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While Aoraki/Mt Cook is officially listed as 3754 metres above sea level, analysis of high accuracy GPS data obtained during an Otago-led climbing expedition in November has revealed that it is actually only 3724m tall at its highest point.
The readings confirm new aerial photography based calculations performed by Otago National School of Surveying researcher Dr Pascal Sirguey and masters student Sebastian Vivero.
Dr Sirguey, the project leader for the research, said the discrepancy between the old height - estimated from aerial photography immediately following a massive rock-ice collapse in December 1991 - and the new height can be explained by a two-decade reshaping process affecting the remnant of the originally thick ice cap.
"By carefully studying photos taken after the collapse, it appears that there was still a relatively thick ice cap, which was most likely out of balance with the new shape of the summit ridge," he said.
"As a result the ice cap has been subject to erosion over the past 20 years. While the effects of climate change may spring to mind as an explanation, it is probably a case of a simple change in the geomorphology of the mountain."
Despite being taken down a peg or two, Aoraki/Mt Cook still towers above its close neighbour Rarakiroa/Mount Tasman, which with its current official height of 3497m, remains the second highest mountain in New Zealand.
The new GPS measurement is only the sixth non-aerial accurate survey of the mountain's height to be taken, with the previous trigonometric measurements made in 1851, 1879, 1881, 1883 and 1889, said Dr Sirguey.
The four-person Otago expedition that obtained the GPS data was led by Dr Nicolas Cullen, a senior lecturer in the Department of Geography at Otago.
Jim Anderson, a recent graduate from the National School of Surveying, and Dr Cullen were responsible for taking the GPS measurements.
To observe Aoraki's tapu status, and as agreed with Ngai Tahu and the three Papatipu Runanga who have a shared mana whenua interest in Aoraki, the climbers did not step on the summit, but instead took measurements with recently acquired state-of-the art receivers while at the top of the ice cap a few metres away and below the true summit.
An additional GPS point was measured at the top of the Summit Rocks for further validation of Dr Sirguey's photogrammetric 3D model.
"It was very exciting to see that the team's GPS data closely matched our photogrammetric calculations from a 2008 aerial survey," he said.
"From early on in this work we suspected that Aoraki was tens of metres lower that the official height, so it is very satisfying to have our estimates validated by GPS."
Graeme Blick, chief geodesist of Land Information New Zealand, among several organisations that provided financial and scientific support for the project, said the expedition had resulted in a "significant change".
"This means LINZ online data will use this new height and it will be incorporated when we next print hard copy topographic maps for the Aoraki/Mt Cook region."