The change in the thickness of the glacier at a defined horizontal location. Thickness can change at a point due to ablation and accumulation at the surface and bottom of the glacier, compaction of snow and firn, or a non-zero emergence velocity or submergence velocity. Thickness change is often used interchangeably with elevation change, but the two are not necessarily the same. For example, elevation can change due to glacial isostatic adjustment or vertical tectonic motions, without a change in glacier thickness. The thickness change at a point is not equivalent to the climatic-basal mass balance at that point because the thickness change may be due in part to emergence or submergence. Thickness change at a point is therefore not a direct indicator of the local climate. The glacier-wide mean thickness change is the volume change of the entire glacier divided by the mean glacier area during the time span of the measurements: 2(h V VS S 21)/(21) , where V is volume, S is area, and subscripts 1 and 2 refer to measurements at an earlier and later time, respectively. The quantity 21( ) V V V is the volume change. Usually the two volumes are not known separately, and: V is obtained from measurements of by geodetic methods. The mean thickness change, if multiplied by the density of the mass gained or lost, is equal to the glacier-wide mass balance over the period of the thickness change. The mean thickness change differs from the change of mean thickness, which is V2/S2 V1/S1.