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The Editor,

Journal of Glaciology

Sir,

Two of the major conclusions of this paper are: (1) the true area of a glacier surface is not the same as the area of its projection onto a horizontal plane (the planimetric area), and (2) the true surface area should he used in computing average mass balance from point measurements.

The first statement is correct but trivial, because it follows from the most basic trigonometry that an element of planimetric area can be obtained from an element of surface area by multiplication with the cosine of the local surface slope angle. The second statement is incorrect if the point measurements are made in the vertical direction, which to our knowledge is always the case, whether one uses poles, aircraft altimetry or even sequential mapping. Average mass or volume change is computed from these data by integration over the planimetric area, not the surface area, as outlined by Paterson (1994), for example. Use of the true surface area will lead to error in average mass balance which is on the order of 15% (depending upon the geometry), as the authors compute.

The point seems quite basic. We caution against the uncritical use of the results of this paper.

12 February 1996

References

Jacohsen,, F.M. and Theakstone,, W. H. 1995. The use of planimetric surface area in glacier mass-balance calculations: a potential source of errors. J. Glaciol., 41 (139), 441-444.
Paterson,, W.S.B. 1994. The physics of glaciers. Third edition. Oxford. etc., Pergamom Press.