...measures the slope gradient as it swells and subsides before and after an eruption. Within the tiltmeter, is a pot of water and as the ground deforms, the water shifts to maintain a perfectly horizontal surface. This is accurate to tiny increments. After mutliple eruptions, a particular angle of slope that is achieved immediately before an eruption may become apparent, making eruption prediction simpler.
...The most senstive types will detect minute chages in an area's topography. Satellite imaging often highlights grrouund deformation not visble from ground level or in areas not monitored by tiltmeters.
Before an eruption, the grouund changes shape - it usually bulges up as magma builds up in the magma chamber. This can be just a matter of millimeters.
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