The U.S. Is Getting Shorter, as Mapmakers Race to Keep Up - The New York Times

Topographic surveying by members of the National Geodetic Survey.Credit...Family of Vice Admiral H.

Arnold Karo/Coast & Geodetic Survey, via NOAA Photo Library.

But across the United States, the heights of structures, landmarks, valleys, hills and just about everything else are about to change.

Blackwell, director of the National Geodetic Survey.

That’s because height is only height compared to a reference point — and geodesists, who calculate the Earth’s shape, size, gravitational field and orientation in space over time, are redefining the reference point, or vertical datum, from which height is derived.

The grand recalibration, called “height modernization,” is part of a broader effort within the National Oceanic and Atmospheric Administration, or NOAA, to establish more accurately where and how the United States physically sits on the planet.

This new National Spatial Reference System, encompassing height, latitude, longitude and time, is expected to be rolled out in late 2022 or 2023, Ms.

One of the few areas of the United States expected to either stay the same height or rise fractionally will be the toe of Florida.

A recalibration of the system for measuring elevation will officially shrink much of the United States, especially the Pacific Northwest.

Approximate change in measured height.

change in measured height.

Approximate change in measured height.

By Jonathan Corum | Sources: National Oceanic and Atmospheric Administration; National Geodetic Survey.

The United States has been measuring its own height since 1807, when Thomas Jefferson, then the president, established the Survey of the Coast, forerunner to the National Geodetic Survey, to chart the waters and coasts on the Eastern Seaboard.

As the country expanded westward through the 19th century, so did the measuring, using the coast, a proxy for sea level, as the reference point for zero elevation.

Surveyors planted metal benchmarks in the land as they traveled, describing each point’s height above sea level, often mile by mile.

Anyone who wanted to measure the height of a building or hill measured it relative to the benchmark and, indirectly, to sea level.

They have adjusted the height reference five times since then, in 1903, 1907, 1912, 1929 and 1988.

The 1988 model remains the standard in both the United States and Mexico.

Zilkoski, a geodesist who is the former director of the National Geodetic Survey.

But it is also capable of telling you where you are in a three-dimensional world: Bank Street and Garden Avenue at 40 feet above sea level

Satellites, and therefore global positioning systems, measure height relative to a smoothed out mathematical approximation of the Earth’s shape called an ellipsoid

Height is distance measured along the direction that gravity points, and the strength and direction of gravity’s pull vary according to the density of what is beneath the terrain and near it

As a result, a height measured only by GPS could be badly inaccurate

In 2007, the National Geodetic Survey launched an ambitious mission — GRAV-D, for Gravity for the Redefinition of the American Vertical Datum — to accomplish just that

Blackwell of the National Geodetic Survey said

Their mass shifts from the land to the ocean, raising sea level and, eventually, changing height, which uses sea level as the reference for zero elevation