric geoid solution. In this study, the most recent and high-resolution GGMs are selected and compared, both geometrically and spectrally with a view to selecting an optimum GGM for future geoid modelling in Kenya. In the first step, thirty-one GGMs are evaluated using 55 GNSS-levelled points scattered over 4 regions and gravity data distributed over the entire territory of Kenya. In the second step, some of the best performing GGMs are further compared using the spectral information contained in their spherical harmonic coefficients. After removal of systematic errors, the EGM2008 model showed some advantage over other GGMs with a standard deviation of 40.89 cm. Other high-resolution geoid models perform well in terms of recovering geoid heights in Kenya with a standard deviation of <42 cm. In terms of residual gravity anomalies, the EIGEN-6C4 model showed the best fit with a standard deviation of 6.892 mGal. In the spectral analysis, the XGM2016 provided the best results among the models evaluated. Based on the overall performance in all areas of evaluation, the SGG-UGM-1 and SGG-UGM-2 were considered best for geoid modelling in Kenya. • Optimal gravimetric geoid determination requires a best-fitting geopotential model. • Internal and external evaluations of geopotential models are important in selecting the best geopotential model. • High resolution geopotential models can recover geoid undulations and gravity anomalies in Kenya with reasonable accuracy. • Overall, SGG-UGM-1 and SGG-UGM-2 geopotential models are most suitable for geoid modelling in Kenya. [ABSTRACT FROM AUTHOR]

We deal with modelling of the geoid undulations for the European Plate by use of topographic and Moho data. Two models assuming linear density stratification in the lithosphere (constant contrast model CCM, constant gradient model CGM) and isostatic balance of the lithosphere were used for calculating the undulation in the flat layer approximation. The results show that the constant contrast model is able to describe the entire oceanic lithosphere, as it indicates the amplitude of thermal density change is in good agreement with the cooling plate model estimation. The constant gradient model gives reliable estimations of the lithosphere properties only in smaller regions of relatively uniform conditions like the Interior of the East European Craton. For continental and oceanic regions the resulting values of the density gradient have some average meaning and they are in interpretable correspondence with characteristic mantle heat flow. In the entire area, both models show strong confusion giving not intermediate and unrealistic lithosphere characterization, which is a result of essential differences of thermal constitution, differences in average crustal density and mineral differences of the lower lithosphere, occurring between the two major tectonic provinces (oceanic and continental). The convention of equivalent linear reduction was discussed extensively and applied as an adequate method of lithosphere thickness estimation. This approach leads to thickness determination similar to other methods (seismic, petrological and thermal). The two concepts allow for the construction of LAB (transitional zone between the lithosphere and asthenosphere) depth maps from topographic and Moho data. [ABSTRACT FROM AUTHOR]

Land Information New Zealand (LINZ) has recently commenced a project to improve the accuracy of the New Zealand Vertical Datum 2009 (NZVD2009), which will primarily be achieved through the incorporation of airborne gravity data into the quasigeoid that defines NZVD2009. This paper describes a trial airborne gravity survey carried out over Canterbury in early 2012 by LINZ, in conjunction with GNS Science. The purpose of this trial was to provide a proof of concept for the capture of a national airborne gravity dataset in the New Zealand environment. [ABSTRACT FROM AUTHOR]

The lithospheric structure of the contact zone between the Iberian and European plates in the transition from the Western Pyrenees to the Cantabrian Mountains is investigated by 3D modelling of gravity anomalies and geoid undulations. The main target of this study is to gain insight into the deep geometry of this plate boundary, especially into the topography of the lithosphere-asthenosphere boundary. The results, still preliminary, suggest that the lithospheric thickness is similar on the Iberian and European sides, with a small and discontinuous lithospheric root in the contact zone between the two plates. [ABSTRACT FROM AUTHOR]

Abstract: We investigate the effect of lateral viscosity variations (LVV) in the mantle on dynamic geoid and near-surface mantle velocities. Contrary to the previous studies, we analyse the effect not only of the lithospheric keels and asthenosphere but also of the whole mantle. A 3D global viscosity model is constructed using (a) the S20 seismic tomography model converted to temperature and (b) assumptions about homologous temperature in the mantle Paulson et al. [Paulson, A., Zhong, S., Wahr, J., 2005. Modeling post-glacial rebound with lateral viscosity variations. Geophys. J. Int. 163, 357–371]. The conversion parameters provide a depth-dependent viscosity profile, which is generally consistent with the existing studies. Therefore, the horizontal variations are produced self-consistently within this approach. We estimate them as ‘conservative’, thus providing a minimum limit for possible horizontal changes; consequently the obtained results show a low limit for dynamic topography, geoid and surface velocities’ disturbances. To estimate the effect of LVV we have employed the perturbation method of Zhang and Christensen [Zhang, S., Christensen U., 1993. Some effects of lateral viscosity variations on geoid and surface velocities induced by density anomalies in the mantle. Geophys. J. Int. 114, 547–551], which is modified to implement mantle compressibility. It has been found that the impacts of the upper and lower mantle are comparable in amplitude. The difference between the initial (only radial viscosity) dynamic geoid and the geoid with implemented LVV reaches −27 to +19m for the whole mantle LVV, while the effects of the upper and lower mantle are equal to −24 to +16 and −20 to +16m correspondingly. In general these effects are not correlated. The difference in the geoid response leads to substantially altered velocity-to-density scaling factors (up to 30% compared to the initial ones) obtained in least squares inversion. The effect of the lower mantle LVV on surface velocities is also substantial, in particular with respect to the toroidal component, which does not exist in the initial model. [Copyright &y& Elsevier]