This photo shows geologists Christina Heliker and Tom Wright using a theodolite to measure the height of a lava fountain. The theodolite is a very accurate surveying instrument that allows you to measure angles, both vertically and horizontally. From their vantage point here, the Pu'u 'O'o vent was about a kilometer away (out of view to the left). By knowing the distance accurately and measuring the angle to the top of the lava fountain they could use simple trigonometry to determine its height, which was often >300 meters!