Surveying at 20,000 Feet—A Challenge of Mountainous Proportions

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Centuries ago, Alaska Natives first christened America’s highest mountain with the indigenous Athabasacan name of Denali, meaning "The Great One." And it is indeed great in both height and stature. From as far back as 1897, the mountain was thought to be the highest peak in North America and at 20,320 ft (6,193 m) above sea level–a measurement derived from a 1953 trigonometric leveling survey–that title still holds true today.

This visibility and cultural significance may be one reason for some of the highly visible disputes that have surrounded the mountain, such as what it should be called. For Alaskans and most alpinists, Denali has been its rightful name. But for the U.S. government it has been Mount McKinley, a name officially given to it in 1917 in honor of the 25th U.S. president, William McKinley of Ohio. Over the past 40 years, many initiatives have been launched to revert the name to Denali. And those desires were finally fulfilled on August 30, when U.S. Secretary of Interior Sally Jewell announced that the mountain’s name will now return to Denali in all federal records.

The timing of the name-change resolution coincided quite remarkably with the resolution of another more recent dispute that arose in 2013 regarding Denali’s elevation. Published results of an Interferometric Synthetic Aperture Radar (IFSAR)-based survey sponsored by the U.S. Geological Survey (USGS) indicated Denali’s height was 20,237 ft (6,168 m), 83-ft (25 m) lower than the long-accepted, published height. That discrepancy sparked renewed interest in definitively resolving the true height of the mountain.

"It didn’t seem possible to me that we weren’t sure about the actual height of the highest peak in North America," says Blaine Horner, a former mountain guide on Denali and a survey account executive with CompassData, a geospatial data and services provider based in Centennial, Colo. "With today’s GNSS technology, you can calculate a summit elevation with centimeter accuracy. All you need is someone to put an antenna on the top."

Horner was that someone. Sponsored and supported by the USGS, the National Oceanic and Atmospheric Administration (NOAA), National Geodetic Survey (NGS), the National Park Service (NPS) and the University of Alaska Fairbanks (UAF), a Horner-led survey team summited Denali in June, successfully setting two GNSS antennas in the summit snow pack and collecting the needed surface point data to accurately pinpoint the peak’s elevation. That information, coupled with better gravity data to improve the geoid model in Alaska, has enabled the expedition partners to confidently resolve questions about Denali’s elevation and solidify its rightful place in the world order of highest mountains.

Pride of place
Situated about 170 miles (275 km) southwest of Fairbanks, Denali is the snow-capped jewel of Alaska’s Denali National Park and Preserve, a six-million-acre expanse of wilderness. Since the first successful climb to the summit in 1913, nearly 40,890 people have attempted to reach the top, but only 52 percent of them have succeeded. Denali is notorious for its punishing cold, which can dip to as low as minus 75 F (minus 60 C) with wind chills of minus 118 F (minus 83 C), as well as its low barometric pressure, which makes acclimatizing at higher elevations difficult.

Apart from being the highest peak in North America, Denali is also the third most prominent peak in the world and is one of the Seven Summits, the elite ranking of the highest mountains of each of the seven continents–summiting all of them is one of the ultimate mountaineering challenges. With such superlatives attached to it, the mountain is a particular point of pride for Alaskans.

"Being able to say that the highest mountain in North America is in your home state is a great honor for Alaskans," says Tom Heinrichs, director of the Geographic Information Network of Alaska at UAF, and a member of the Denali summit survey team. "Denali holds considerable cultural value for the state, so it’s important to everyone that the facts about the mountain, particularly its height, are correct."

So when the IFSAR data of Denali was collected through the USGS and the Alaska Statewide Digital Mapping Initiative (SDMI) and estimated a summit height of 20,237 ft (6,168 m), many communities took notice, including Dave Maune, a senior project manager with Dewberry, the Fairfax, Virginia-based geospatial company leading the IFSAR mapping effort.

