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Home arrow Archives   The American Surveyor     

Surveying Beyond the Mist—A Sacred Mountain's True Height Revealed Print E-mail
Written by Frances Mortimer   
Sunday, 29 June 2014

A 4.108Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

Late in 2013, researchers from the Survey School at New Zealand's Otago University embarked on a physically demanding survey to validate the height of New Zealand's highest mountain, and confirm the accuracy of a photogrammetric 3D model made of the area. The project team sought to honor the status of the mountain, the people to whom the mountain is supremely important (see sidebar), and the work of the surveyors who came before them.

A mist of cloud frequently enshrouds the sacred summit of Aoraki/Mt Cook, New Zealand's highest mountain. Like a great Maori chief, the mountain shows its face to only a privileged few. But when it chooses to, on a rare clear and sunny day, the sight of the mountain is breath-taking.

The enigma of Aoraki/Mt Cook, has increased in recent years by uncertainty over its true height. So in November 2013, an intrepid team of researchers, led by Dr Pascal Sirguey of the Surveying School at Otago University in Dunedin, set out to climb the mountain and accurately measure the summit's true reach.

A Strong History of Surveying, then Disruption
Aoraki/Mt Cook's height wasn't always unknown. The first trigonometric measurement of the mountain was performed in 1851, with four subsequent surveys occurring later that century. An 1881 survey calculated the mountain's height at 3,764 m (12,349 ft). This measurement was confirmed and recognized as the official height until 1991. Further confirmation of the mountain's height was obtained in 1986 when a comprehensive photogrammetric survey was performed using aerial images of the area plus ground control points.

Then on December 14, 1991 Aoraki/Mt Cook experienced a massive rock avalanche. Approximately 14 million m3 (490 million ft3) of rock travelling up to 200 km per hour (124 mph) plunged down the mountain's eastern face and onto the Tasman Glacier below. The cascade carried with it a portion of the mountain's actual summit, leaving a peak that was extremely narrow and unstable.

Shortly after the avalanche, new aerial photos were taken of the summit area. Ground points were not obtained, but the Department of Survey & Land Information, now Land Information NZ (LINZ), reassessed the height of the summit by tying the updated photos to the triangulation of the 1986 aerial survey. A new official height of 3,754 m (12,316 ft) was estimated and new topographic contours were derived for the affected area.

New Zealand Aerial Mapping Ltd performed another aerial survey in the area in 2008. However, no ground control points were collected at this time.

2013 Photogrammetric Survey Reveals New Height
In 2013, Otago MSc student Sebastian Vivero, supervised by Dr Pascal Sirguey and Prof Sean Fitzsimons, embarked on a project to measure changes in Tasman Glacier. As part of his research, Vivero collected GPS coordinates from all mountain huts (climbers' shelters) in the area. These ground control points, collected with support from GNS Science of NZ, enabled the team to triangulate the 2008 photos and thus generate a photogrammetric model of the area.

The photogrammetric 3D model was determined to have sub-meter accuracy. And after analysis, the team made a surprising discovery--that in the previous two decades the summit of Aoraki/Mt Cook had eroded to a new height of just 3,724 m (12,218 ft). That measurement was 30 m (98 ft) lower than the 1991 post-avalanche estimate.

A Validation Project Worthy of the Mountain
The Otago University team had confidence in the accuracy of their photogrammetric model. However, they believed the results warranted a new project to validate the newly discovered height for Aoraki/Mt Cook. As Dr Sirguey explains, "We are talking about the highest peak in all of Aotearoa/ New Zealand, and a mountain with tapu (sacred) status and of supreme importance to Maori people. Aoraki/Mt Cook deserved the effort that it took to validate its height to the best possible accuracy."

And so Dr Sirguey began seeking support for the survey project, and assembling a team to climb the mountain.

