Determination of datum level

Abstract

After the earthquake, large subsidence of ground level was observed in the wide area on the Pacific coast in the Tohoku region. The Geospatial Information Authority of Japan reported the subsidence in some areas over 1m measured by GPS observation. A number of quays and tidal stations have been destroyed in the ports, and most of the benchmarks that were necessary to determine a datum level, have significantly subsided or been lost. Possibly earliest re−determination of the datum level at these ports was required because the datum level is used for not only depth sounding for nautical charts but also construction works in the ports.The JHOD, collaborating with port authorities and local municipalities, has been working to determine the datum level since mid−March, carrying out tidal observations at these affected ports. We have determined the datum level at the Ports of Onahama and Hachinohe on 18 May and at other 16 ports by early July.