Map comparison or traditional mass-balance measurements: which method is better?

A number of Norwegian glaciers were selected in the 1960s for long-term mass-balance measurements, to produce necessary hydrological information for hydropower exploitation. Special large-scale glacier maps were produced for field work and data processing, and some glaciers have been mapped more than once. Thus, comparison of glacier maps can be used to calculate changes in glacier volume for some of the glaciers, provided they are of sufficient accuracy. Conventional mass-balance measurements were carried out on all the selected glaciers. A cumulative calculation of net balances for a series of years is used to indicate the change in a glacier's volume during that period. However, various errors originate in the field, some of which are systematic, particularly on glaciers with large winter accumulation. The present study indicates that certain errors are difficult to define and determine, For the maritime glacier Alfotbreen, a cumulative mass-balance calculation gives a positive total balance (+/-3.4 m water equivalent in the period 1968-88), whereas the map comparison indicates a total negative balance (-5.8 m water equivalent). This indicates a discrepancy between the methods, which must be accounted for. Determination of errors in mass-balance measurements is difficult. Sinking of stakes in the accumulation area and the use of sounding sticks (steel probes) in heavy snowlayers cause problems.