It is known that on the occasions of big eruptions, an enormous volume of earth crust material extrudes to the earth surface in the forms of lava flows and fragmental ejecta. The extrusion of lava is always accompanied by a movement of substances of the earth crust down beneath, resulting in the deformation of the earth surface. The changes of the state of the earth crust seem to be closely related to the mechanism of volcanic eruptions. Injection of magma beneath or into a volcanic edifice usually results in a small but detectable inflation of the edifice (Decker & Kinoshita, 1971); such inflation can be either temporary or permanent. A relaxation of stresses, either by eruptions or by the migration of magma into lateral fissures, may result in a broad deflation. An area of deformation is a function of size, shape, and depth of the magma body; a magnitude of a measured deformation is a function of these variables, along with the change in the volume of the magma body as well as the distance of the measuring station to the magma body. A measured rate of deformation is a function of all of the above, plus the rate of magma• injection.
Under a cooperative programme in volcanic studies between the Volcanological Survey of Indonesia and the US Geological Survey which has been in existence for some years, geodetic monitoring using tilt and trilateration techniques has been carried out at some potentially active volcanoes in Java. Networks of tilt measurement at Tangkubanparahu, Merapi, and Kelut volcanoes have been in existence since 1981 and at Galunggung volcano since 1983. EDM has been employed at Galunggung since 1982, at Tangkuban parahu since 1983, at Merapi since 1984, and at Kelut since 1955 A trilateration network was established in the west part of Lamongan volcano in response to a recent seismic swarm. The network will be remeasured constantly to recognise any subsurface magma movement. Until 1984, earth of the volcanoes were measured, normally once in 3 months unless continuous data were required at times of crisis like with Galunggung in 1982 and Merapi in 1984.

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