Modeling topography on active volcanoes is unlike doing so in any other setting, because dramatic changes can occur on timescales far shorter than a human lifetime. For example, in 2018 at Kilauea, approximately 1 cubic kilometer of rock volume (0.25 cubic miles) was lost at the volcano’s summit and deposited on the lower East Rift Zone. So, topographic models can become outdated relatively quickly, and we need to update them accordingly.
From an article in West Hawaii Today.
Due to the vastness of the areas to be modeled, measuring elevations on the ground using GPS is not feasible, except as a verification of other measurement techniques. Therefore, remote sensing — the measurement of ground features from the air — is preferred.
For many decades, aerial photography was the preferred remote sensing technique that the USGS employed in modeling topography. Overlapping aerial photos taken from slightly different positions along a flight line can be viewed to make the observer think they are seeing a three-dimensional scene rather than a couple two-dimensional images, similar to how the two human eyes sense depth. Using a large and complicated instrument called a stereoplotter, a user could draw outlines of lava flows and lines of equal elevation, or contour lines, on such a projected scene.
Aerial photography began to fall out of favor near the end of the 20th century as new technologies emerged to replace it. As such, the last aerial photography surveys to cover the entire Island of Hawaii were completed in the early 1980s. Years later, the data were compiled into a digital elevation model (DEM) of the island at 10-meter (yard) resolution, which is still being used today.
However, Kilauea does not always cooperate with geographers’ desire to maintain current topographic data. When the Puu Oo eruption started on Jan. 3, 1983, it marked the onset of 35 years of near-continuous topographic changes at Kilauea that reached a climax in 2018. Some reasonably successful remote sensing occurred during this time, most notably a National Oceanic and Atmospheric Administration (NOAA) survey in 2005, but with an ongoing eruption it is inevitable that some of the data will be instantly rendered obsolete.
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