Height is height, isn’t it? Look at the Manhattan skyscraper, or the Washington Monument, or a mountain peak in California, and imagine that tomorrow will be the same height as today.
But in the United States, the heights of structures, landmarks, valleys, hills, and just about anything else are about to change. Most will be cut. Parts of the Pacific Northwest will shrink by as much as five feet, and parts of Alaska by six and a half, according to Juliana P. Blackwell, director of the National Survey.
“The United States, on a large scale, is a big deal,” said Chris Rhizos, president-elect of the International Union of Geodesy and Geophysics and emeritus professor of geodesy at the University of New South Wales in Sydney, Australia.
The large-scale calibration, called “altitude modernization,” is part of a broader effort within the National Oceanic and Atmospheric Administration or NOAA to determine more precisely where and how the United States physically sits on the planet. This new national spatial reference system, covering altitude, latitude, longitude and time, is expected to be introduced by the end of 2022 or 2023, Ms Blackwell said. It will replace the reference systems of the 1980s, which are slightly sloping, derived from calculations made before the advent of supercomputers or global navigation satellite systems such as GPS.
Height errors increase as a person moves diagonally across the country from southeast to northwest. One of the few areas in the United States that is expected to remain at the same height or rise partially will be the soil of Florida. “There is indeed a slope, which shows that all the accumulated errors in our vertical network have been pushed up to the northwest,” Ms Blackwell said.
But height has long been attached to the ego. Some Coloradans are worried that several of their mountain peaks will fall below the eulogy threshold with the new altitude system, Ms Blackwell said.
“They are very proud of how tall these things are, and I know it will be a little less if they start to be a little shorter than previously thought,” she said. She added that she is still not sure exactly what the new measurements of the Colorado peaks will be.
And near Beaumont, Texas, citizens are faced with the unwanted news that certain areas have calmed down so much from previous altitude calculations that these regions are now sitting in the floodplain. As a result, some landowners may need to insure against flood losses, said Daniel R. Roman, chief surveyor at NOAA. “They didn’t want to know that the heights had changed,” he said, “because when they make flood maps, they’re like, ‘Well, I’m that height – it hasn’t changed.’ “
A brief history of altitude
The United States has been measuring its own height since 1807, when Thomas Jefferson, then president, established the Coastal Survey, the forerunner of the National Survey, to map the waters and shores of the East Coast. The study was the first civilian scientific agency and aimed to make shipping safer.
As the country expanded westward in the 19th century, the measurement using the coast was a proxy for sea level as a reference point for zero elevation. Surveyors plant metal benchmarks on land as they travel, describing the height of any point above sea level, often a kilometer per mile. Anyone who wanted to measure the height of a building or a hill measured it against the reference value and indirectly against the sea level.
Geodetic leveling, as the process is called, was thorough and expensive. The rationale is to ensure that altitudes are measured in the same way across the country over time, and that not every county or country has its own system. For example, if engineers from two countries are building a bridge across government lines, they need to know that they will meet in the middle.
And by 1900, geodesy became even more complex. Instead of using the coastline as a protrusion for sea level, surveyors have developed a mathematical model representing sea level based on tidal readings. They have since adjusted the reference height five times, in 1903, 1907, 1912, 1929 and 1988. The 1988 model remains the standard in both the United States and Mexico.
But the 1988 version was brief with accurate information about California and parts of Texas and North Carolina, said David B. Zilkoski, a surveyor who is the former director of the National Survey. This is because the crust there has moved significantly up or down, as a result of the tectonic activity of the plates and the removal of oil, gas and water from the ground.
The solution, Mr Zilkoski decided, could be to use global navigation satellite systems such as GPS, which then began to spread. GPS is great for locating you in a flat, two-dimensional system – say, on the corner of Bank Street and Garden Avenue. But it is also able to tell you where you are in a three-dimensional world: Bank Street and Garden Avenue 40 feet above sea level. By the mid-1990s, Mr Zilkoski said, the goal of using GPS to upgrade altitude had been achieved.
It had the advantage of being cheap and easy. Satellites, and therefore global positioning systems, measure altitude relative to a smoothed mathematical approximation of a terrestrial shape called an ellipsoid. (Imagine basketball smeared on top and bottom.)
But there was a big trick. “GPS doesn’t know much about gravity,” said James L. Davis, a geophysicist at the Lamont-Doherty Earth Observatory at Columbia University in New York.
A matter of some gravity
Gravity is important to the surveyor. Height is the distance measured in the direction of gravity, and the force and direction of gravity vary depending on the density of what is below and near the terrain.
“Every time I give a public lecture on gravity, half of the conversations make them think about it differently,” Dr. Davis said.
As a result, altitude measured with GPS alone can be poorly accurate. An engineer who lays pipes with GPS only, without measuring local differences in the effect of gravity, may not make the water flow where it should have gone.
But making very detailed measurements of the gravitational field to divide them into altitudes captured by GPS is no small task. In 2007, the National Survey launched an ambitious mission – GRAV-D, for Gravity for Reedefinition of the American Vertical Date – to achieve just that.
Surveyors will then use these gravitational readings to make a model that best represents the mean sea level anywhere in the world, even on land. Because the attraction of gravity varies everywhere, this pattern, called a geoid, looks like a lump of potato. All heights will subsequently be measured taking into account.
Once the new altitude system is created, people will find unexpected applications for it, said Ms. Blackwell of the National Survey. She referred to the Jetson, a futuristic animated television sitcom from the 1960s that features cartoon characters chirping around their cities in small spaceships. The basic technology – the ability to calculate altitudes and other positional coordinates quickly and accurately – was unimaginable at the time. Today, with the proliferation of drones, self-propelled vehicles and remote-controlled air systems, the ability to accurately navigate in three dimensions is becoming increasingly important. “I think he will be adopted really quickly,” she said.
Even as surveyors get better at calculating the shape of the Earth, people change it. As we warm the planet, we melt glaciers and ice sheets. Their mass shifts from land to ocean, raising sea levels and eventually changing altitude, which uses sea levels as a reference for zero elevation. The change in mass also affects the configuration of the planet.
“This mass on the Earth’s surface is pushed down onto the Earth and actually changes shape,” said Dr. Davis of Columbia University.
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In fact, through climate change, our species is changing gravity across the planet. “We do it by making chemical changes in the atmosphere that cause mass displacement,” Dr. Davis said. “And the amount of mass is huge now. This can be seen in the shape of the geoid. This is also observed in the rotation of the Earth. “
Dr. Davis and other scientists are vying to figure out exactly how to calculate the impact of the human footprint in the coming years.
“Several hundred years ago, it was all about what it is on the shape of the Earth, he said. “And now it is: Can we measure the changing shape of the Earth and the amount of mass in the glaciers and where it comes from, well enough to say what will happen in this place in the next few years?” We are in a race. “