"IFSAR is an excellent mid-accuracy remote sensing technology for the SDMI, but in certain conditions such as narrow peaks and steep slopes, it is less accurate," says Maune. "When I saw the result of the Denali survey, I questioned its validity, along with others. I knew the only way to have faith in the true elevation of Denali was to do a GNSS summit survey." Following an Alaska survey and mapping conference in February 2015, a chat among Maune, Heinrichs and Horner about the height discrepancy eventually led to Horner and Heinrichs raising their hands to scale Denali and record its summit elevation with GNSS technology.

"This project is a great example of how public-private partnerships can cooperate to provide geospatial data for our nation," says Brant Howard, CompassData’s CEO. "We were honored and proud to support such an important surveying expedition."

With financial and technical support from the USGS and NGS, GNSS equipment support from Trimble, in-kind support from the UAF and the critical support of the National Parks Service and Alaskan mapping, mountaineering and resident communities, Horner, Heinrichs and two other experienced climbers from CompassData flew to Alaska in mid-June to set the height record straight.

In search of a number
After arriving in Talkeetna, Alaska on June 15, the team flew to Camp 1 at 7,800 ft (2,377 m) on the Kahiltna Glacier and spent the next several days moving gear and supplies up the mountain, eventually basing themselves at 14 Camp, the upper mountain main base camp at 14,200 ft (4,328 m). The team used their time there to acclimatize, ensure their technology and gear were functioning correctly in the sub-zero temperatures and to cache the equipment at 17 Camp at 17,200 ft (5,242 m), the last base camp before the summit. Confirming the functionality of the GNSS technology at both 14 Camp and 17 Camp was an essential part of preparations, says Horner, because the team needed to feel confident that the GNSS technology was working at those elevations prior to summiting.

"As mountaineers, we know anything can happen on summit day," he says. "And as surveyors, we know what we need from the technology to be successful. The equipment needed to be lightweight, and simple enough to operate with gloves on yet robust enough to withstand extreme cold, provide centimeter accuracy and have a long battery life. We felt our chosen technology would meet those criteria. An additional benefit is we could preset all of the GNSS receivers so on summit day we could simply hit "on" and they would immediately start collecting data."

The initial plan was for the entire team to climb to 17 Camp and remain there for two days before ascending. However, inclement weather in the forecast forced them to skip this step and Horner and Augustin Karriere climbed to the summit from 14 Camp in one day.

At 6:30AM on June 24, Horner and Karriere set off from 14,200 ft and took the standard West Buttress route to 17 Camp to dig up the stored survey equipment, which included one Trimble R10 GNSS receiver, one Trimble NetR9 GNSS receiver, a Trimble Zephyr-2 antenna, two sets of batteries and an avalanche probe. The two hit the well-trodden trail about 9:30 and by 3:30 that afternoon, Horner and Karriere were standing on top of Denali.

Although they were the lone climbing team on the summit, and it was a surprisingly balmy 5 degrees Fahrenheit and calm, they wasted little time getting to work. First, Horner found the highest point on the summit. Moving about 19.7 in (50 cm) northeast from that point, he then hammered a 1-m range pole 34 in (86 cm) into the snow pack, leveled the top of the range pole from that point and installed the Zephyr 2 antenna on the pole. He dug a small pit at the pole’s base and placed the NetR9 and its battery inside, connected the two and immediately began acquiring static data. At an 8.2-ft (2.5-m) distance southeast of the NetR9, Horner pounded another 1-m range pole down 22 in (56 cm) and leveled that to the top of the other range pole, ensuring that both GNSS units were at the same elevation. He mounted the R10 on the range pole, dug a small pit for its battery and connected the antenna to the battery, initiating the R10’s data collection. Both pits were then backfilled with snow to protect the gear from the elements.

"From the outset we wanted to have two different GNSS systems on the summit," says Horner. "That was not only for technical reasons in case one unit failed, but it would provide data redundancy for a more precise measurement."

While the two GNSS receivers were collecting data, Horner used a 10-pound, steel avalanche probe to penetrate the snow pack around the two instruments in an effort to estimate the snow pack depth–a value never before measured. Around the R10 he was able to probe to a depth of 155 in (394 cm) and near the NetR9 he went to 163 in (415 cm) before hitting definitive resistance. However, Horner cannot state with 100 percent certainty that he hit rock.

