The modern monitoring system on Mauna Loa is constituted not only by its locally seismic network but also of a large number of GPS stations, tiltmeters, and strainmeters that have been anchored on the volcano to monitor ground deformation due to swelling in Mauna Loa's subterranean magma chamber, which presents a more complete picture of the events proceeding eruptive activity. The GPS network is the most durable and wide-ranging of the three systems, while the tiltmeters provide the most sensitive predictive data, but are prone to erroneous results unrelated to actual ground deformation; nonetheless a survey line across the caldera measured a increase in its width over the year preceding the 1975 eruption, and a similar increase in 1984 eruption. Strainmeters, by contrast, are relatively rare. The Observatory also maintains two gas detectors at Mokuʻāweoweo, Mauna Loa's summit caldera, as well as a publicly accessible live webcam and occasional screenings by interferometric synthetic aperture radar imaging.