Estimation of the elevation of Mean High Water and the High Tide Line


Notice of Change

In 2017, we revised our HTL calculation methodology using the approach described on our new Tidal Datums and the HTL for Massachusetts page. Our new interactive tidal datum viewer for Massachusetts has HTL values for Buzzards Bay that differ from this page by up to 0.4 ft in certain harbors where the HTL calculation was biased to a single tidal datum station. The new data viewer also uses Geoid12B data. This page will be updated in 2017 to reflect the new calculation approach and dataset.

Explanation and Caveats

This page is set up to allow government officials, scientists, and engineers to find the estimated elevations of mean high water (MHW), mean low water (MHW), and the high tide line (= "annual high tide" or "king tide") in the NAVD88 datum. However you should use our new interactive tidal datum viewer for updated calculations for the HTL elevation. On the maps on this page, although the elevations are classified into 32 color variations, differences in most datum elevations across most of Buzzards Bay are less than 1 ft. When you zoom in the map, you can click individual points to see the various tidal elevations for that site. LiDAR correction values should be subtracted from the tidal datums to determine the LiDAR set elevation.

This data set was developed for our study of projected salt marsh migration with sea level rise. The Buzzards Bay NEP estimated the high tide line because it defines the regulatory boundary of salt marshes. The high tide line is equivalent to the "annual high tide" or "King Tide" for a particular year. For this analysis, we are primarily using a tidal elevation model developed by NOAA called VDatum. Using this model, we can estimate the elevation of tidal benchmarks around Buzzards Bay in the NAVD88 datum. One limitation with VDatum is that the highest tidal datum elevation it calculates is Mean Higher High Water (MHHW). This is the 19-year average (tidal epoch) of the higher of the two high tides that occur each day. To estimate the High Tide Line around Buzzards Bay, we first looked at NOAA's predicted highest annual tide in 18 stations in and around Buzzards Bay. Using the GIS software program ArcMap by ESRI, we interpolated these values to determine how much higher the annual high tides were above the MHHW around Buzzards Bay. By combining the high tide line elevation above MHHW with MHHW elevations, we generated a map of the elevation of the high tide line around Buzzards Bay in the NAVD88 datum. This methodology is described in detail in the QAPP report (link above right under "more information". As noted above, we have changed our HTL elevation calculation methodology as described on our new Tidal Datums and the HTL for Massachusetts page.