Mount St. Helens, the volcano in southwest Washington state that erupted in 1980, is again showing signs of life. Scientists with the US Geological Survey and NASA are using photonics technology to track the growth of a new volcano dome with an accuracy of inches. Thanks to remote sensing, the researchers can do this with less clambering over steaming rock or peering though ash. Instead, they are using airborne lidar systems as well as photogrammetry measurements made from aerial photographs to create a digital elevation model that provides a visual representation of the volcano.Linda Mark, a hydrologist with the US Geological Survey's Cascades Volcano Observatory in Vancouver, Wash., explained that the digital elevation models created by lidar and photogrammetry complement each other and confirm their respective findings. In the case of lidar, a plane flies over the site and pings the surface below with thousands of laser pulses each second. Based on the time it takes for the reflected pulses to be received, the system calculates the distance to the ground. That information, along with plane location measurements, yields surface feature heights with an accuracy of about 10 cm. In photogrammetry, two images of the same location are shot at different angles, and the images are combined to create a three-dimensional reconstruction of the surface.Both techniques have their advantages. Photogrammetry, which is older and more readily available from commercial sources, produces traditional photographs that are useful in determining rock types. Lidar is newer and still evolving, but geophysicist David Harding of NASA's Goddard Space Flight Center in Greenbelt, Md., noted that the elevation measurements do not depend on illumination by the sun, so they can be made anywhere and at any time.Repeated flights using these techniques determined that the volcano's new uplift rose 110 m from late September to early October and covered 130,000 m2. Continued measurements are planned, and the data should provide information about volcanic hazards and processes at Mount St. Helens. It also may find application at other similar volcanoes.