How high?
New GPS system seeks to get better read on what is – and isn't – a 14er

by Tom Hesse

Colorado Public Radio

Kate Dignan has summited one 14er – she thinks it was probably Longs Peak, but she has done her best to forget the experience.

That trip and countless photos from hikers around the state got her wondering: what are those round, metal markers that dot Colorado’s mountains? 

“Who actually manages those; determines where they go on the actual mountain?” Dignan asked.

Derek van Westrum works for the National Geodetic Survey, an agency under the National Oceanic and Atmospheric Administration that has the biggest catalog of those survey markers at around 1.5 million. He said the history behind the markers is extensive, but the future is fascinating, including an extensive project that will define a new sea level. 

Van Westrum, the gravity program manager for the NGS, said survey markers are used for measuring two things: horizontal location, like where something would be on a map, and elevation. 

“Survey markers go back thousands of years,” van Westrum said. “I think there’s evidence of Egyptians using survey markers to plot out land so they could redistribute it after the Nile would flood.”

When it comes to elevation, the markers are meant to establish baselines for which something can be measured off of. If you know the height of one mountain, you can stamp a survey marker into the rock and measure the height of the neighboring mountain based on the elevation you know to be set in stone. 

“Because we had a map of the coast, we had an idea of where the average tide line was at some location. And if you call that a zero elevation, you can then work your way inland into the mountains to find out how high things are,” van Westrum said. “You don’t want to go back to the ocean each morning to start over. So you put benchmarks in as you go to keep track.” 

It took about half a century to establish all those marks in the rock before they could do the math to measure elevation relative to everything else. The pieces themselves are crafted specifically for measurement, van Westrum said. They’re dome-shaped so that surveyors using them can find the highpoint on the marker to measure off of. They’re largely made from brass or bronze to be corrosion-resistant and durable. And the information on them is typically to help find them in archives. Someone could call the NGS, for example, and get more information on the marker based on what’s engraved on it.

Unsurprisingly, the scientific and map-making community is moving beyond brass domes epoxied into rock. The survey markers have a tendency to move or be affected by changes around them. 

“There’s a famous poster in the San Joaquin Valley, California, where the land, because of the pumping out of the groundwater, has sunk now by something like 50 meters, just a huge amount. So any benchmarks that were placed in the 1970s or 1980s are now woefully out of date,” van Westrum said. 

For horizontal measurements, GPS has largely taken over. But GPS is not as good at determining elevation, partly because it’s not working off the right data.

Knowing the elevation is particularly important when it comes to flood mapping and construction of large projects like dams or railroads. The gravity program van Westrum is working on hopes to get those elevation measurements as precise as possible. This means moving away from the current number used for sea level, which was just an average. 

Getting that new sea level, as van Westrum’s job title suggests, involves measuring gravity itself. 

“What we’re doing now is we’re picking basically a value of gravity times a distance from the center of the Earth and combining those,” he said. “That has a numerical value ... We’re basically just defining that to be sea level.”

Getting that number for the whole country involved more than a decade of survey flights. 

“We have devices that measure the acceleration of gravity, and we make a map of that by putting instruments in airplanes and literally flying the entire United States on a 6-mile grid. It took 12 years,” van Westrum said. 

That new baseline of sea level could then be fed to GPS data, providing the appropriate baseline to measure off of and thus, a truer elevation. 

“This is probably 10 times more accurate than the old system that was established in the 1980s. It really will help those flood-prone places to predict where water will flow,” van Westrum said. “And in principle, it should or could change the height of a few 14ers, maybe shuffle things around a bit.”

For more from Colorado Public Radio, visit cpr.org