Taking a mountain's measure.

Mount Everest looms a little taller now, at least to mapmakers. Climbers and scientists this spring remeasured its peak at 29,035 feet--7 feet higher than its official elevation.

The mountaineers, working with Bradford Washburn of the Boston Museum of Science, carried receivers that pick up signals from Global Positioning System (GPS) satellites. The men measured the height of the snow-covered peak but not of the rock below the snow. "The rock summit is still unknown," says Charles Corfield of Palo Alto, Calif., science manager of the team.

Washburn announced the new measurement on Nov. 11, 1999. The National Geographic Society, which helped support the work, has adopted that figure and will include it on maps and globes.

The satellite data determined the peak's position relative to Earth's center to within a few centimeters, says geophysicist Frederick Blume of the University of Colorado at Boulder, who assisted the team. The mountain's elevation, however, has an uncertainty of roughly 7 feet because of difficulties in telling exactly where the peak is relative to sea level. Sea level varies around the world depending on features, such as mountains, that alter gravity's strength. Scientists need to map Everest's gravity in much finer detail to fix where sea level would be in that region.

The official elevation of Everest had been 28,028 feet, a figure established by Indian surveyors in 1954. Done before the discovery of plate tectonics, this older survey had included data from 1850 and had not accounted for changes in the position of India and Asia since then. When Blume corrected this omission, the previous elevation agreed with the GPS one.

Roger Bilham of the University of Colorado, who also worked with the team, argues that the new results suffer from the same sea level uncertainties as the older measurements do. "I would have advised against the National Geographic issuing a new official height," he says.