The height of Aoraki / Mount Cook was established in 1881 by G. J. Roberts (from the west side) and in 1889 by T. N. Brodrick (from the Canterbury side). Their measurements agreed closely at . The height was reduced by when approximately 12–14 million cubic metres of rock and ice fell off the northern peak on 14 December 1991. Two decades of erosion of the ice cap exposed after this collapse reduced the height by another 30 m to 3724 m, as revealed by new GPS data from a University of Otago climbing expedition in November 2013.