Chart Datum 3 : The tidal datum to which soundings on a navigation chart are referred. In order to provide a factor of safety, some level lower than mean sea level is generally selected, such as mean low water or mean lower low water
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}