Mean Lower Low Water (MLLW)

Mean Lower Low Water (MLLW) is the average of the lower of the two daily low waters over the 19-year National Tidal Datum Epoch. It is the chart sounding datum for US waters, the reference for charted depths, predicted tide heights, and bench-mark publication adopted by NOAA. Because it takes only the lower low of each tidal day, MLLW sits below Mean Low Water wherever diurnal inequality exists. Paired with Mean Higher High Water it defines the great diurnal range Gt. The 19-year epoch removes the 18.61-year nodal cycle from the datum.