New NOAA Tide Datums: What the Epoch Update Means for Dock Depth and Clearance

Every tide prediction, charted depth, and dock clearance number is measured from a reference plane called a tidal datum. NOAA computes these datums from 19 years of water level records, a window it calls a datum epoch, so the averages smooth out the moon's 18.6-year cycle and ordinary weather swings.

The epoch now in force covers 1983 through 2001. Every tide table, marine chart, and depth app you use today is referenced to the water levels recorded in those years.

NOAA updates the epoch when mean sea level at the nation's coastal gauges has changed enough since the last update, historically by an average of more than about 0.10 feet, which has happened roughly every 20 to 25 years. Long-term sea level rise means the averages locked into the current epoch no longer match today's water.

The next epoch will use water level observations from 2002 through 2020. NOAA is coordinating the rollout with the National Geodetic Survey and the Federal Geodetic Control Subcommittee, and as of this writing it has not published a single effective date. The change is coming, but it is not yet in force.

Along southeast Florida, average water levels in 2002-2020 are higher than they were in 1983-2001. When the new epoch is adopted, the low-water datums used at stations such as Lake Worth and Port Everglades will be computed from those higher years.

Charted depths are measured down from the low-water datum, so the same physical spot can read shallower once the new datum applies, and tide predictions will be referenced to the new plane. The shift at any one station is small, but it is a real change in the number your depth sounder, tide app, and dock plans are compared against.

Keep your own measured records. A depth reading taken at mean low tide, logged with the date and the gauge it was measured against, stays your property record even when published references change.

If your dock or boat lift was designed around an older low-water reference, review the clearance once the new datums are in force. And before quoting a depth to a buyer, an appraiser, or a contractor, say which datum and which epoch the number comes from.

Seawall elevations are usually written in NAVD88, a fixed geodetic datum that does not shift when tidal datums change. The new epoch does not change the elevation a seawall must be built to; it changes the tidal reference that water levels are measured against below that.

The practical point: tidal numbers will be re-based, and owner records that also capture the physical measurement will stay valid either way.

A qualified crew can measure your water depth at mean low tide in a free site consultation and tell you how your slip, canal, and seawall stand today, including whether sediment has built up enough to plan maintenance dredging.

To request free resources about dredging, call (561) 299-1323 or email slselleck@gmail.com. Tell us about your dock, slip, or waterway and we will help you understand what it needs.