Any method for determining mass balance by repeated mapping of glacier surface elevations to estimate the volume balance; cartographic method and topographic method are synonyms. The conversion of elevation change to mass balance requires information on the density of the mass lost or gained, or an assumption about the time variations in density (see Sorge's law). Elevation changes are commonly measured using repeated altimetry, photogrammetry or ground surveys. In the past, glacier mapping relied on ground surveying with theodolites and similar instruments, but global navigation satellite system receivers are now usual, offering more rapid and more accurate coverage. The entire glacier surface may be mapped, but more sparse elevation measurements, for example along a central flowline, are often extrapolated to the full glacier surface.