Analysis and Comparison of Mean Sea Level Discrepancy in Manila and Legazpi Bays using Geodetic Leveling, Gravity Field and Oceanographic Approaches
Keywords:Geodetic leveling, Gravity field model, Mean Sea Level, Mean Sea Surface, Orthometric height, Unification of vertical datum
The determination of the mean sea level (MSL) is very important in geodetic measurements. However, the accelerating rise in sea level brought about by climate change introduces uncertainties in present observations. While the effects of accelerated sea level rise vary according to geographical location, local climatological and geophysical conditions (e.g. geodynamics or tectonic activities) may contribute significantly to its overall effect. This effect may result to elevations that are not homogeneously distributed throughout the country, meaning that benchmark height determined from two different reference MSLs will give different values.
Geodetic leveling is the classical approach in propagating heights landward but this is not applicable on lands separated by large bodies of water. This is a problem for an archipelagic country such as the Philippines. As an alternative to geodetic leveling approach, gravity field and oceanographic approaches were tested to determine the discrepancy between local MSLs. The results from these alternative approaches were compared with the result of the geodetic leveling approach that was used for validation. The MSLs used for Manila and Legazpi Bays were the averaging of 19 years tidal observations from 1990-2008 and 1989-2007 respectively.
Based on the results of this study, the MSL in Manila Bay is higher by 23.6 cm than the MSL in Legazpi Bay determined from geodetic leveling approach. The oceanographic approach showed a 19.5 cm difference between the two bays, which is 4.1 cm off with the with geodetic leveling approach value. Among the three gravity field models evaluated, EIGEN-6C4 gave the closest value to the geodetic leveling approach computed MSL difference. The gravity field approach computed difference between the two locations is 24.2 cm, which differs by only 0.6 cm than the geodetic leveling approach value. Therefore, it can be concluded that the gravity field approach can be used as an alternative method to for inter-island height unification. However, further investigation should be conducted since only two locations were considered.
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