How tall is Rainier, really?

Mountain climbers scale peaks because they're there.

Surveyors do it to show off their tools.

Nearly two decades after Global Positioning System (GPS) technology was first used to measure Mount Rainier, a team of surveyors will hike to the summit this month to see if the latest generation of GPS can pin a more accurate number on the Northwest's tallest mountain.

"I fully expect that we'll get a different elevation," said Gavin Schrock, administrator of the Washington State Reference Network and project coordinator.

How different?

Probably just a few inches, Schrock conceded.

"But who knows?" he asked. "What if we see a couple of feet?"

The mountain gained a foot and a smidgen in 1988, when that initial GPS measurement yielded an elevation of 14,411.1 feet. The same surveyors group came within a half-inch of that number when they remeasured Rainier again in 1999.

Most official maps still stick with 14,410 feet — the number derived in 1956 by U.S. Geological Survey (USGS) scientists using old-fashioned survey methods based on triangulation.

A foot more or less is just noise for the 5,000-some folks who trudge to the summit each year, said Chuck Young, chief ranger at Mount Rainier National Park. Absolute precision will always be elusive with a volcano that swells and contracts, and whose high point is cloaked in an ever-changing layer of snow and ice, he pointed out.

But Young still feels the project is worthwhile.

"I think what's intriguing is that it's always up for some debate, depending on how you measure."

The debate dates back more than 150 years, with estimates ranging between 12,330 feet and 15,500 feet.

The low number came from naval officer Charles Wilkes, who eyeballed and triangulated the mountain from Fort Nisqually in the early 1840s. Near the end of the century, a young chemistry professor carted a three-foot mercury barometer to the summit, using changes in atmospheric pressure to calculate an elevation of 14,528. He fell to his death on the descent. Another survey team used as a yardstick the boiling point of water, which also changes with elevation.

Rainier was among the first major mountains to get the GPS treatment. Established in 1973, the system pinpoints location based on signals from a network of satellites orbiting 12,000 miles above the Earth. In 1988, the technology was still young and the network consisted of six satellites, said surveyor Larry Signani, who organized the pioneering expedition.

His team included 150 surveyors and volunteers. The battery-operated receivers they hauled to the summit weighed 80 pounds each. It took a month for multiple groups to process the data.

By 1999, the instruments had shrunk to 10 pounds and the team was down to 40 people. Signani did the number crunching himself — but it still took about two weeks.

Today, there are more than two dozen satellites in orbit and GPS has become so commonplace it's embedded in every new cellphone and many cars.

A dozen people will set out from Paradise on July 21, carrying three receivers. Each weighs about two pounds and is capable of spitting out elevations instantaneously, down to the level of a few millimeters.

These aren't your typical handheld GPS units, which can be off by several yards. The professional instruments, which can cost up to $12,000, are owned mainly by construction companies.

Another key to the accuracy possible today is a ground-based network of 100 GPS stations installed across the state over the past decade — the reference network that Schrock oversees. The fixed stations provide reference points that can be used to factor out errors in GPS measurements caused by atmospheric interference and other glitches.

Contractors tap into the reference network when they need precise measurements to lay a water pipe or grade a roadbed. Farmers use it, too, to set the most efficient grids for planting.

The Rainier team, drawn from the Land Surveyors' Association of Washington, will take elevation measurements at several points on the mountain. They'll also be measuring gravity as they go — which isn't quite as odd as it sounds.

As anyone who has cursed the weight of their pack while plodding up a mountain can attest, there's no high-elevation boost from low gravity, à la the lunar surface. But gravity does vary subtly across the Earth.

In general, the denser the rock, the stronger the pull, said USGS geophysicist Carol Finn, who's sending a scientist from her Denver lab to participate in the Rainier project.

Because of that, gravity measurements across Rainier can provide a kind of X-ray view into the volcano, revealing pockets of crumbly rock that might collapse in an eruption.

Gravity measurements are also important in establishing the baseline for gauging the elevation of everything from mountains to flood control levees, Schrock said.

Traditionally, elevations are described as being "above sea level."

But gravity affects sea level. And what is sea level, anyway, if you're in the middle of a continent, or under a mountain like Rainier?

"Sea level is more of a philosophical thing than a reality right now," Schrock said.

Map makers are building a catalog of gravity measurements from around the world, to provide better benchmarks for sea level. It's logistically difficult to get gravity data from big mountains, so the Rainier measurements will help fill a gap, Signani said.

"In the future, all elevations may be related to gravity, rather than sea level."

Signani, who works at WHPacific in Bothell, won't be climbing the mountain for the new survey. But he's advising the team and is eager to see how modern GPS performs.

As he planned the 1988 expedition, Signani tracked down one of the USGS surveyors who measured the mountain in 1956 — and whose results have held up remarkably well.

"He never asked: 'What are you measuring it again for?' " Signani said. "He just thought it was exciting that we were using something new to measure a mountain. Now I'm the old-timer — and I feel just like he did."

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com