Tidal Datums and the HTL for Massachusetts

by Joe Costa.  First posted June 12, 2017; revised June 28, 2017.

Subpages:  Tidal Datum Viewer  |  Salt Marsh Lower Boundary Elevation

On our subdomain climate.buzzardsbay.org, in 2013 we posted a number of webpages that discuss and explain geodetic and tidal datums, and their use in environmental regulations, the high tide line (defined by “king tides“), the location of salt marshes within those tidal zones, site specific tidal elevations in Buzzards Bay, and information about how to use online tools like VDatum to help review site plans and permit applications.  On this page, we consolidate that information in a simpler form, and expand the maps to show mean high water, mean low water, the estimated high tide line, and other tidal elevations of interest for the entire coast of Massachusetts.  The elevations reported on the tidal datum maps at the bottom of this page are in feet NAVD 88 (the datum or reference elevation). This NAVD 88 datum is the reference elevation used in FEMA flood insurance rate maps, and by most federal and state agencies today.  It is about 10 inches higher than the NGVD 29 datum used

You can view tidal datums for any coastal location in Massachusetts on our interactive tidal datum viewer page.

The maps on this page, and data in the interactive viewer, were prepared in ArcGIS using a coordinate and elevation data matrix developed with NOAA’s VDatum software tool using the GEOID12B.

Salt Marsh Vegetation Elevations

Salt marshes are generally divided into two habitat types that are defined by tidal inundation and species composition: the low marsh and the high marsh. The low marsh is dominated by smooth cordgrass, Spartina alterniflora. In scientific and technical publications, the low marsh is often defined as beginning at the local mean sea level (LMSL) or sometimes at the tidal elevation “mean high water at neap tide” (MHWN). The actual elevation that the low marsh begins varies depending on numerous physical and biological factors within an estuary, but seems to generally range between LMSL and MHWN. On the salt marsh lower boundary elevation page, we show the LiDAR elevations of low marsh boundary at selected sites in Massachusetts.

The low marsh continues to about the mean high water (MHW) line, which generally defines its upper boundary. The high marsh is dominated by salt meadow cordgrass Spartina patens, this species typically begins to be the dominant plant species at the mean high water (MHW) elevation.  The high marsh continues grading into other species, to the high tide line (HTL, now popularly referred to as the King Tide), which is the regulatory boundary definition of salt marshes in Massachusetts.  The upper marsh boundary consists of a transition zone grading into upland or fresh wet species, and the actual boundary may be defined by vegetation rather than elevation, as established in Massachusetts case law.

 

Important Regulatory Tidal Datums

In Massachusetts, tidal wetlands permits issued pursuant to Chapter 91 of  Massachusetts General Law, are required for fill and construction for projects between the mean high water (MHW) and mean low water (MLW) lines.  The legal definition of these boundaries, legal cases about the zone, and the rights of the public to use this zone for fishing, fowling, and navigation are  discussed at this Massachusetts Law Library beach rights info page. The “high tide line” (defined as the highest tide of the year) establishes the legal boundary for salt marshes in the state’s Wetland Protection Act, and is the boundary for federal Navigable Waters where there are no bordering vegetated wetlands.  While the high tide line (together with biological indicators) establishes the regulatory boundary of salt marshes. The two figures below summarize both regulatory boundaries and salt marsh habitat types relative to tidal and geodetic datums.

 

The relationship between tidal datums, land elevation datums, and salt marshes in Buzzards Bay.

The relationship between tidal datums, land elevation datums, and salt marshes in Buzzards Bay. (click to enlarge)

 

Tidal datums, salt marsh boundaries, and coastal wetlands and their regulation under state and federal laws.

Tidal datums, salt marsh boundaries, and coastal wetlands and their regulation under state and federal laws. (click to enlarge)

 

Calculating the High Tide Line for Massachusetts

VDatum does not calculate the high tide line (HTL), which is the highest predicted tide of the year. As per the figure below, the HTL of 2016 was roughly equal to the highest astronomical tide (HAT, the highest predicted tide during the last tidal epoch, 1983-2001) for nearly all stations in Massachusetts examined. This happened to be the case because the moon had one of its closest orbital approaches to the earth in many decades, although in practical terms, the highest tide of the year varies only a few inches among years in Buzzards Bay and the south side of Cape Cod. This finding also mean HAT elevations reported by NOAA are also an excellent estimate of the HTL.

The predicted highest tide of 2016 (the high tide line) versus the highest astronomical tide for the last tidal epoch.

The predicted highest tide of 2016 (the high tide line) versus the highest astronomical tide for the last tidal epoch.

