Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from R. Greg Vaughan, U.S. Geological Survey research scientist, and Brett Carr, U.S. Geological Survey Mendenhall postdoctoral fellow.

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Yellowstone's thermal areas are some of the most dynamic landscapes in the Park, with frequent changes in not just activity, but also in shape and size. How can such changes be monitored given the large number and vast expanses of thermal areas in Yellowstone? It turns out that a digital camera and a view from the sky can do the trick!

Like most animals, humans have 2 eyes, which we use together to sense information about our surroundings. Because our eyes are located at different places on our heads, each eye sees objects from a slightly different point of view, resulting in two slightly different images. Our brains quickly process those two images into one. This binocular vision gives us the ability to perceive the depth and 3-dimensional structure of the world around us. 3-dimensional depth perception that results from 2 eyes looking at the same scene from different positions is called stereopsis. And it works because the apparent position of an object viewed along two different lines of sight differs based on how far away the object is from the observer. Nearby objects show a larger apparent displacement than farther objects, and we can use this to judge distance. You can demonstrate this to yourself by looking outside: hold your head still, close one eye and then the other, back and forth repeatedly. Objects that are closer to you appear to move back and forth a lot; objects that are farther away appear to move very little.

While stereopsis is natural when looking at a 3-dimensional scene with 2 eyes, it can also be simulated when looking at 2-dimensional photographs. Just as 2 eyes that view the world from slightly different perspectives allow us to perceive distance, two single images of the same location from different perspectives can be combined to perceive depth of objects at different distances. Stereoscopy is a method that shows two different images of the same area to each eye separately. These images can be viewed with a stereoscope, which shows the left image to the left eye and the right image to the right eye. Some people have the ability to view stereo-pair images just by changing their focus. Using this approach, it is possible to calculate distances from photographs, providing the basic data needed to make a map!

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If you were flying in a plane or helicopter and looking down at the Earth's surface, the tops of hills would be closer to you than the bottoms of valleys. The varying distances between different places represent Earth's topography (the shape of the land), and you would be able to see this clearly with 2 eyes. We can simulate this same effect using a regular camera!

If you have a series of overlapping pictures of the same area taken from slightly different positions, then you can reconstruct the topography of the surface. Using modern camera and computer technology, a technique called Structure from Motion (SfM) photogrammetry does just this. SfM uses a series of overlapping 2-dimensional images that are acquired from a moving platform, like a plane, helicopter, or unoccupied aerial system (sometimes called a UAS or drone) to determine the 3-dimensional structure of the surface, which is called a digital elevation model (DEM).

The technique ingests hundreds (sometimes thousands) of overlapping images and automatically finds common features among the images to "stitch" them together into a seamless mosaic. Then it determines surface elevation variations based on the different perspectives from each image. With additional information about the elevation of distinct features in the area—for example, a road intersection—this process is much more quantitative than what you can determine with your eyes alone.

So, what does all this have to do with detecting and monitoring changes in Yellowstone?

It turns out that the topography of Yellowstone's thermal areas changes rapidly over time. At Mammoth Hot Springs, for example, travertine deposits can accumulate at a rate of up to 1 meter (about 3 feet) each year!

In 2013, a series of digital photos over Mammoth Hot Springs was acquired from a helicopter. Many of the images overlapped, covering the same areas but from different perspectives. In 2016 a similar series of photos was acquired. Using SfM, YVO scientists put all the images together into a mosaic and generated DEMs of the lower terraces at Mammoth Hot Springs.

By comparing the DEMs from 2016 and 2013, they measured how much the travertine terrace had grown. They found that surface elevation increases of up to 2 meters (over 6 feet) from 2013 to 2016 occurred in the outflow region downslope from Palette Spring, indicating travertine growth in this area.

This experiment showed that images acquired using inexpensive cameras from airborne platforms can be used to monitor surface changes in thermal areas over a variety of spatial and temporal scales. Given the importance of preserving and protecting thermal area resources—one of the mandates of Yellowstone National Park—regular aerial monitoring using SfM provides important data that might not otherwise be possible!

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