Calculation of geoid undulations and gravity anomaliesin the Northwest Pacific by using the Topex/Poseidonand Geosat altimeter data

The geoid undulations in the Northwest Pacific were calculated by usingtheTopex/Poseidon and Geosataltimeter data. Firstly the bias between two types of the altimeter data was removed and the geoid undulations in theNorthwest Pacific were acquired by a long wave bias diminishing model with a resolution of 30 km and precision of 14cm. Then an algorithm of inversion of gravity anomalies was derived , and the gravity anomalies in the East China Seawere calculated by using the algorithm and the geoid undulations. The rms of difference between the in situ measure-ments the gravity anomalies from altimeter data was 3 .8× 10-5 m/s2. A method to colculate the gravity anomaliesin a larger area was developed which combined gravity anomalies in four subregions overlapping each other into one data set in a larger region. The error analysys shaws that the model and result of the inversion of gravity anomalies were reliable.

Impact of the formation and ablation of Antarctic ice sheet on the global geoid and sea level

It is convenient to investigate the gravimetry using a harmonic spheric function for the description of the distribution and thickness of the Antarctic ice sheet. The gravitational theory and the Stokes' harmonic spheric function formula were used to determine the impact of the Antarctic ice cap on the global geoid. The Antarctic ice cap is formed from the condensation of seawater vapour whose mass is equal to a layer of seawater 59 m thick of covering the earth's surface, i.e. 2.7×10 19 kg. This will cause the global averaged geoid to decrease for around 23 m. The authors' computations show that the Antractic ice cap has a great impact on the global geoid, which increases (+) in some regions, but decreases (-) in other reigions. The geoid is +115 m, -37 m and +8 m at the South Pole, the 25°S parallel and the North Pole, respectively. If the Antarctic ice cap melts completely, on the rigid Earth's surface the seawater and geoid will return to its original position (and height) due to the balancing force of the fluid. Since the crust is almost in a state of isostasy, assuming that the crust is an elastic solid and the mantle is an incompressible fluid, the load of seawater will deflect the crust and drive the mantle material to flow. The material above the isostatic surface compensates mutually. If the densities of the mantle and seawater are 3270 kg/m 3 and 1030 kg/m 3, respectively, then the variation in the elevation of the continent is only 2.8 m with respect to the sea level after the Antarctic ice cap melts;it is not larger than that estimated by some people.It is worth noting that the above results were derived from an ideal Earth model. In the real Earth, the mantle and crust are visco elastic.

Local Geoid Model Generation Using the Geometrical Approach

The global navigation satellite systems(GNSS)provide an accurate three-dimensional positioning including the geodetic(ellipsoidal)height(h),which is,in most cases,must be transformed to the local/regional orthometric height(H)to have physically the desired meaning of the elevation above the national vertical datum(e.g.,MSL(Mean Sea Level)).Usually,the local orthometric heights are determined based on the value of geoid undulation(N),which is obtained by methods of gravimetrical observations or derived by methods of interpolation utilizing the local geoid models(LGM).The current paper highlights one of the methods of generating LGM that is based on the geometrical relationship between the global datum WGS84 and the Iraqi datum Karbala 1979.The DGPS(Differential GPS)method is used to get the geodetic coordinates of about 97 selected points to cover the experimental area,within the boundaries of Erbil municipality,which has been partitioned into four parts.Therefore,four LGM were generated individually for each one of the four parts with an estimated uncertainty equal to±0.076 m.

Geoid Deformation and Elevation Influence Caused by Building Loading

The paper studies the ground vertical deformation and the geoid undulation caused by loading of neighboring buildings, based on the loading tides theory. The influence on elevation is also considered. The results show that the ground vertical deformation and the geoid undulation both reach millimeter magnitude. Therefore, it is obvious that the building loading significantly affects the precise engineering surveying, and it must be seriously considered in application.