Initiation of the project began with the team presenting their proposal to Ngai Tahu, the largest iwi (Maori tribe) in the South Island and for whom Aoraki/Mt Cook has enormous cultural significance. In consultation with iwi leaders, the research team agreed to not step on the actual summit of the mountain. Instead they would measure a control point about 45 m (150 ft) north from the highest point. This point would be used to validate the 3D photogrammetric model. For the type of survey they were seeking, one that simply validated the existing and reliable photogrammetric survey, agreeing to not stand on the summit was perfectly acceptable. Says Dr Sirguey, "Other climbers can and do step on the summit, but because we wanted this project to honour the mountain's status, we sought to defer to Ngai Tahu's preferences."

The National School of Surveying, GNS Science, Land Information New Zealand (LINZ), and the Federated Mountain Clubs of NZ (FMC) provided financial support for the climb, while sponsor Southern Approach Ltd provided each team with a GoPro camera to document the attempt.

A Daunting Climb
Even some hobbyist mountain climbers would decline an opportunity to tackle Aoraki/Mt Cook. The mountain is a challenging, often dangerous peak with dynamic glaciers and steep ice and rock faces to negotiate. New Zealand's temperate maritime climate and the geography of the Southern Alps create changeable alpine weather. And because the summit of Aoraki/Mt Cook is less than 20 km (12 mi) from the Tasman Sea, sudden and severe storms are common.

So Dr Sirguey selected members who were also experienced and capable mountain climbers. He created, in fact, two teams of two. If one team could not complete the ascent-- due to, for example, inclement weather or injury--the other team would still be able to carry out the survey. Each team was made up of one very experienced mountain guide and one person responsible for gathering the data. (Standard protocol is one guide per climber.)

A Customized Survey Methodology
The complete research team was flown by helicopter to Plateau Hut, which is situated on Aoraki/Mt Cook at 2200 m (7,218 ft). Shortly after midnight on November 23rd, 2013--after a day of snow-safety training-- the four-person party left the hut to begin the 1500 m (4,921 ft) climb to the summit. The weather conditions were cold and clear, and perfect for the climb.

Each team carried with it a Trimble R10 GNSS receiver recently acquired by the School of Surveying. The lightweight Trimble R10 receivers, which have integrated antennas, were programmed before the climb, so no controller or other equipment was required.

The receivers were programmed to log data every five seconds from the moment they were powered on until their batteries died. Says Dr. Sirguey, "We had no idea how long our guys would be able to stand on the summit, let alone if they would even make it. So just in case, we planned for each team to simply set the receiver in place at the top of the mountain for a minimum of 20 minutes, and then again at one backup point on their downward climb. In between, the receivers would be continuously logging data, even while our guys descended, in order to ensure that a long enough session was captured so it could be postprocessed."

Aoraki/Mt Cook indulged the teams with good weather and a trouble-free climb, albeit one that included hours of glacier and crevasse crossing, steep rock and ice faces, and exhausting cramponing up the summit's wind-hardened ice cap. At times they had to move quickly beneath ice cliffs to minimize the risk of being in the wrong place at the wrong time during one of the mountain's frequent ice falls.

"But," says Nicolas Cullen of the ascent, "after climbing in darkness for several hours, we experienced that moment of brilliance when the sun came up and suddenly we realized how high we were. The vista of the New Zealand landscape suddenly appeared and it was a brilliant feeling. Absolutely beautiful."

The final ascent to the summit was straightforward, and all four men safely reached their destination approximately 10 hours after leaving the hut. Says Cullen, "The summit area has re-formed itself into quite a nice area to sit down. So we could place the GPS receivers exactly where we needed them."

Cullen and Jim Anderson spiked the two Trimble R10 receivers into the ice cap about 45 m (150 ft) north and 5 m (16 ft) lower from the actual summit as agreed with Ngai Tahu, and performed a static survey for about 20 minutes each. But they lingered at the highest point in New Zealand for almost an hour.

