Measuring accurate heights across a country

March 29th, 2019, Published in Articles: PositionIT, Featured: PositionIT

A look at how the National Oceanic and Atmospheric Administration (NOAA) scientists are changing the way heights is measured accurately across the United States.

Scientists at NOAA’s National Geodetic Survey (NGS) maintain a nationwide coordinate system that defines latitude, longitude, height, scale, gravity, orientation, and the national shoreline throughout the United States. This reference system ensures that those who need accurate positioning information – whether they are in Alaska or Guam or Key West – are all working from the same reference points. The height component of this nationwide system, or vertical datum, is a collection of specific points on the earth with known heights either above or below mean sea level.

Fig. 1: Visualisation depicting a valley and several houses before and after a flooding event, illustrating how accurate heights are needed to understand how water flows across the land.

Fig. 1: Visualisation depicting a valley and several houses before and after a flooding event, illustrating how accurate heights are needed to understand how water flows across the land.

Measuring elevation is more complex than most people realise. It sounds easy enough: the elevation of a point on land is a measurement of how high it is above sea level, right? The problem is that sea level is not level at all: it is lumpy and irregular.

If the earth were a perfect sphere with no mountains or valleys, sea level would also be the same everywhere. But the earth, of course, is far from uniform. It bulges at the equator due to the long-term effects of the earth’s rotation. And, at a smaller scale, topography also affects the mass of the earth at different locations – a mountain has more mass than a valley, so the pull of gravity is stronger near mountains.

Fig. 2: In addition to a network of continuously operating reference stations supporting 3D positioning activities (shown here), the reference system includes a network of permanently marked points; a consistent, accurate, and up-to-date national shoreline; and a set of accurate models describing dynamic, geophysical processes that affect spatial measurements.

Fig. 2: In addition to a network of continuously operating reference stations supporting 3D positioning activities (shown here), the reference system includes a network of permanently marked points; a consistent, accurate, and up-to-date national shoreline; and a set of accurate models describing dynamic, geophysical processes that affect spatial measurements.

All of these large and small variations to the size, shape, and mass distribution of the earth cause slight variations in the strength of gravity in different places around the globe. These variations determine the shape of the planet’s water. This shape is what we call global sea level.

Think of it this way: If we could remove the tides and currents from the ocean, it would settle into a smoothly undulating shape (rising where gravity is high, sinking where gravity is low). This irregular shape is called “the geoid.” It is the surface which defines zero elevation. Using complex math and gravity readings on land, surveyors extend this imaginary line through the continents. And this model is used to measure surface elevations.

Improving the current system

How accurate heights based on mean sea level is measured today is good, but it is quite complicated, expensive, and time-consuming. Scientists at NGS are in the midst of a 15-year project to make accurate height measurement dramatically better. The result will be a new vertical datum, slated to be released in 2022, that will allow users to calculate heights to within about an inch (2,54 cm) for most locations around the country.

Fig. 3: NGS survey technician Justin Dahlberg collects gravity measurements aboard NOAA’s Gulfstream IV aircraft, shown in top image, over the Hawaiian Islands in February 2019.

Fig. 3: NGS survey technician Justin Dahlberg collects gravity measurements aboard NOAA’s Gulfstream IV aircraft, shown in top image, over the Hawaiian Islands in February 2019.

The new model will not only give more accurate height measurements, it will serve as the baseline to update height data over time as gravity values around the country shift with the changing landscape of the earth. The new model will also be tied into GPS, so it will be easier than ever to get accurate height information. Last but not least, users will be able to see more easily how heights are changing over time for a given area. That is a big deal when monitoring things like levee heights, hurricane evacuation routes, or the elevation of your home.

More accurate measurements are expected to pay big dividends. A recent study estimated that this project, the Gravity for the Redefinition of the American Vertical Datum (GRAV-D for short), will provide about $4,8-billion in social and economic benefits to the nation over 15 years through improved floodplain mapping, coastal resource management, construction, agriculture, and emergency evacuation planning. The study estimates a savings of $2,2-billion in improved floodplain management alone, because better height measurements will result in less property damage from flooding events.

Surveying the entire gravity field of the United States, including its territories, to update the country’s vertical datum is one of the most ambitious projects ever undertaken in the more than 150 year history of the National Geodetic Survey. While it is not an easy task, the result will transform how we measure accurate heights for decades to come.

Acknowledgment

This article is republished with the kind permission of NOAA’s National Ocean Service.

Contact Dr. Yan-Ming Wang, NGS, yan.wang@noaa.gov

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