Editor's note:Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Michael Poland, geophysicist with the U.S. Geological Survey and Scientist-in-Charge of the Yellowstone Volcano Observatory.

At the tops of many mountains and along numerous roads across the USA are small brass disks called benchmarks. These survey points are critical for mapping the landscape, determining boundaries, and documenting changes, and there are hundreds of them in Yellowstone National Park!

If you’ve gone hiking in Yellowstone—or really anywhere in the forest or mountains of the USA, especially out west—you’ve probably come across brass or aluminum disks that are set in rock outcrops or sometimes concrete. These are called benchmarks, and they provide a foundation for mapping the landscape of the country.

Benchmarks are reference points with precise and known vertical elevation and/or horizontal position. The elevation or position is usually established relative to some other reference, for instance, sea level or a latitude or longitude baseline. Surveying methods, like leveling and triangulation, are used to determine the elevation or position of a mountaintop in relation to the original reference. A benchmark can then be installed on the mountain to serve as the data point for that specific elevation and/or horizontal position.

Classic disk-like benchmarks are typically about 3.5 inches (9 centimeters) across and usually stamped with identifying information—for example, the station name, the year it was installed, and sometimes the precise elevation. The organization that installed the marker is also usually indicated, and some benchmarks note that disturbing the mark can result in a “$250 fine or imprisonment.” The amount of this fine, specifically for disturbing U.S. Coast and Geodetic Survey (now the National Geodetic Survey) benchmarks, was established over 100 years ago and in today’s dollars would be more than $4,500!

Not all benchmarks are disks. Some are rods that were “driven to refusal,” meaning that they were pounded into the ground until they would sink no further. These rods often have a hinged cover, like a small door, placed over them and are right at ground level, so that they aren’t inadvertently disturbed by, for example, a snowplow. In rare occasions, benchmarks are symbols, like crosses or squares, that are chiseled directly into a rock outcrop or boulder.

Once established, benchmarks serve a variety of purposes. For instance, they can be used as base stations from which other locations can be surveyed, establishing precise elevations and positions for other mountains or places of interest. This is extremely important when mapping boundaries, like the map lines that distinguish states, or various types of land, or even property owned by different people or entities. Benchmarks are also frequently used as reference points during construction, especially for roads and infrastructure.

There are hundreds of benchmarks in Yellowstone National Park. The oldest is a granite block near the Lake Lodge that was set up in 1892–1893, with the position established by means of astronomical observations (although the block has been moved by an unknown amount since it was originally placed). Numerous brass-disk benchmarks were installed throughout the park in 1923 as part of a major survey to map elevations along the developing road network. Some of these markers are easily viewed today—for example, on a concrete pier near the front steps of the Albright Visitor Center in Mammoth Hot Springs. Additional markers were set up along roads and on mountains in the decades that followed, including during another major road survey in 1975–1977. Comparing the elevations from that survey to those from 1923 revealed that Yellowstone Caldera had uplifted by about 28 inches (72 centimeters) during the time spanned.

Benchmarks have also served as reference points for GPS surveys that have monitored ground motion in the region since the late 1980s.

Today, new tools to measure ground deformation have largely superseded methods that require physical benchmarks, and many of the markers installed in the 1900s have been obliterated by road construction and occasionally vandalism. But the remaining brass disks and other survey points throughout Yellowstone National Park are still sometimes used to establish control points for road construction, and even for gravity surveys that are designed to measure subsurface mass changes over time.

The next time you come across a benchmark, whether on top of a mountain, in a concrete post by the side of the road, or in a sidewalk or bridge abutment, take a moment to have a look. Is there a year or elevation stamped into the disk? What organization installed the mark? In Yellowstone, these marks are an important part of the park’s history—not only for establishing boundaries and infrastructure, but also for documenting the changes associated with magmatic and hydrothermal activity over the past century.