Remeasuring 'zero:' Great Lakes datum survey in motion

Aug. 14—TRAVERSE CITY — It's easy to see how nothing holds still in the Great Lakes, even on days when wind-whipped waters aren't lapping at Grand Traverse Bay's shore.

One number, the International Great Lakes Datum, helps keep track of everything from the dynamic and dramatic — wild swings between record high and low lake levels — to the essential — exact elevations for building bridges, dredging shipping lanes and adjusting flows through hydroelectric dams, said Ryan Hippenstiel, chief of National Geodetic Survey's field operations branch.

It's part of the National Oceanic and Atmospheric Administration, and one of a cluster of agencies working to recalculate the number that shows up even in everyday spots, like fishing charts and site plans for a rebuilt dock, Hippenstiel said.

By providing a common starting point all throughout the Great Lakes, the agency can ensure that a foot of water elevation as measured in Duluth means the same thing as a foot measured in the mouth of the St. Lawrence Seaway.

The number last computed for 1985 is so ubiquitous that people might not even know to take it for granted, said Hans Van Sumeren, director of Northwest Michigan College's Great Lakes Water Studies Institute.

"It's an important number, and it's used in all navigational charing purposes, for floodplain management as well as shoreline development," he said. "It's just unfortunately not a very exciting number, but it's critical to all things that are exciting."

John May recently came to Traverse City and Northport to take a set of measurements, each one on an established marker, he said. His equipment — a global positioning satellite antenna and high-precision receiver — sat for up to 48 hours at each location, both of which had a solid record of lake level data taken nearby.

He and several other technicians are visiting more than 350 survey monuments all along the U.S. and Canadian shore, plus connecting channels and the St. Lawrence Seaway — during one interview, he stood on the breakwall in Manitowoc, Wisconsin, near the dock for the S.S. Badger.

The "zero" point for so many measurements needs refiguring every 35 years because the Earth's crust itself is moving, and has been since the end of the Ice Age, Van Sumeren said. Glaciers that once pressed down on the Great Lakes region sunk the Earth's crust below them.

It's called isostatic rebound, and the crust keeps slowly rebounding in some places while sinking in others, Van Sumeren said — a rise of about four millimeters on Lake Superior's northeastern shore, versus sinking by a few millimeters at Lake Michigan's south end.

Accurately pinning the whole Great Lakes to one spot on the Earth gets complicated, and seriously so — that rebound rate occurs unevenly across the Great Lakes, just for starters.

Figuring exact coordinates along the lakeshores through standard surveying techniques like leveling would be extremely time-consuming, May said. And tiny errors over long distances add up, throwing off the final accuracy.

The pull of Earth's gravity, and the gravitational forces of land masses themselves, impact those surveying measures, May said.

"Basically when you drop something, it goes down toward what everybody would consider the center of the Earth," he said. "But if you have a great big mountain next to you, the angle is going to be slightly different."

How slight? May said the difference is measured in fractions of an arcsecond.

An arcsecond is 1/3,600 of a degree, so picture two 7-mile-long strings, one hanging straight down and another pulled out at a half-arcsecond's angle. The angled string's far end would hang just over an inch from that of the other. (Thanks, trigonometry!)

A constellation of U.S. and Russian global navigation satellites make an effective and efficient way to pinpoint coordinates over long distances, Hippenstiel said. Those measurements aren't thrown off by differences in gravity.

But using just satellite data or on-the-ground surveying wouldn't give an accurate enough picture of how the land is moving over time, Hippenstiel said.

That's because global navigation satellites measure elevations as if the Earth were a spheroid, Hippenstiel said — picture a basketball being squeezed from the top and bottom point. Those measurements are accurate within millimeters at the point they're taken.

But the Earth isn't a perfect sphere — far from it, considering miles-high mountain ranges, ocean trenches that sit more than a mile deeper than the tallest of them and continents that jut like plateaus above the oceans.

Those changes in gravity matter, especially when water movements are concerned, Hippenstiel said. So surveyors compute coordinates and elevations based on a geoid concept, as is the name for an irregular shape like Earth truly is.

"We're trying to basically create a large fabric of accurate coordinates, especially horizontal as you would kind of pin them to the Earth," he said.

By combining both geoid- and spheroid-based measurements, surveyors are tying in what the lake levels measurements show at each measuring station, Hippenstiel said. Isostatic rebound impacts lake levels, too, so adding in those observations makes for an even more accurate model to compute the new datum.

It's been a long process, Hippenstiel said — the first, smaller-scale surveys began in 1997. And a National Geodetic Survey timeline shows there are many more tasks ahead before the new datum's set to be released in 2027.

That's not the end, either. Another, smaller survey is set for them to validate the new datum.

Once the datum is recomputed, and aligned with a newly calculated sea level benchmark set to wrap this year, Great Lakes water levels could change on paper by as much as two feet, according to NOAA. The difference represents corrections to errors in the 1985 Great Lakes datum.

Hippenstiel said the measures may not change much at all in some places.

Whatever the difference, getting the most accurate baseline possible is crucial for making meaningful observations of an ever-changing environment, Van Sumeren agreed.