The vertical distance between any two surfaces, and in particular between the glacier face and the summer surface, or the glacier surface and the bed. Glacier thickness is measured ideally by interpolating from a dense array of point measurements, constructed for example from ground-penetrating radar traverses. However the measurement density is often less than ideal, as when the array consists of a single traverse or even just a small number of boreholes. On most glaciers there are no thickness measurements at all and the thickness must be estimated, for example by volume-area scaling or as a function of surface slope and estimated basal shear stress. The definition of thickness as a vertical distance is adopted almost invariably in studies of mass balance, but not in all branches of cryospheric science. For example Fierz et al. (2009) define thickness as the coordinate normal to the slope, measured from the base of a layer of snow.