However, HAT values are reported for only a few tidal stations in Massachusetts. Is there a way to estimate the HTL for any portion of the coast or within embayments?  In reviewing the tidal records for the 2016 HTL for all stations in Massachusetts, the HTL elevation for 2016 correlated very well with both the MHW line and MHHW line elevations (relative to MLLW) for all stations. This finding means that the observed regression curve can be applied to the outputs of the VDatum software package to produce the HTL line on this page, and the HTL elevation data reported on the interactive tidal datum map page.

The predicted highest tide of 2016 (the high tide line) versus the mean high water elevation relative to the tidal datum.

The predicted highest tide of 2016 (the high tide line) versus the mean high water elevation relative to the tidal datum.

A Note about Nautical Charts

The shoreline on nautical charts is mean high water (MHW), except for areas of salt marsh, where the apparent shoreline is shown, which is  generally about mean high water neap (MHWN), which is roughly 1/3 of the mean tidal range added to LMSL. Around Buzzards Bay MHW is mostly near 2.5 ft NGVD 29 or 1.6 ft NAVD 88.  You can of course use the tidal datum viewer to find the exact elevations for your site.

On nautical charts the bathymetry, however is relative to the tidal datum. Before 1990, the tidal datum on nautical charts (and used for other NOAA calculations) was set to MLW; after 1990, it was set to MLLW. This is only a 4-inch difference in most of Massachusetts.

This difference in datums between land elevation and bathymetry means a bathymetric depth of -1 foot is at a land elevation of about -3 feet in NAVD 88 in Buzzards Bay, and this elevation difference is greater north of Cape Cod. Land elevations on maps produced by the federal government before 1991 are NGVD 29 (which is a foot below NAVD 88, so land elevations are a foot higher), and after 1991 are typically NAVD 88.

Tidal datums associated with boundaries, elevations, and bathymetric depths as shown on nautical charts.

Tidal datums associated with shoreline boundaries, land elevations, and bathymetric depths as shown on nautical charts.

Useful Links

2014 GPS on Bench Marks Campaign: https://www.ngs.noaa.gov/heightmod/2014GPSonBM.shtml (Reports geoid accuracy for Massachusetts)

OPUS: Online Positioning User Service: https://www.ngs.noaa.gov/OPUS/view.jsp (crowd sourced by engineers; GPS elevation data for benchmarks in each state)

NOAA Tide Predictions Page: https://tidesandcurrents.noaa.gov/tide_predictions.html

Look up tidal datums at benchmarks: https://www.ngs.noaa.gov/Tidal_Elevation/

Online VERTCON Calculator (NGVD 29 to NAVD 88): https://www.ngs.noaa.gov/cgi-bin/VERTCON/vert_con.prl (just for geodetic elevation conversion; enter decimal Lat and Long. Longitude should be entered without a sign, and with a leading zero for values under 100 degrees west. For example, 70.1234 degrees west should be entered as 070.1234)

VDatum online tool:  https://vdatum.noaa.gov/vdatumweb/ (70.1234 degrees west is entered as -70.1234)

State-wide maps

Click on one of these maps to view a larger version, or use our tidal datum interactive viewer for elevations near a specific site of interest.

Mean tidal range (MN = MHW -MLW) elevations around Massachusetts. Elevations are in feet.

Mean tidal range (MN = MHW -MLW) elevations around Massachusetts. Elevations are in feet above the “NAVD 88” reference datum.

Mean high water (MHW) elevations around Massachusetts.

Mean high water (MHW) elevations around Massachusetts. Elevations are in feet above the “NAVD 88” reference datum.

Estimated high Tide Line (HTL) elevations around Massachusetts. The HTL is the highest tide of the year, and is sometimes called the "King Tide."

Estimated high tide line (HTL) elevations around Massachusetts. The HTL is the highest tide of the year, and is sometimes called the “King Tide.” The HTL elevations shown are very close to the highest astronomical tide (HAT) reported for the current tidal epoch (NTDE 1983-2001). Elevations are in feet above the “NAVD 88” reference datum.

Local mean sea level (LMSL) elevations around Massachusetts. Elevations are in feet above or below the "NAVD 88" reference datum.

Local mean sea level (LMSL) elevations around Massachusetts. Elevations are in feet above or below the “NAVD 88” reference datum. LMSL is generally the lowest elevation that a marsh will be observed.  The actual lower boundary of a marsh may exist up to 1 to 3 feet above LMSL depending upon the tidal range as described on our low marsh boundary page.

Mean low water (MLW) elevations around Massachusetts. Elevations are in feet above or below the "NAVD 88" reference datum.

Mean low water (MLW) elevations around Massachusetts. Elevations are in feet below the “NAVD 88” reference datum.

Estimate elevation range (in feet) of salt marshes around Massachusetts.

Predicted maximum potential elevation range (in feet) of salt marshes around Massachusetts. The range was calculated by the equation Marsh Range = HTL – LMSL as defined in the other maps. See our low marsh boundary page for more information about variability in the range of the low marsh boundary.