Explore

For Developers

Understanding Tide Datums

A datum is the reference level that tide heights are measured from.

LAT Lowest Astronomical Tide Recommended

The lowest tide level that can be predicted under normal conditions. International standard (IHO). All tide heights are positive.

MLLW Mean Lower Low Water

Average of the lower daily low tides over 19 years. Used in the United States.

MSL Mean Sea Level

Average water level. Heights are both positive and negative.

Which should I choose?

LAT is recommended for most users. MLLW matches US NOAA tables. MSL shows oscillation around the mean.