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National Tidal Datum Epoch

What is the National Tidal Datum Epoch?

• A set of reference points for water levels at different stages of the tide, such as mean high water and mean sea level.
• NOAA collects data on a 19-year cycle (called an epoch) for locations on U.S. coasts and averages the data to create these reference points.
• How high the water goes at high tide is important information for flood maps and planning projects on shore, like where to put a road or housing development.
• How low the water goes at low tide is critical to ensure ships don’t run aground.
• Scientists also use the data to track sea level rise, to track long-term oceanographic trends, and to predict the effects of severe weather.

What are we doing?

• Every 15-20 years (an epoch), we take the data from 200+ National Water Level Observation Network stations on the U.S. coasts and calculate new averages.

Why does it need to be updated?

• Water levels don’t stay the same. Sea level rise continues to accelerate.
• The earth’s crust is also rising and sinking in different areas at different rates. The land is rising in parts of the West Coast and Alaska. It’s sinking on the East and Gulf coasts.
• We have to update the data regularly to keep it accurate.

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2

International Great Lakes Datum

What is the International Great Lakes Datum? How is it different from National Tidal Datum Epoch (NTDE)?

• Like the NTDE, this datum is a set of reference points for water level heights. Unlike the NTDE, each location only has one water level measurement — mean water level. The Great Lakes are considered non-tidal, so there are no high or low tide levels.
• This data comes from the Great Lakes and connected waterways, like the St. Lawrence River, that span locations in the United States and Canada.
• This datum is used for nautical charts, flood maps, and shoreline planning, just like the NTDE. It’s also very important for dredging and other projects that maintain the important shipping lanes in the Great Lakes.

What are we doing?

• Using permanent and seasonal water level stations, NOAA oceanographers and Canadian counterparts are collecting water level data and will calculate new averages.

Why does it need to be updated?

• The land in this area is also shifting. During the last ice age, the area was covered by glaciers, and the weight of the glaciers caused the land to sink. Though the glaciers are long gone, the earth’s crust is still bouncing back from this extra weight, particularly in the northern part of the Great Lakes region. The shifting land changes the water levels.
• We have to update the data every 25-30 years to keep it accurate.

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3

Gravity for the Redefinition of the Vertical Datum (GRAV-D)

What is the geodetic vertical datum?

• The datum is a set of height measurements for the U.S. and its territories, generally referred to as the height above sea level.
• While the national tidal and international Great Lakes datums measure the height of water, this datum measures the height of the land.
• The Gravity for the Redefinition of the American Vertical Datum (GRAV-D) project is using satellite data and gravity measurements to calculate more precise vertical heights. The resulting updated datum will replace an older datum, NAVD88, which was made with old-fashioned leveling and surveying techniques.

What are we doing?

• We are flying across the U.S. and territories and collecting gravity measurements from the air.
• We have to measure gravity because it’s not uniform. Local topography and rock density create subtle changes in local gravity.
• Using gravity allows us to determine where sea level would be, even in areas far from the coast. This can tell us where water will flow.
• The gravity measurements will be combined with satellite data and data collected on the ground to create accurate height measurements across the U.S. and territories.

Why does it need to be updated?

• The old datum, NAVD88, is no longer as accurate as it should be. The old leveling techniques used to create the datum can cause inaccuracies and make it difficult to update or re-check data as time passes.
• NAVD88 is particularly inadequate in places where ground surveying is difficult or impossible — like parts of Alaska.
• With GRAV-D, we will have more accurate height data for more places in the United States and territories.
• Using gravity data instead of leveling and surveying gives us information on where water will flow. This is critical for floodplain maps, planning building projects and evacuation routes, and can even help cut down on pollution from fertilizer and pesticide runoff.
• Using gravity measurements and satellite data also makes it easier to update the datum as land shifts, and to track those changes over time.