ESTIMATION OF LONG-TERM SEA LEVEL VARIATION TAKING INTO CONSIDERATION GROUND DEFORMATION BY GNSS SURVEY

For estimating long-term sea level variation, periodical corrective adjustments by measuring the degree of ground deformation are required which are done by surveyors' levelling from the 1st class benchmark to the levelling spot at the tidal station. However, due to the issues of increasing costs of such manual levelling works and shortage of young surveyors, finding a simpler method of measuring ground deformation has become an urgent issue. In this study, we estimated sea level variation using a GNSS (i.e. global navigation satellite system) survey to grasp the level of ground deformation. At the Port of Mikawa, annual sea level variation was estimated to be -3.20 (mm/year) based on the tide observation record, while the same decreasing tendency was observed at the Ports of Toba, Nagoya and Onizaki which are all located along Ise bay. However, due to ground deformation, the GNSS survey shows that the height of the levelling spot at the tidal station at the Port of Mikawa increased by 68.4 mm in 5 years (13.7 mm/year) while the ground elevation estimation using the semi-dynamic correction of the geodetic framework shows that the area of Port of Mikawa rose by 120 mm in 19 years (6.32 mm/year). Based on the magnitude of the ground deformation, the sea level at the Port of Mikawa is estimated to be rising at a rate of 3.12 mm per year. Furthermore, the authors proposed a method to control the height of the levelling spot at the tidal station when recovering from the disasters.