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Digital Elevation Models (DEMs) map changes to a volcano’s topography
Observing and measuring changes in the shape of a volcano’s surface (topography) helps scientists understand the processes that cause landforms to grow and erode. Scientists create highly accurate maps of the ground surface called digital elevation models (DEMs). Two or more DEMs covering the same area are used to monitor the topographic changes of several volcanoes. By comparing DEMs made at different times, scientists can calculate the volume of lava ejected or the rate of growth of an active volcano’s dome, monitor changes in glaciers, measure the thickness of debris flows, understand how sediments are transported in a river or stream bed, and monitor changes in channel shape.
DEMs are generated from three-dimensional topographic 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 photographs or overlaid satellite images of the ground surface or with instruments that take high-resolution elevation measurements using lasers.
DEMs help scientists locate ideal monitoring sites and produce maps of volcanic hazards.
On several volcanoes, topographic analysis using geographic information system (GIS) techniques has been used to determine appropriate locations for the installation of monitoring instrument stations. GIS calculations are used to map radio transmission paths for data to be sent from monitoring sites to the Volcano Observatory offices, to determine the optimal location for the interaction of the sun with solar panels, to identify helicopter landing zones and to measure the height of tree canopies.
Modelling is very important for understanding volcanic hazards. These models use DEMs as the base surface for computer simulations of pyroclastic flows, debris avalanches, lahars, floods and river sediment transport. The models allow scientists to produce maps of volcanic hazards, predict flooding from rainfall, snowfall and lake outbursts, and reduce the impacts of sediment accumulation in rivers and streams.
The problem remains that the cost of these monitoring operations is possible for so-called privileged countries and often unthinkable due to lack of resources in disadvantaged countries.
Dome building episode from 2004-2008 at Mount St. Helens can be observed and measured with these two digital elevation models (DEMs) developed before and after the eruptive episode.
Digital photographs are a cheap and fast way to create DEMs.
In a process called photogrammetry, VHP scientists take digital overlay photographs of a volcano, which can be taken from the air or the ground. Thanks to recent advances in camera and computer technology, these images are used to construct DEMs quickly and accurately. This method is similar to the older technique of using analogue stereo cameras and comparing pairs of overlapping aerial photographs taken at the same time. The collection and processing of photographs to create DEMs can be done at relatively low cost.
Lasers can be used to map the ground surface under vegetation.
Light detection and ranging (lidar) technology uses laser scanners to measure ground surface elevation, which is used to generate very high resolution DEMs. Lasers are able to penetrate the forest canopy to the ground, so unlike photogrammetry methods, maps made using lidar reveal features of the land surface that are masked by digital photographs taken in densely vegetated or forested areas.
Lidar DEMs have been used to locate previously unknown faults, map pyroclastic flow deposits and groundwater. Most lidar scanners use near-infrared lasers that are absorbed by water, allowing scientists to map the characteristics of surface waters such as springs, rivers and lakes.
Highway 504 is shown in upper right.
Lidar images of Shastina cone, west flank of Mount Shasta, California. Details of lava flows and other surficial features are best seen in the image to the right with vegetation removed.
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