Frequently Asked Questions


Who is revising the NTDE?

The Center for Operational Oceanographic Products and Services (CO-OPS), an office of the National Oceanic and Atmospheric Administration, is leading the effort to update the National Tidal Datum Epoch.


What is the NTDE?

To learn more about the National Tidal Datum Epoch (NTDE), we invite you to visit the CO-OPS NTDE landing page.

What triggers an update to the NTDE?

The average national difference of 0.10 feet (0.03 meters) is generally used as the threshold difference to warrant an update to the NTDE. As sea levels change, over time the existing NTDE becomes less accurate in representing those levels at many coastal stations. Updating the NTDE will ensure tide predictions are more accurate as they will more closely account for sea level changes since the last NTDE period. With the present average rate of sea level rise, the 20-25 year time period has been adequate to capture the 0.10 feet / 0.03 meters changes. However, a more frequent review period may be needed if there is an acceleration in sea level rise. To determine if this is necessary NOAA will look at the changes in the relative sea level trends over time using analyses such as those in the periodic sea level trends reports (Sea level Variations for the United States 1855-1980 (Jan. 1983); Sea Level Variations for the United States 1855-1986 (February 1988); Sea Level Variations of the United States 1854-1999 (July 2001); Sea Level Variations for the United States 1854-2006 (Dec. 2009)).

What will change in the update?

All currently published tidal datums and bench mark sheets will be updated, and all water level data products that reference published tidal datums will have updated references.


When will the finalized update be released?

The NTDE update will be considered complete once all currently published datums and BM sheets (~2,000 stations) have been re-processed. CO-OPS is currently scheduled to complete this work after 2026. We will communicate progress on a periodic basis and any changes to the schedule as necessary.

When will data be collected from stations?

Data for the NTDE update has already been collected for most stations, except for the 130 19-year control stations. These stations will have collected 19 years of 2002-2020 data at the end of calendar year 2020.


Why is this update necessary?

The 2002-2020 NTDE has been adopted to ensure that all tidal datums throughout the United States are based on one specific common reference period. This action is necessary to provide the latest up-to-date information for applications essential to supporting Federal, State and private sector coastal zone activities. These activities include: hydrographic surveys and coastal mapping, navigational safety, wetland restoration, marine boundary determinations, coastal engineering, storm warnings and hazard mitigation, emergency management, and hydrodynamic modeling.

Why does the NTDE follow a 19-year time period?

The 19-year period encompasses an 18.6 year astronomical cycle that accounts for the most significant long-term variations in the moon’s orbit. This variation, called “regression of the moon’s nodes,” is a slowly changing orientation of the plane of the moon’s orbit, and results in a slight change in the declination of the moon (Parker, 2007). This causes changes in the range of tide with 18.6 year repeat cycles.


How will the update be accomplished?

Once the 130 control stations have collected their 19-years of data at the end of 2020, CO-OPS field crews will conduct survey leveling at these stations. This will ensure data quality through the end of the NTDE time period. CO-OPS data processors will then perform quality control reviews for each control station. Once reviews are complete, datums team members will re-compute new datums and bench mark (BM) sheets for all 130 control stations and continue to re-process datums as well as BM Sheets for the remaining 1,870 stations. After all ~2,000 datums and BM sheets have been re-computed, they will be made available to the public via the CO-OPS Tides & Currents website and GIS and web services data access tools. The datums and BM sheets will be released simultaneously after 2026.

How can the NTDE support habitat conservation and restoration?

Coastal engineers rely upon the latest accurate tidal datums to successfully restore marshlands. Because marsh grasses are sensitive to how long and how much they are flooded, high water frequency and duration of inundation analyses provide critical information to support successful restoration.


Who can I contact with additional questions?

For media and communications-related NTDE queries, please contact Alexis Sluder at For technical and programmatic questions, please contact Kelly Kriner at or Michael Michalski at


How will the NTDE update impact professional mariners and recreational boaters?

Water-level data will be updated with the new tidal datum references at the time of the NTDE roll out. Please visit our Impacts to You section for more information on this question.

How will the NTDE update impact professional land surveyors?

Professional land surveyors (PLS) will see the following changes. When accessing tidal datum information they will need to ensure that they are using the currently accepted NTDE for all projects. Historical projects may need to be updated to reflect the new datum. The NOAA VDatum tool will be updated in time to include the new tidal as well as geodetic datums to enable PLS to obtain tidal geodetic relationships for the coastal zone of the Contiguous US, Puerto Rico, US Virgin Islands, and SouthEast Alaska.

How will the NTDE update impact shoreline use planning?

Due to the small vertical changes (0.10 meters / 0.3 feet) standard, oceanographic products are not significantly modified as a result of tidal datum updates. Tidal prediction products put out by CO-OPS will change as a result of the NTDE update. It may be the case that downstream products relying on tidal predictions may not need to be changed as the resultant change in predictions will likely be minimal (roughly 2.5 inches / 6-7 cm) for most locations. Additionally, tidal datums and bench mark elevation relationships will be noticeable when re-establishing tide stations using high accuracy survey techniques.