Observing and measuring changes to the surface shape of a volcano (topography) helps scientists understand the processes that cause both growth and erosion of landforms. Scientists create highly accurate maps of the ground surface called Digital Elevation Models (DEMs). Two or more DEMs that cover the same area are used to monitor topographic changes at several U.S. volcanoes. By comparing DEMs made at different times, scientists can calculate the volume of lava erupted or the rate of dome growth at an active volcano, monitor change at glaciers, measure debris flow thickness, understand how sediment is transported in a river or stream bed, and monitor changes to stream channel shape.
DEMs are generated from three-dimensional topographic point measurements made using remote sensing techniques, which means that the measuring devices or sensors are not in direct contact with the objects being measured. For example, DEMs can be made from overlapping photographs or satellite images taken of the ground surface or with instruments that capture high-resolution elevation measurements using lasers.
At several U.S. volcanoes, topographic analysis using Geographic Information System (GIS) techniques has helped determine suitable locations to install monitoring instrument stations. Calculations made in a GIS can map the radio-transmission pathways for data to be sent from monitoring sites back to Volcano Observatory offices, determine the optimal location for sun to interact with solar panels, identify helicopter landing areas, and measure tree canopy height.
Creating models of how a material flows over the ground is very important to understand volcanic hazards. These models use DEMs as the ground surface for computer simulations of pyroclastic flows, debris avalanches, lahars, floods, and fluvial sediment transport. The models allow scientists to produce volcanic hazard maps, predict flooding caused by rain, snowfall and lake-breakouts, and reduce the impacts from sedimentation build up in rivers and streams.
In a process called photogrammetry, VHP scientists take overlapping digital photographs of a volcano, which can be taken either from the air or ground. Using recent advances in camera and computer technology these images are used to build DEMs quickly and accurately. The method is akin to the older technique of using analog stereo cameras and comparing overlapping pairs of aerial photographs taken at the same time. Collecting and processing photographs to create DEMs can be done with relatively low cost and in near real-time (minutes to hours), which is an advantage when monitoring volcanic activity.
Light Detection and Ranging (lidar) technology uses laser scanners to measure the elevation of the ground surface, which is used to generate very high-resolution DEMs. The lasers are able to travel through tree canopy to the ground, so unlike photogrammetry methods, maps made from lidar reveal features of the earth's surface that are obscured by digital photographs taken of densely vegetated or forested areas.
DEMs made from lidar have helped locate previously unknown faults; map pyroclastic flow deposits and ground water at Mount St. Helens, glacier extent at Mount Rainier, and debris flow deposits at Mount Hood; and calculate the volume of Crater Lake. Most lidar scanners use near-infrared lasers that are absorbed by water, which allows scientists to map surface water features such as springs, rivers, and lakes.