Tidal Datums and Benchmarks for Buzzards Bay

Overview

Whether you are interested in historical storm surges, flood plain mapping, relative sea level rise, or are an engineer designing structures along shore, it is important that you have a good understanding of tidal datums, geoids datums, local benchmarks, and how they relate. Over the years we have come to realize that many scientists and engineers that work on the coast do not have a full understanding of these elevations, their relationships, or how definitions have changed over time. Although differences in the various datums are less than a foot or two, such discrepancies in designing structures like culverts under roads can have profound implications on tidal flushing and flooding characteristics if these structures are not installed at their appropriate elevations.

We have set up this page to help planners, town engineers, government officials, and scientists quickly find the data and information they need to relate tidal elevations, local benchmarks, and vertical elevation datums for sites they are interested in. We have also have included sections below that explain the definition of coastlines and maps, and summarize historical storm surges recorded in Buzzards Bay

Relationship Between Tidal Datums, Vertical Datums, and Local Benchmarks

The relationship between tidal datums (average tide heights, the datum is MLLW=0), vertical datums (land elevation, datum=0, sometimes incorrectly referred to as mean sea level), and local benchmarks (brass elevation markers found on bridges, roads, and occasionally large rocks) varies up and down the coast of the US and elsewhere.

Below is a generalized diagram of this relationship for our area. Refer to this diagram as you read the sections below it.

Mean Sea Level

Prior to the 1980s, "Mean Sea Level" (MSL) was one of the original names for the NGVD29 datum, and thus MSL had an elevation of zero. In 1973, the federal government realized "Mean Sea Level" was a misnomer for the datum, and renamed this reference elevation NGVD29 instead. However, some engineers still incorrectly refer to the zero elevation on plans as "mean sea level". The actual MSL varies around Massachusetts and is typically 3 to 6 inches above zero for NGVD29 and 3 to 6 inches below zero for NAVD88.

Tidal Datums

In our area we have diurnal tides. These are slightly asymmetrical tides that occur twice daily. In a 24 hour period, we have two high tides and two low tides. One of these low tides is lower than the other, and one of the high tides is higher than the other. These two tides are described as the "lower low" tide and the "higher high" tide respectively. The heights of these diurnal tides change over the course of the month because of the relative alignment of the moon and sun which both exert gravity on the oceans to create the tides. When the moon and sun are aligned (new moons and full moons), the higher highs are higher and the lower lows are lower, and these are referred to as "spring tides." New moon spring tides are more extreme than full moon spring tides. These occur every two weeks). The midpoint between spring tides (also every two weeks), are called "neap tides", which are the periods of the smallest time changes.

Tidal datums are 19-year averages of all tides in an area. The zero value of the datum is the average of the Mean Lower Low Water (MLLW) heights over the 19 year period. The values of the Mid tide level, as well as Mean Low Water (MLW), Mean High Water (MHW), and Mean Higher High Water MHHW are the average of each of these terms for the 19 year period, relative to the MLLW average. Every 25 years or so, the federal government revises these tidal datums by examining new "epochs" of data to account for relative sea level rise.

Tidal elevations recorded at the Woods Hole tide station, including highest values recorded can be found at: NOAA Tides and Currents Woods Hole Benchmarks

In the review benchmarks and tidal datums, it is important to recognize that as "mean sea level" is not the same as "mean tide level" for any given site. Mean sea level is a term tied to the NGVD29 tidal datum, which was really the average sea level at 26 stations in North America. This elevation was originally referred to as the "Sea Level Datum of 1929." Sometime in the 1960s it was realized that mean sea level was not really the mean sea level and the term caused considerable confusion, so in 1973 the name mean sea level officially became the "National Geodetic Vertical Datum (0 foot elevation) of 1929" (NGVD 29)". If you see a engineering plan that labels "mean sea level", recognize that this is not the same as mid tide level for the current tidal epoch for that location. It is just the 0 foot elevation of NGVD29. Some engineers today still mistakenly refer to the NGVD29 datum as mean sea level, when MSL (based on tidal data) may be either higher or lower than NGVD or NAVD.

