Prior to 1974, glaciologists used gravity surveys as a means of determining ice thickness, but the density variations in the underlying rocks were inherent sources of errors in their measurements. Because of these errors and because of the poor resolution of the gravity techniques, better geophysical tools for obtaining ice depths were sought. In polar regions, where the ice is below its melting temperature throughout most of the ice mass, radio echo sounders were used successfully starting about 1960. Until 1974, however, radio echo sounder experiments on temperate glaciers were unsuccessful. Temperate glaciers are found throughout the mountains of Washington, Western Canada, and Alaska. In many of these areas, terrain is so rugged that a helicopter-borne gravity crew can find very few landing sites which are not on glaciers (Figure 1).