The Difference Between Predicted and Observed Tide Heights

About Tide Predictions

The data in our tide tables and tide graph is provided by NOAA–and others–and covers most of the coastal United States. These predictions are derived mathematically from numerous inputs, including historical or nearby stations where real-time observations have been or are currently being recorded. An example of this is Portland, Maine where a physical sensor is located that monitors water levels in real-time, while NOAA is also providing calculated projections (e.g., “predictions”) about what the future heights of the tide will be. Tide predictions only account for the astronomical tide and do not account for weather. For example, during stormy conditions, we expect that measured water level will be higher than predicted water level. The National Weather Service provides total water level forecasts that include the impacts of tides and weather.

NOAA tide predictions are also calculated based on tidal datums (mean sea level, mean lower low water, etc.) over a standard 19-year period called the National Tidal Datum Epoch. The current epoch is 1983-2001. If tidal datums have changed at your location since this time period due to factors such as sea level change, dredging, opening or closing of inlets, or vertical land motion (subsidence or uplift), tide predictions will consistently be biased high or low. NOAA is updating the National Tidal Datum Epoch to 2002-2020, and new tidal datums and tide predictions will be released in 2026.

About Real-Time Tidal Monitoring

Real-time monitoring data, sometimes referred to as ‘observational’ data provides an historical record but cannot provide information about what the future has in store. However, this type of data is very important in helping NOAA improve their tide predictions, measure extreme high and low water levels, and track long-term changes in water level. Additionally, the National Weather Service (NWS) and other weather and climate-focused agencies, use this data to improve their total water level forecasts and understand the possible impacts high-tide flooding and extreme weather may have on coastal communities.

Our tidal predictions come from working closely with NOAA, relying on their great and ever-improving work. 

Our real-time observational data—where available—currently comes either from NOAA stations or from those of our partner, Hohonu. We will integrate other stations as time and technology allow and continue to encourage communities to get this technology in their harbors. However it is important to be aware that you may encounter gaps in the observational data due to various factors, such as sensor failures, environmental conditions like ice formation, or communication issues. We advise refreshing your browser as a first option, and after that unfortunately all we can do is wait for the sensor to come back online.

There are currently 210 long-term monitoring stations in our National Water Level Observation Network (NWLON), which are part of a wider network of sensors in the Integrated Ocean Observing System (IOOS). The IOOS program supports many different types of sensors, not just those that measure water levels. NOAA’s CO-OPS group also installs ‘temporary’ stations to support of a variety of programs including hydrographic and shoreline mapping, marine boundaries, navigation systems, coastal habitat and marsh restoration projects, and more.

Unfortunately, advanced water-level sensors, like the type NOAA uses for their real-time monitoring, are extremely expensive and require trained staff to maintain them. US Harbors is been proud to have initiated some of the earliest pilot testing of affordable alternatives that can be deployed easily and don’t require significant maintenance. We work with partners that can provide not only quality sensors but excellent data cleansing and the other things needed for accurate and reliable tide stations. Our team, along with others around the United States, have been working with local communities and state agencies to get these observational sensors in place so we can augment the current gaps between NOAA’s 210 sensors. Our team at US Harbors is committed to strengthening our national water level monitoring network and helping our coastal communities have more accurate water-level information to use for planning and safety.

Need more information on getting a sensor for your harbor or coastal business?