Vertical changing glacier surface elevation (altitude), typically derived from two elevation measurements, adjusted if necessary for the difference of their respective datum surfaces, at the same (or nearly the same) horizontal coordinates. The elevation of the surface can change due to (i) ablation and accumulation at the surface and bottom of the glacier; (ii) compaction (densification) of snow and firn; (iii) emergence and submergence resulting from ice flow; (iv) changes in subglacial water pressure; (v) tectonic and isostatic movements of the glacier bed; and (vi) geomorphic processes (abrasion, plucking; lodgement of sediment) at the bed. Changes due to (iv) and (vi) can usually be neglected in mass-balance studies, although a correction is sometimes applied for glacial isostatic adjustment (GIA). Surface elevation change is usually similar to thickness change, but (ivvi) above produce elevation changes without changes of the thickness or glacier mass, while (ii) above produces a decrease of thickness with no accompanying change of mass. See continuity equation, geodetic method. In turn, large changes of glacier thickness lead to isostatic changes of the bed elevation.