Half-Tide Level 5 : The level midway between high water and low water for any series of tidal observations. It is generally different from mean sea level because of the existence of semi-annual, and longer-period, partial tides.
Intergovernmental Oceanographic Commission IOC technical series 37, Tsunami Glossary (A glossary of terms and acronyms used in the tsunami literature - English), , United Nations Educational, Scientific and Cultural Organization UNESCO, Paris, France 1991. http://unesdoc.unesco.org/images/0008/000885/08853 8eb.pdf, (ND/ Tsunami Sci - Brit) {590}