(1) A sequence of columns of infinitesimal cross section, each extending vertically from base to surface of the glacier, arranged so that each column but the first gains mass by flow from an upglacier
neighbour and each column except possibly the last loses mass by flow to a downglacier neighbour. (2) The trace of such a sequence on the glacier surface. Ideally, the upglacier and downglacier walls of all the columns would be at right angles to the local horizontal velocity vector. It is assumed that flow through the other two walls of the columns may be neglected, by allowing an implicit relative width of the flowline to vary and thus to account for transverse straining. In practice, velocity measurements are usually sparse or lacking and it is necessary to construct the flowline from the surface topography. The topography is averaged within a radius of the order of the glacier thickness, to suppress the effect on calculations that might be exerted by short-wavelength topographic features that are not due to the glacier flow. The definitions may be extended to accommodate interrupted glaciers, in which part of the 'flow' is by avalanching from an upper part to a lower part.