Mt Cook's height settled by survey

The official height of New Zealand's tallest peak, Mt Cook, will soon fall by 30m, after updated measurements by University of Otago researchers.

The peak is still officially listed as 3754m above sea level, but analysis of high-accuracy GPS data gained during an Otago-led climbing expedition last November shows the mountain is, in fact, only 3724m tall.

The readings confirm new calculations, based on aerial photography, made by Otago National School of Surveying researcher Dr Pascal Sirguey and master's student Sebastian Vivero, with support from GNS Science and New Zealand Aerial Mapping Ltd.

This is believed to be the first time the height of a major New Zealand mountain has been adjusted after a study conducted by university researchers.

And this is also understood to be the first time a modern, relatively light-weight, survey-grade GPS device has been carried close to the summit of a New Zealand mountain to measure its height, officials said.

Dr Sirguey, the research project leader, said the old height had been estimated from aerial photography immediately after a massive rock-ice collapse on the mountain on December 14, 1991.

The discrepancy between the old and new heights could be explained by a subsequent two-decades-long erosion and reshaping process affecting the remnant of the originally thick ice cap.

Despite being taken down a peg or two, Mt Cook still towers above its close neighbour Mt Tasman, which, at 3497m high, remains New Zealand's second highest mountain.

The four-person Otago expedition that obtained the GPS data took place on November 23 last year and was led by Dr Nicolas Cullen, a senior lecturer in geography.

To observe Mt Cook's tapu status, and as agreed with Ngai Tahu, the climbers did not step on the summit, but instead took GPS measurements while at the top of the ice cap, a few metres below the true summit.

"Dr Sirguey said it had been ''very exciting'' to see that the team's GPS data ''closely matched our photogrammetric calculations from a 2008 aerial survey''.

''From early on in this work we suspected that Aoraki was tens of metres lower that the official height,'' he said.

There had then been ''a bit of a jump into the unknown'' to investigate the actual height.

Second-year surveying student Tyler Hager also undertook a related ''tribute'' trigonometric survey of the peak, as part of his summer scholarship work.

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