Repeated electronic distance measurements across Kilauea Caldera with Tellurometers and Geodimeter show definite horizontal expansion related to the vertical uplift and outward tilting of the summit prior to an eruption, and contraction during and after a flank eruption. Measurements started in October 1964, along a 3098 meter line between Uwekahuna and Keanakakoi, indicate a relatively uniform lengthening of 12 centimeters during the interval October 22, 1964 to March 1, 1965. Rapid shortening of the line by 28 centimeters was measured 4 days after the beginning of a flank eruption which involved emission of approximately 29 million cubic meters of lava during the period March 5 to March 15, 1965.
During the expansion, the standard deviation of 10 Tellurometer measurements from a least-squares srtaight line solution is ± 2.0 centimeters (6.5 ppm) whereas 9 Geodimeter measurements have a standard deviation of ± 1.1 (3.6 ppm) centimeters. Absolute distance readings between the two instruments differ by 4 centimeters (13 ppm), but relative changes in distance were the same on both instruments. Changes in distance across Kilauea Caldera can, therefore, be easily measured to accuracies of 4 to 7 parts per million with standard electronic distance measuring systems.
On active volcanoes where ground surface deformation exceeds 10–100 parts per million with changes in subsurface magma pressure or volume, repeated horizontal distance measurements can be a most useful technique.