Reaching the summit was only the half-way point of the effort. After the initial static survey on top of Aoraki/Mt Cook, the team carefully negotiated their descent with the Trimble R10 receivers logging data every five seconds the entire way down. For the planned backup point, the team had decided on a rock outcrop 200 m (650 ft) below the summit that could be easily identified in the photogrammetric images. At this point both teams paused to each perform a further 15-minute static survey, eventually reaching the Plateau Hut approximately 18 hours after first heading out.

A New Official Height
Back at the university the data was postprocessed using Trimble Business Center office software. Using reference signals from nearby stations from Land Information New Zealand's (LINZ) PositioNZ continuous GPS network, the team achieved decimeter accuracy. The highest point the team measured on Aoraki/Mt Cook was 3719 m (12,201 ft), which was less than a meter from that indicated by the 3D model. This measurement was consistent with the height from the computer model, which then allowed the height of the high peak to be calculated and confirmed as 3724 m (12,218 ft) above mean sea level (established using the NZGeoid2009 and vertical datum NZVD2009. The difference between this datum and the earlier Lyttelton 1937 one that was used is 47 cm).

Says Dr Sirguey, "It was very exciting to see that the team's GPS data closely matched our photogrammetric calculations. From early on in this work we suspected that Aoraki/Mt Cook was tens of meters lower than the official height, so it is very satisfying to have our estimates validated by GPS." Dr Sirguey suggests that the new height can be explained by a two-decadeslong reshaping process affecting the remnant of the originally thick ice cap.

Despite its new height, the mana (authority) of Aoraki/Mt Cook remains intact, with the sacred mountain still towering above its brother Mt. Tasman (Rarakiroa). Mt. Tasman, which at 3497 m (11,473 ft) is still only the second highest mountain in New Zealand. Dr. Sirguey's team presented its results from the survey first to Ngi Tahu before making them public.

Frances Mortimer is a freelance writer specializing in high-tech positioning solutions, including conventional, GNSS and spatial imaging survey systems. Frances is based in New Zealand.

Sidebar:
Tribute Trigonometric Survey

In the process of determining Aoraki/Mt Cook's new height, Dr Sirguey and his team referred frequently to the work of the surveyors before them, particularly those of the 19th Century. For this reason, an additional aspect of the height validation project involved a trigonometric survey performed as a tribute to those who came before.

This survey was carried out by secondyear surveying student Tyler Hager with sponsorship from LINZ. Hager used a precision theodolite to gather trigonometric observations from several locations with a line of sight of the summit.

Tyler took advantage of new technologies such as GIS and geo-visualization to help identify appropriate viewing points. Even so, Tyler's measurements reproduced some of the complexities that the early surveyors encountered when estimating the height of a prominent peak.

Dr Sirguey says, "As well as being a valuable teaching opportunity, Tyler's project really helps place our survey findings within New Zealand's rich history of surveying and acknowledges the legacy of early surveys, surveyors, and survey techniques.

Sidebar:
Aoraki, a Sacred Ancestor

According to Maori Legend, Raki the Sky Father wedded Papa-tui-nuku the Earth Mother. Afterwards, Ao-raki and his three brothers came down from the heavens in a waka (canoe) to greet their father's new wife. When it was time to return home, the karakia (incantation) to lift the waka back to the heavens failed and the vessel fell onto its side in the sea. Aoraki and his brothers clambered on to the high side of the waka. The waka, now made of stone and earth, formed the South Island of Aotearoa/New Zealand. The brothers also turned to stone, transforming into the peaks of the Southern Alps.

Ngai Tahu, the principal Maori iwi (tribe) of the South Island, view themselves as descendants of Aoraki. They believe their connection with the mountain gives vitality to their culture and mana (authority) to their status as tangata whenua (the people of the land). The mountain is therefore tapu (sacred).

As a gesture of goodwill, in 1998 Ngai Tahu gifted Aoraki/Mt Cook to the people of New Zealand and it is now a National Park. However, its use is restricted in order to honor and preserve the mountain's tapu status.

A 4.108Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

 
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