Anyone determining or calculating MHW elevations will find these web pages especially useful:

NOAA tides and Currents Page (enter a location, the resulting page includes links to tidal benchmarks)

NOAA online NGVD29 to NADD88 calculator (enter a long, lat, and a height to convert)

USGS NGS Datasheets

USGS NGS Tidal Benchmarks

NGS Geodetic Tools Fact Sheet

In Massachusetts, Chapter 91 waterways licenses require plans to use either a NGVD or NAVD datum (as opposed to a relative benchmark), and the Chapter 91 regulations (310 CMR 09) define the mean low water mark as "the present mean low tide line, as established by the present arithmetic mean of water heights observed at low tide over a specific 19-year Metonic Cycle (the National Tidal Datum Epoch), and shall be determined using hydrographic survey data of the National Ocean Survey of the U.S. Department of Commerce.

What is the Coastline on Maps?

On both USGS topographical "Quad" maps and NOAA nautical charts (as well as GIS data files derived from these), the coastline is generally the Mean High Water (MHW) mark. The coastline is not the 0 ft land elevation contour. The land elevation varies around the coast. In Buzzards Bay, MHW is roughly at the 2 ft land elevation (NAVD88 datum).

We say the coastline is "generally" the MHW mark because there are important exceptions. One of these exceptions is where salt marshes occur. Where there is a salt marsh, the "apparent coastline" is shown, not the MHW mark. The apparent coastline is the extent of the salt marsh vegetation. Because salt marsh can grow down to just below the mid-tide level, the coastline on maps is closer to the 0 foot land elevation (NAVD88 datum) in Buzzards Bay. The apparent and MHW coastline is illustrated in the portion of a nautical chart shown below.

Land elevations versus bathymetric depth

Land elevations on federal government maps produced after 1990 are generally given as feet above NAVD88 (North America Vertical Datum 1988), which is the most recently accepted "zero" elevation for North America. Earlier maps use NGVD29 (National Geodetic Vertical Datum-1929). However, most benchmark data, FEMA flood maps, and data used by local engineering firms is in NGVD29, so the elevations on most site plan submitted to municipal boards will be in NGVD29. However, newer FEMA maps are now being released in NAVD88 (go to our FEMA maps page) In the table below is a comparison of the two datums for three sites.

Location	Elev MLW NAVD88 meters	Elev MLW NAVD88 feet	Elev MHW NAVD88 meters	Elev MHW NAVD88 feet	NAVD88 minus NGVD29 meters	Elev MLW NGVD29 meters	Elev MLW NGVD29 feet	Elev MHW NGVD29 meters	MHW NGVD29 feet	Source*
Woods Hole, MA	-0.372	-1.217	0.173	0.566	-0.262	-0.110	-0.360	0.435	1.423	NOAA Tides and Currents
Oak Bluffs, MA	-0.413	-1.351	0.153	0.500	-0.266	-0.147	-0.480	0.419	1.370	CLE Engineering Inc study
Newport, RI	-0.580	-1.897	0.477	1.560	-0.270	-0.310	-1.014	0.747	2.443	NOAA Tides and Currents

Bathymetric depths for nautical charts and topographic quads in our area are generally given as feet below mean low water (MLW) on maps prior to 1990, and depth below mean lower low water (MLLW) after 1990, however, always check the tidal datum definition for the chart because it varies. In Buzzards Bay, the difference between the MLW mark and MLLW mark is generally around 4 inches in Buzzards Bay, but varies around the bay.

As might be apparent from the above definitions, there are areas in the intertidal which have negative land elevations and negative bathymetric elevations.

Keep in mind, when reviewing plans for sites around Buzzards Bay, subtract 10.2 +/- inches from NGVD29 elevations to get NAVD88 elevations. Use the NOAA online NGVD29 to NADD88 calculator to determine the exact conversion.

Legal Case on MLW definition

An interesting recent Lands Court case that helps define ownership of intertidal areas was upheld by an Appeals Court in 2010 (and affirmed in 2011 by the Massachusetts Supreme Court in its failure to take the case on an appeal). The case was a dispute in ownership between the town of Manchester-by-the-Sea and a private property owner, who removed some private moorings from tidal flats that he believed he owned. The case was decided based on a history of ownerships and land grants going back to colonial times, but the most important transferable outcome of the case was the fact that the court adopted a specific legal definition of the "Mean Low Water" for coastal property boundary cases. The judge concluded that the boundary line of MLW should be set to an elevation based on the North American Vertical Datum. The court wrote, "'mean low water' as established by the National Geodetic Vertical Datum (NGVD), is the appropriate standard for determining the low water mark". Read more about the case in this Massachusetts Bureau of Municipal Finance Law article on Spillane v. Adams, 76 Mass. App. 378 (2010) (pdf), and at the Massachusetts Law Library beach Rights page.

