English: New Zealand Geological Survey volcanologist Brad Scott conducts theodolite measurements at the Crater Lake vent in 1988 at the summit of Ruapehu volcano. Measurements of the lake height, temperature and chemistry are made routinely, and along with seismic instrumentation, are used to help forecast future activity of the volcano. Intermittent steam explosions from beneath the lake have produced lahars, which have damaged ski facilities on the upper flanks and structures in river valleys below the volcano.