If one quantity is larger than the other, the ice sheet must be either thickening or thinning. We compare these measured vertical velocities with the rate of snow accumulation, which comes from the study of layers in ice cores. We call this experiment the coffee-can method because we used to use empty coffee cans as the special markers. Nowadays we use wires instead of coffee cans but the name stuck. The coffee-can method is a great way of studying mass balance, but it only provides a local measurement where the markers are installed.
Mass balance varies from location to location.
Earth-observing satellites, on the other hand, can collect measurements over Antarctica very quickly. NASA plans to launch ICESat in 2001 to study the mass balance of Greenland and Antarctica. This satellite will use a laser to measure the elevation of the ice sheet surfaces. By making repeat measurements of the same locations over the course of several years, the satellite will provide a record of ice sheet elevation changes. Ice sheet elevation changes are almost the same as mass balance, but not quite.