What does the zero (0) on a tide chart represent?
Tide heights are given relative to the “datum” which in most cases is one of several benchmarks corresponding to low tides of varying extremeness. The preferred benchmark in the U.S. is Mean Lower Low Water (MLLW). The odds of the predicted tide getting below MLLW on any given day are about half. The preferred benchmark in the Netherlands is Mean Low Water Springs (MLWS). MLWS is lower than MLLW. The predicted tide will get below MLWS on average only about twice a month. The preferred benchmark in Germany is Lowest Astronomical Tide (LAT). LAT is the lowest tide predicted over a 19 year period. The predicted tide will not get below LAT in that 19 year period, and is unlikely to get below it by any significant amount ever. In harmonics-dwf, some U.S. locations for which a MLLW benchmark was unavailable use an estimated value of MLLW that is derived from the predictions. These estimates tend to yield predictions that differ from National Ocean Service published tables by (0.1–0.2) ft. Ol
