Deformation of the ground surface above and around a volcano before, during and after activity can give some indication of pressure changes in the magma chamber through wide-area deformation (GPS and EDM) measurements. The pressure in the upper conduit and dome can be monitored through near-field deformation (EDM and tiltmeter) measurements. In some cases, large-scale instabilities on the flanks of the volcano due to the weight of erupted material can also be tracked.
Ground deformation monitoring using Global Positioning System (GPS) techniques has been the most important method used at the Montserrat Volcano Observatory. Using a constellation of satellites and radio waves, the position of a fixed pin on the flank of the volcano can be measured to within a few millimetres in just a few minutes.
The Electronic Distance Meter (EDM) measures the distance to fixed targets high on the flanks of the volcano and, when combined with the theodolite, can be used for very accurate surveying around the volcano.
Tiltmeters are electronic spirit levels which send a radio signal every few minutes so giving real-time measurement of localised ground deformation. They are most useful in the near-field (close to a centre of eruptive activity), for instance on the crater rim where they are used to monitor pressurisation of the dome.