عنوان مقاله [English]
A vertical reference surface was used as a reference to measure the heights of the points on the earth surface. A vertical datum can also be defined by computing the geopotential number of the origin point (tide gauge station) using its ellipsoidal height and absolute gravity value. In this research, after determining the mean geopotential for the sea level, three local vertical datums (LVDs) were described for three tide gauge stations namely, Bushehr, Hormozgan, and Chabahar, on the southern coast of IranSince the mean sea level is not constant, the equipotential surfaces which create the local datums are not coincided and show some deviation from each other to an undefined extent. These offsets are calculated by using a Global Navigation Satellite System (GNSS) with ellipsoidal height, normal-orthometric height and height anomaly of the datum. Therefore the data can be related to each other. One goal of the modern geodesy is the global unification of vertical data so that height data from them can be properly integrated. The unification of these LVDsmay be performed by using a regional gravimetric quasigeoid model and also the ellipsoidal height data on each datum. For this purpose, the iteration method was applied. Using the LVD offset of the related datum compared to quasigeoid, the gravity anomalies of each datum was reduced to a quasigeoid model. The quasigeoid was computed by combining two global geopotential models, namely EGM96 and EGM2008, with a set of the gravity data obtained from Bureau Gravimetrique International (BGI) including the total number of 8582 stations across Iran and the digital elevation model (DEM) with three arc second resolution. The reductions were applied to the gravity observation to produce the free-air and complete Bouguer anomalies.
Because many countries do not have gravity observations along all the preciseleveling routes, the computation oforthometric or normal heights isnot strictly possible. To overcome this limitation, thenormal-orthometric height system was developed. In this research, the normal-orthometric height system was used to reduce the measured gravity values from the earth surface to the quasigeoid.The normal-orthometric heights for the BGI stations as well as for the tide-gauge stations in Bushehr, Hormozgan and Chabahar were calculated based on a program prepared in Matlab. The solution converged after three iterations. Creating these data, and unification of them, the height of the stations located in the area of these three local vertical data can be calculated. A relation for determination of the dependency of these offsets to the heights of the data was also presented.
It should be noted that the mean offsets and the information relevant to each datum were calculated separately. The base of this calculation was the determination of reduced anomalies for all points of the data and the related preliminary values of the offsets for each datum.
Finally, it may be concluded that the quasigeoid models resulted from the iteration method perform a vital role in the vertical datum unification. Comparison of such quasigeoids with those obtained in previous researches, and also considering the standard deviation of 0.6m showed that the iteration method may be a suitable method to determine the quasigeoid in coastal areas. Furthermore, the combination of the described quasigeoids with the obtained offsets can be applied to transfer the reference ellipsoid as a datum to each one of the LVDs.