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In this research, a series of field tidal observations inside the Keelung Harbor had been carried out to investigate the change of elevation induced by tidal waves, and also their influences to bathymetry surveying and to the calculation of the amount of dredging material had been discussed. Three tidal gauges were deployed along the main path of the harbor, and measured synchronously with the tidal station of Keelung Harbor. The results show that the changes of tidal elevation inside the harbor can be divided into two zones: the area adjacent to harbor entrance (outer harbor), and the area at inner harbor. The water elevation at outer harbor is affected by the propagation of tidal waves, and the change of water elevation at inner harbor is the results of water body balance. The tidal difference between outer harbor and inner harbor might have a large value with 16 cm, but varied with the tidal type outside the harbor. The mean water levels in these two areas are also different with around 8 cm difference. The influence of such variation of water elevation inside the harbor is dominant to the accuracy of bathymetry surveying, and should be studied in detail.
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