Chart Datum

A chart datum is the level of water that charted depths displayed on a nautical chart are measured from. A chart datum is generally a tidal datum; that is, a datum derived from some phase of the tide. Common chart datums are lowest astronomical tide and mean lower low water.

The Chart Datum always refers to the date at which the soundings were taken regardless of method. Thus all NOAA charts, say in the SE United States, will indicate "North American Datum 1983 (WGS 1984)". This is of great importance where tidal current or meteorological events like hurricanes, can and will change the depth contour. This is also of great importance where the Chart Datum was collected more than fifty years prior, as for example in the Bahamas (1950's) or in Cuba (1930's) and has not since been updated, verified, or sounded electronically. Chart datums achieved via dragging of chains adversely impacted the ecosystem and were rarely executed over sensitive bottoms, say coral reefs. Thus an older Datum will often be purposefully incorrect. They also relied on optical triangulation from fixed land points, and thus if taken over the horizon, fixes were taken from floating objects and are not necessarily correct. Lat/Longs determined via Satellite of land masses (ports of entry specifically) can be off by as much as 90ft in the Bahamas, Caribbean and Cuba. Datum is also the latin word for given, thus, "It is given that in 1984 the depth contour is such" does not indicate that in 2014 the depth contour is as indicated. Chart Datums boxes on paper charts will also usually indicate "updates" from the issuing organization, "corrections" to official charts by the publisher, and suppliers of electronic charts will also provide their own "corrections" that deviate from data 'given' from official charts. GPS receiving units also provide offsets to reconcile the Datum to satellite triangulation. Where the chart indicates "Entry by visual navigation only", it must be assumed that the bottom contour is not 'given'.