With more climbers starting to arrive–250 climbers were on the mountain that day–and a big descent ahead, Horner and Karriere started to descend one hour after reaching the top of Denali, leaving the two GNSS instruments to collect data for the next 18 hours. They safely arrived back to base camp at about 8:30 that evening.

Critical redundancy
In addition to the summit survey, the team established a third survey site at "Windy Corner," a flatter area at 13,400 ft (4,084 m), to both collect data in parallel with the summit collection and to help establish a baseline for researchers to determine Denali’s velocity of change over time.

"Although researchers at UAF have measured tectonic motion statewide, there haven’t been observations on Denali itself to help determine how fast the mountain is changing in elevation," says Heinrichs. "By capturing a very accurate location on the mountain, we can begin to detect if Denali is still rising and moving and by how much."

Prior to summit day, Horner went to Windy Corner, found exposed bedrock and placed a second R10 on the rock several hundred meters away from the main high-traffic trail. Located about 6,560 ft (2 km) below the peak, the R10 recorded continual observations for six days total, the last two coinciding with the summit survey, enabling them to strengthen the elevation measurement. During the summit climb, Heinrichs and team member Rhett Foster checked the GNSS instrument to ensure it was logging data.

"We had nearly perfect weather on the mountain," says Horner. "And the ascent and set up all went really smoothly. The true success of this survey summit is the accuracy of the data, which has enabled us to issue a centimeter accuracy summit elevation."

And the real height is?
Indeed, to ensure consistency and confidence in concluding the new height of Denali, the expedition partners created their own "data-redundancy triangle," tasking experts from CompassData, UAF and the NGS to process the same GNSS data independently using their own software solutions.

Philipp Hummel, a professional land surveyor and CompassData’s technical director, led the GNSS data processing using Trimble Business Center (TBC), a software suite of geospatial data analysis, processing and editing tools.

Based on nearly 18 hours of observations, Hummel first calculated independent network adjustments for the R10 summit data and NetR9 summit data based on ten permanent CORS stations located 25-93 miles (40-150 km) away from Denali. With the second R10 receiver at Windy Corner only 2 km from the peak, he could use that unit as another reference point to strengthen the geometry of the network. Hummel says the results of the two independent calculations were accurate enough that he could combine together the summit data, and the data collected at Windy Corner, and process one single network adjustment using TBC. The result was a network adjustment with an accuracy of plus/minus 5 mm.

"The tricky thing about determining mountain elevations is that you can never be absolutely sure of their height because there are so many variables involved," says Hummel. "But with the precision of today’s GNSS technology and advanced processing software, we have the tools to measure the height to an extraordinarily high degree."

By mid-August, all three expedition partners had processed the GNSS data and they compared their results–all of them were within 3 cm of each other. Based on comprehensive analysis and review of their methods and results, CompassData, UAF, NGS and the USGS determined a final above-sea-level height for Denali of 20,310 ft (NAVD88), just 10 ft lower than the 1950’s survey. The new, top-of-snow elevation was officially recognized and published by the USGS in September–the survey information can be accessed through NGS’ Online User Positioning Service–finally putting to rest any lingering questions about the height of the highest peak in North America–for a while at least.

"Determining the updated elevation for the summit of Denali presented extraordinary challenges," said Kari Craun, director of the USGS’ National Geospatial Technical Operations Center. "We used the best available modern GNSS survey equipment and techniques and some of the best experts in the fields of geodesy and surveying to ensure this new elevation is as accurate as possible. Having to climb the mountain to determine its height seems an extreme measure but it leaves little room for ambiguity."

Indeed, as the overwhelming "need to know" continues to fuel more GNSS summit surveys, the USGS and other agencies may see more hands raised ready to measure mountain heights.

Mary Jo Wagner is a Vancouver-based freelance writer with 20 years experience in covering geospatial technology.

A 4.118MKb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE