A pole or rod that has been emplaced in a vertical hole drilled into the glacier surface; may also be referred to as a mass-balance stake, or as an accumulation stake or ablation stake as appropriate. The change in height of the glacier surface relative to the top of the stake is the basis for a measure of the sum of accumulation and ablation (that is, of surface mass balance). The five quantities measured (Figure 13) over the course of the mass-balance year are at t0, when by definition there is no snow, the distance d0 from the stake top to the summer surface; at tw, the distance dw from the stake top to the surface and (in a nearby snow pit or with a coring device) the mean density w of the snow (if any; the winter balance is not necessarily positive); and at t1, the distance d1 from the stake top to the surface and the mean density 1 of the snow (if any). The layer thicknesses hw and h1are obtained by subtraction, as beneath the figure; note that in the right part of the figure the Annual balance is negative because the thickness h1 is negative. When the balance is positive the stake measurements are often supplemented by digging or probing to sample local variability. It is not possible to measure the density of mass that is lost between any two survey dates. For example the summer balance is commonly evaluated as ba bw; and when h1 is negative an appropriate value (usually, outside the firn area, the density of ice) must be assumed for 1. At the instant following t1, any residual snow is deemed to become firn and the glacier surface, at d1, becomes the summer surface d0 of the next balance year.