Highest Astronomical Tide Line - Frequently Asked Questions

Q1. What is the Highest Astronomical Tide?     Back

According to NOAA, the Highest Astronomical Tide is the elevation of the highest predicted astronomical tide expected to occur at a specific tide station over the National Tidal Datum Epoch, or NTDE. The NTDE is a specific 19-year period adopted by the National Ocean Service as the official time segment over which tide observations are taken and reduced to obtain mean values (e.g., mean lower low water, etc.) for tidal datums. It is necessary for standardization because of periodic and apparent secular trends in sea level. The present NTDE is 1983 through 2001 and is actively considered for revision every 20-25 years.

Q2. How is the Highest Astronomical Tide calculated?     Back

Highest Astronomical Tide elevations along the Maine coastline were derived from two different sources: at select tidal stations, the elevation of the highest astronomical tide is a value from the NOAA COOPs Datums page. At other tidal prediction station locations, the Highest Astronomical Tide is estimated by using published tidal height differences and offsetting the Highest Astronomical Tide from reference tidal stations. These Highest Astronomical Tide water elevations were then adjusted (using VDATUM) to land elevations all along the coast, giving a continuous coast-wide representation of the landward limits of the Highest Astronomical Tide.

The Highest Astronomical Tide is also an important proxy for a regulatory boundary in Maine. The extent of the highest tides is included in Maine’s definition of the upper boundary of coastal wetlands through Maine’s Mandatory Shoreland Zoning Act

Q3. What data is visualized in this viewer?     Back

The Highest Astronomical Tide is visualized by a line representing the inland extent of the highest astronomical tide along the Maine coast. The line can be queried (by clicking on a section of the line you might be interested in) for the predicted Highest Astronomical Tide along that section of coastline, along with the nearest tidal prediction station used to adjust Highest Astronomical Tide values. Also shown in the viewer are the locations of tidal prediction stations and their predicted Highest Astronomical Tide values.

Q4. Does the Highest Astronomical Tide viewer account for tidal restrictions?     Back

Tidal range and amplitude is impacted by tidal restrictions, such as bridges, gates, and culverts, which restrict full tidal exchange with the open ocean, bay, or estuary. NOAA’s tide prediction locations do account for changes in tidal range by providing high and low offsets from “reference” tidal stations that have verified water level data. For example, the tide prediction at Phippsburg on the Kennebec River has published offsets of *0.88 feet for the highs and lows. This means that the tidal range at Phippsburg is 0.88 times the predicted high and low water values from Phippsburg’s reference station, which is Portland, ME. So if the tide at Portland is predicted to be a high of 12 feet MLLW at Portland, the predicted tide at Phippsburg is 12 feet x 0.88 = 10.56 feet. In this way, the HAT viewer does, to an extent, account for changes in tidal range. However, the viewer does not account for localized changes in tidal range or amplitude in areas where no offsets have been calculated - the viewer simply interpolates predicted water levels into these areas. Thus, the HAT viewer should be used for general site planning only. In areas with suspected tidal restrictions, site-specific water level observations, in addition to site-specific observations of coastal wetland vegetation, should be used to accurately delineate the highest astronomical tide boundaries.

Last updated on December 7, 2020