Chart Datum Establishment for Mandaitivu Island

Abstract

Mandaitivu is an island situated near Jaffna which has only the land surveying datum. The purpose of this study was to establish an accurate tidal datum for Mandaitivu Island to be used for hydrographic surveying. This research evaluated three methods of establishing tidal datum. First method was transferring datum by running a level line from a known station. As the second method, the datum was established by considering the tidal pattern in Mandaitivu Island against the nearby standard port at Kankesanturai harbour. The third method was by analysing long term observed tide data at the island itself. Mean Sea Level (MSL), Indian Spring Low Water (ISLW) and Lowest Astronomical Tide (LAT) datums were derived at the new tidal station at Mandaitivu. The MSL was used as the common datum for all three methods. Here, the levelling transfer method is only valid as there was no difference in tidal pattern between the two stations. However in the real world, the tidal pattern differs due to local characteristics. Therefore the results from the simultaneous tidal observation method and tidal analysis method were more appropriate. The differences between the computed datums were around +/- 5 cm for all the methods and are within the International Hydrographic Organization (IHO) standards. Finally, the LAT was established as the chart datum for Mandaitivu with the analysis results of 10 months tidal data and it is 0.47m bellow the MSL.