To those familiar with the application of elevation datums, this decision may at first seem inappropriate. The judges in the case also called it the "NGVD mean low water datum", but this is not the correct terminology. NGVD(29) is a datum (zero elevation), and it does not set the mean low water mark, which are based on tidal datums. Although the wording of some elements of the decision are imprecise from a technical perspective, the outcome of the case can be understood by a careful reading of the decision, and by reviewing references to the Land Court decision. Specifically, Mean Low Water (which is an elevation based on separate local tidal datums, a fact not explicitly mentioned in the decision), needs to be set to a particular land vertical elevation datum (in this case NGVD29) and applied to plans. This contrasts with demarcating a MLW boundary based on low tide observations on any particular day (as happened in this case). The map below shows the change in property bound set by the Land Court decision, and the table to the right shows the elevation of MLW based on the tidal datum near Manchester by the Sea as contained in a National Geodetic Survey database. As shown in that figure, MLW in the area of Manchester by the Sea is -4.31 ft NGVD29 and -5.12 ft NAVD88. Note that tidal datums (0 feet) is set to Mean Lower Low Water (MLLW), and in this case, MLW is 0.34 feet above the tidal datum.

For the purposes of local regulations, municipalities should require MLW be demarcated on site plans using National Geodetic Survey database tidal datums referenced against plan elevations (typically NGVD29 or NAVD88).

MLW boundary set in Spillane Case

Tidal datum versus other vertical elevation datums

Left: Diagram from Appeals Court decision showing demarcations of MLW considered. Right: National Geodetic Survey database tidal datums compared to NGVD for a station near Manchester by the Sea, Massachusetts.

Past Hurricanes Tidal Surges in Buzzards Bay

NOAA Summaries for the four worst storms affecting southern New England are in the links below.

The 1938 Hurricane

Hurricane Carol, 1954

Hurricane Edna, 1954

Hurricane Bob, 1991

Environmental Management Issues Sea Level Rise

Sea level has repeatedly risen and fallen over geologic time with the formation of large glaciers during ice ages, and the melting of this glaciers and polar ice during intervening warm periods. Since the last ice age 10,000 years ago, sea level has continued to rise, with the rate of relative sea level rise in recent centuries of roughly 10 inches per century. This rate of sea level rise is expected to increase with global warming caused by green house gas emissions, with rates projecting to possibly increase to 20 inches or more per century.

Storms and high seas erode the coast and flood low-lying areas. For environmental managers and regulators today, construction and development in storm flood zones, and areas of high erosion is an important concern, especially in terms of threats to human safety. Also important are the high public financial costs and economic impacts associated of replacing roads, loss of personal property, and public financing of flood insurance programs and emergency response, particularly in areas repeated damaged by storms. The links to the articles below discuss some of these public policy issues.

Massachusetts Coastal Zone Management Office Shoreline Change Project.

What would be the impacts of a Hurricane like Isabel on Buzzards Bay? We need only consider the impacts of Hurricane Bob in 1991 (NOAA summary of Hurricane Bob).

Massachusetts Coastal Zone Management Office Coastal Hazards Assessment (pdf file).

National Assessment of Coastal Vulnerability to Sea-Level Rise (pdf file).

National Academy of Sciences: Sea Level Rise and Coastal Disasters (pdf file).

Cape Cod Coastal Erosion: A Case Study..

URI fact sheet on coastal Erosion.

Beach Nourishment: A Guide for Local Government Officials.

For several thousand years, relative sea level rise in Buzzards bay has averaged about 10 inches a century. Actual documented sea level changes in Woods Hole since 1930 and data from other sites is available at the NOAA Sea Levels Online website. Actual Sea level rise in Woods Hole, MA

Buzzards Bay National Estuary Program

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