Millions of dollars are invested to collect accurate elevation information across the
United States and along our coasts.
Cartographers, engineers, planners and other professionals then use this information to
help ensure that ships travel in and out of ports safely; roads, bridges, and levees are
constructed correctly; and evacuation routes stay dry during inundation events.
To correctly use maps, charts, and geospatial tools, it’s important that we understand
where the elevation and height information comes from, how it fits together, and how
it will be applied.
Let’s say we are conducting long-term planning for a coastal area.
We need to collect elevation information to ensure safe navigation and to support coastal
construction projects.
NOAA airborne topobathy lidar uses laser technology to provide accurate height information for
land surfaces, as well as shallow water depths, and objects—such as houses and vegetation.
NOAA hydrographic survey ships use sound waves in the form of sonar to measure water depths.
In order to align elevation data from various sources, we need to pick a common starting point.
The most commonly used reference surface (or datum) is an ellipsoid—an idealized representation
of Earth’s shape.
Because Earth is not smooth, the ellipsoid may lie above or below Earth’s surface at
any given location.
The ellipsoid is the basis for satellite navigation systems such as the U.S. Global Positioning
System (or GPS).
Lidar and sonar data tells us that our pier is 35 meters below the ellipsoid and the seafloor
is 50 meters below the ellipsoid.
With this information, we might think we know enough to keep our ships from running aground.
But... because water levels are always changing with tides and other meteorological and hydrodynamic
effects...we need to use a different reference frame that accounts for changes in water level.
Tidal datums are local reference planes defined from water-level observations.
For navigation applications, the generally accepted vertical datum of reference is “Mean
Lower Low Water”—the average of the lowest daily tides over the 19-year period referred
to as the National Tidal Datum Epoch.
So, we need to transform our elevation information from an ellipsoid reference to Mean Lower
Low Water to best support navigational needs.
Fortunately, NOAA’s Vertical Datum Transformation Tool—known as VDatum—allows us to realign
existing datasets from one vertical datum to another!
Using VDatum to transform our heights from an ellipsoidal to a tidal datum, we see that
our pier is 10 meters above Mean Lower Low Water... and the seafloor is only 5 meters
below Mean Lower Low Water.
This means our ship will have to have a draft less than 5 meters...or find another pier
at low tide!
Now, turning our attention to on-shore construction...
Before a new house is built, a surveyor needs to make sure the foundation is built above
the high-risk floodplain.
The surveyor uses professional GPS equipment to compute a highly accurate height of the
ground... relative to the ellipsoid reference frame.
Based on the survey, we know the base of our house is 25 meters below the ellipsoid!
But what does that tell us about the floodplain?
Unfortunately, ellipsoidal reference frames do not accurately represent topography or
gravity, which are both important in determining the direction water will flow.
As a result, for applications like floodplain mapping, we need our data referenced to an
orthometric height system, such as the North American Vertical Datum of 1988 (NAVD 88),
which accounts for differences in gravity.
Again, NOAA’s VDatum tool allows us to transform the ellipsoid height of the base of our house,
which was negative 25 meters, to an orthometric height of 11 meters in NAVD 88.
Our floodplain map, which is published in NAVD 88, has a base flood elevation of 8 meters.
So... we know this house is built above the high-risk flood zone!
VDatum can also transform our 11 meter NAVD 88 height to a height referenced to local
Mean Higher High Water (or MHHW).
MHHW is the average of the highest daily tides over the National Tidal Datum Epoch.
The Mean Higher High Water datum is helpful for inundation modeling and storm surge forecasting.
As these examples illustrate, it is important to understand which reference frame our elevation
and height information is based on—so we can appropriately use different data sources
together.
NOAA’s VDatum tool—which covers many parts of the United States—makes it easy to convert
and align datasets for mapping, charting, and geospatial applications...so we can be
sure critical and strategic decisions are based on accurate information!