VOLCANO, Hawai'i — Scientists from the U.S. Geological Survey's Hawaiian Volcano Observatory are engaged in their tracking of earth movement on the Big Island's active volcanoes.

Data obtained from global positioning system surveys at about 100 locations supplement information from more than 60 continuously recording GPS receivers that have been permanently installed on the volcanoes, the scientists say.

"GPS, a satellite-based technology, is used to measure ground movements, or surface deformation, on the volcanoes," the observatory said Monday. "Deformation is a key indicator of changes within a volcano that indicate where magma is located and whether or not it is likely to erupt, so tracking it is an important monitoring tool."

Magma is lava before it reaches the surface.

The GPS monitoring can detect horizontal and vertical ground motions to within fractions of an inch, observatory geologist Mike Poland said.

The annual surveys of Hawaii's two most active volcanoes, Kilauea and Mauna Loa, are to continue through May. The biennial survey of Hualalai was completed in early April. Kilauea has been erupting since Jan. 3, 1983.

The surveys involve setting up a GPS receiver on at least two days at the same site and allowing it to run for up to 24 hours each day. As many as eight GPS units could be collecting data in a particular area on a given day.

Observatory personnel can reach most of the sites by vehicle or on foot, but some are accessible only by helicopter.