Using Digital Elevation Models (DEMs) to Map Changes in Topography
At Mount St. Helens, scientists use Digital Elevation Models (DEMs) to monitor changes to topography around the volcano. For instance, overlapping DEMs are used to calculate the volume of lava erupted and the rate of dome growth, volume and growth of Crater Glacier, measure debris flow thickness, study sediment transport in streams and rivers, and monitor changes to stream channel shape.
Remotely-sensed data are collected using measuring devices that are not in direct contact with the objects being measured. Depending upon the collection methods, these data may be used to generate DEMs.
DEMs from Aerial Photos
Photogrammetry surveys acquire overlapping aerial photographs. Using recent advances in camera and computer technology these images are used to build digital elevation models quickly and easily. 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.
Photogrammetry during 2004-2008 eruption activityThroughout the 2004-2008 eruption, DEMs were created to calculate the volume of lava erupted and the rate of growth of the lava dome. Measurements of changes in dome volume and eruption rate were used in conjunction with measurements of deformation and seismicity to model the behavior of the magma stored beneath Mount St. Helens. In addition, the DEMs were used to track the evolution of Crater Glacier. The erupting lava dome severed the horseshoe-shaped glacier in two, squeezed the two glacier arms, doubled the thickness of each arm, and caused ice and rock to flow northward toward the mouth of the crater.
DEMs from LiDAR
One of the most rapidly-advancing remote sensing technologies is Light Detection and Ranging (LiDAR). LiDAR data includes elevation and other information about distant objects and can be collected from the air and from the ground. LiDAR equipment is relatively expensive and collecting and processing data can often be time consuming. However, DEMs generated from LiDAR data can have very high resolution and be very accurate. Unlike aerial photography, LiDAR scanners collect data at a very-fine (centimeter) scale resolution and can gather information about the ground surface underneath vegetation. This is extremely useful for geologic mapping and for measuring geologic features. Also, LiDAR data is highly sensitive to water, which allows scientists to map surface water features such as springs, rivers, and lakes.
LiDAR at Mount St. HelensAt Mount St. Helens, LiDAR based DEMs are used to map Crater Glacier, map pyroclastic and debris flow deposits, and provide base maps for modeling the impacts of volcanic hazards. Also, by analyzing topography with these highly detailed maps, scientists responsible for installation and maintenance of remote monitoring stations can easily determine suitable locations to install monitoring stations by mapping data radio telemetry paths and identifying helicopter landing areas.
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 debris avalanches, lahars, floods, and fluvial sediment transport. The models allow scientists to produce volcanic hazard maps, predict precipitation-driven and lake-breakout flooding, and help to reduce the impact of excess sedimentation.