Comparison of geodetic and glaciological mass-balance techniques, Gulkana Glacier, Alaska, USA

The net mass balance on Gulkana Glacier, Alaska, U.S.A., has been measured since 1966 by the glaciological method, in which seasonal balances are measured at three index sites and extrapolated over large areas of the glacier. Systematic errors can accumulate linearly with time in this method. Therefore, the geodetic balance, in which errors are less time-dependent, was calculated for comparison with the glaciological method. Digital elevation models of the glacier in 1974, 1993 and 1999 were prepared using aerial photographs, and geodetic balances were computed, giving 6.0 +/- 0.7 m w.e. from 1974 to 1993 and -11.8 +/- 0.7 m w.e. from 1974 to 1999. These balances are compared with the glaciological balances over the same intervals, which were 5.8 +/- 0.9 and -11.2 +/- 1.0 m w.e. respectively; both balances show that the thinning rate tripled in the 1990s. These cumulative balances differ by < 6%. For this close agreement, the glaciologically measured mass balance of Gulkana Glacier must be largely free of systematic errors and be based on a time-variable area-altitude distribution, and the photography used in the geodetic method must have enough contrast to enable accurate photogrammetry.