Sir,

In his recent paper on the definition of glacier mass budget terms, Reference MeierDr. Meier (1962) has brought some much-needed clarity of thought into this rather confused and confusing subject. However, there is one point that he has left rather vague which seems to me to be of importance, particularly where glacier mass budgets are determined by photogrammetric means.

When defining specific budget quantities, Meier refers to the accumulation and ablation measured at a point. He does not make it clear whether this is a point on the glacier and moving with it, or whether it is a point fixed in geographical coordinates. From what follows it is apparent that Meier had in mind a point on the glacier and moving with it, for this is what is defined by an accumulation or ablation stake or by a pit. For such a point it will be reasonable to find a net budget in the way Meier describes, and to refer to the result as the apparent accumulation if it is positive and the apparent ablation if it is negative. A relatively simple glacier will then have apparent accumulation in its upper part and apparent ablation in its lower part, the two being separable into the accumulation area and the ablation area.

If, however, a particular point in geographical coordinates is considered, as for example if the height at the surface of the glacier is determined photogrammetrically for a particular point on the map, then a glacier which was in a steady state would have its surface at the same height in the same place each year (this is the annually repeating state defined by Reference NyeNye (1958, p. 142)). If we applied Meier’s definitions to this situation without modification we would reach the surprising conclusion that there was, summed over the budget year, no apparent accumulation and no apparent ablation anywhere on the glacier. Similarly in a year in which the ablation had been considerably less than normal, it is quite thinkable that a glacier might at all points be higher than it had been the preceding year—and a direct application of Meier’s definitions would then mean that we spoke of an apparent accumulation over the whole of the glacier, and that the whole glacier was in the accumulation area—despite the fact that large amounts of ice from preceding years had been melted away from the surface in the lower parts.

I think these simple examples are sufficient to show that the question of whether we concentrate attention on a particular parcel of ice moving with the glacier or on a particular point on the map is of great importance. Of course, when the budget quantities are integrated over the whole glacier, the totals will be the same whichever method is used, provided the boundaries for the integration are correctly chosen; both will give the same cumulative budget quantities over the whole area of the glacier in Meier’s terminology.

The solution that I would like to propose is that Meier’s terms, apart from the cumulative totals mentioned in the last paragraph, should be used only for measurements made with probes that move with the ice. For measurements made by photogrammetry, or any other method that works by comparing the situation from time to time at one particular point in geographical coordinates, different terms should be used. In particular, I suggest that the terms accumulation, ablation and specific net budget should not be used in such measurements, but rather terms such as height variation of the surface.

I am well aware that many people who have measured changes in glacier surface height have kept this distinction in mind, this is particularly clear in the discussions of glacier mass budget determination by photogrammetric means given recently by Reference FinsterwalderFinsterwalder (1961, Reference Finsterwalder1962). My purpose in writing this letter is simply to prevent a possible confusion entering in the proposed definition of terms.

I would like to thank Professor H. C. Hoinkes and Dr. M. F. Meier for useful discussions on this topic.

19 December 1962