Taking America to New Highs and Lows

The National Geodetic Survey is giving the United States a gravity check.

By Craig Collins

In a couple of years, when Seattle gets shorter, Mount Rainier, the tallest of the Cascade peaks, will shrink also, from 14,411 feet to about 14,407.

As unlikely as this may sound, it will actually be the end of a long and complicated process called “height modernization.” It all started in 1807, when President Thomas Jefferson directed the first Superintendent of the Survey of the Coast, Ferdinand Hassler, to chart U.S. shorelines. Hassler, realizing these charts would have to be anchored to reference points on land, laid the foundation for American geodesy: a combination of disciplines that together measure and represent the size and shape of the Earth.

The geodesic tools of Hassler’s Coast Survey – the theodolite, a kind of telescope for measuring angles, and bars and tapes for measuring distances – were used to conduct line-of-sight Earth-based surveys that relied on a geometric method known as “triangulation”: using the angles between two lines of sight to establish the distances between two points. These distances were then used to assign longitudinal and latitudinal coordinates to geographic locations, anchored at benchmarks – which are still functional and visible today as cast-metal disks set in stone or concrete, or at the ends of rods sunk into the Earth, displaying a site’s longitude, latitude, and height above sea level.

As the nation expanded to the west, benchmark heights used the coast as the reference point for zero elevation. Americans who measured the heights of things, such as buildings or mountains, tied their measurements to benchmarks and, indirectly, to sea level.

More than 1.5 million of these benchmarks eventually formed a network used to position other points of interest. By the late 19th century, geodesists had begun to record and observe other geodetic criteria, such as elevation, gravity and magnetic variation. By the turn of the 20th century, geodesists had developed a sophisticated mathematical model to represent sea level, based on tidal readings. The nation’s height reference, or vertical datum, has been adjusted several times since then – but not since 1988.

Ferdinand Hassler directs the movement of the great theodolite to the station while working on Fire Island, about 1837. Sketch by John Farley.

Over this time, for several reasons, slight errors in calculating heights have accumulated, generally worsening diagonally from the tip of Florida – which requires minimal height adjustments – to the Pacific Northwest, where readings are off by as much as five feet. Some parts of Alaska are more than six feet too tall.

Today, geodesists with NOAA’s National Geodetic Survey (NGS) use space-based tools, such as the Global Positioning System (GPS), to measure the distance between coordinates and to observe and record other parameters. The NGS’s core responsibility is to define, maintain, and provide access to all of this geodetic information, which is known collectively as the National Spatial Reference System (NSRS). Traditional benchmarks have been supplemented by more sophisticated reference points, known as continuously operating reference stations (CORS), that receive geodetic information broadcast by the Global Navigation Satellite System (GNSS). Today there are about 2,000 CORS online.

NOAA’s National Geodetic Survey defines and manages the National Spatial Reference System (NSRS) – a consistent coordinate system that defines latitude, longitude, height, scale, gravity, and orientation throughout the United States. In addition to a network of continuously operating reference stations (CORS) supporting three-dimensional positioning activities (shown here), the NSRS 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.

The new effort at height modernization, once completed in 2022 or 2023, will enable NOAA geodesists to more accurately describe how geodetic coordinates, heights, and other criteria change over time. Rather than rely on fixed benchmarks for vertical and horizontal datums, the new NSRS will rely on satellite sensors and a new model for measuring local variations in gravity. A modernized NSRS will offer new, more accurate, time-stamped geodetic coordinates at CORS, with coordinates updated every five years – more accurate, complete, and timely measurements of positions.

The nation was literally built on the NSRS and its predecessors: Surveyors, mappers, engineers, and others have used it to establish property boundaries; construct buildings, roads, bridges, and levees; create accurate maps and charts; and much more. A 2009 study estimated that the NSRS and other NGS services provide more than $2.4 billion in economic value to the nation.

The value of such a self-updating system is likely to increase significantly in the future, as the planet continues to undergo rapid change. As glaciers and ice sheets melt, they not only change sea level, the geodesist’s baseline for height measurement; they also make subtle changes to the planet’s shape and configuration. The new National Spatial Reference System will be able to tell us, with confidence, where we are on this changing Earth.