Editor’s note:The Yellowstone Caldera Chronicles is a weekly column written by scientists and contributors to the Yellowstone Volcano Observatory. This week’s contribution is from Michael Poland, geophysicist at the US Geological Survey and scientist-in-charge of the Yellowstone Volcano Observatory.

On the tops of many mountains and along many roads in the US are small brass discs called landmarks. These survey points are essential for mapping the landscape, determining boundaries, and documenting change, and there are hundreds of them in Yellowstone National Park!

If you’ve hiked in Yellowstone—or really anywhere in the woods or mountains in the US, especially in the West—you’ve likely encountered brass or aluminum discs that are set into rock outcroppings or sometimes into concrete. These are called landmarks and provide a basis for mapping the landscape of the country.

Benchmarks are reference points with precise and known vertical elevation and/or horizontal position. The altitude or position is usually set relative to another reference, for example, sea level or a baseline of latitude or longitude. Surveying methods such as leveling and triangulation are used to determine the elevation or position of a mountain peak relative to the original reference. A reference point can then be installed on the mountain to serve as a datum for that specific elevation and/or horizontal position.

Classic disc-like benchmarks are usually about 3.5 inches (9 centimeters) in diameter and are usually stamped with identifying information—for example, the name of the station, the year it was installed, and sometimes the precise elevation. The organization that installed the marker is usually listed, and some landmarks note that disturbing the mark can result in a “$250 fine or jail time.” The amount of this fine, specifically for disturbing US Coast and Geodetic Survey (now National Geodetic Survey) waypoints, was set over 100 years ago and in today’s dollars would be over $4,500!

Benchmark T366, was installed in 1987 a few miles north of Canyon Junction in Yellowstone National Park. Instead of being a brass or aluminum disc, the marker is a rod that has been hammered into the ground until it sinks lower. The precise height of the shaft tip was determined by surveying methods, and a ground-level access cap was installed so the shaft would not be inadvertently disturbed/USGS, Michael Polonia, June 26, 2002.

Not all benchmarks are discs. Some are rods that have been “pushed to the curb,” meaning they have been hammered into the ground until they 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 are not inadvertently disturbed by, say, a snow plow. On rare occasions, landmarks are symbols, such as crosses or squares, that are chiseled directly into a rock outcrop or boulder.

Once established, benchmarks serve a variety of purposes. For example, 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 borders, such as map lines that separate states, or different 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.

I am hundreds of landmarks in Yellowstone National Park. The oldest is a block of granite near Lake Lodge, which was established in 1892–1893, with its position established by astronomical observations (although the block has been moved an unknown amount since it was originally placed). Numerous brass disc markers 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 easy to see today, for example, on a concrete pier near the front steps of the Albright Visitor Center in Mammoth Hot Springs. Additional markers were installed along the roads and on the mountains in the decades that followed, including during another major road study in 1975-1977. Comparison of the shares in that survey with those of 1923 showed that The Yellowstone caldera rose about 28 inches (72 centimeters) during that time.

Benchmark C9, installed by the US Coast and Geodetic Survey (now the National Geodetic Survey) in 1923 near Apollinaris Spring in Yellowstone National Park. The number stamped in the marker, “7337,580,” is the elevation in feet that was determined by surveys in the year the reference point was established/USGS, Michael Polonia, September 4, 2024.

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

Today, new tools for measuring ground deformation have largely replaced methods that require physical landmarks, and many of the markers installed in the 1900s have been destroyed by road construction and occasional vandalism. But the remaining brass discs and other survey points in Yellowstone National Park are still sometimes used to establish checkpoints for road construction and even gravity surveys that are designed to measure changes in groundmass over time.

The next time you come across a landmark, whether it’s on top of a mountain, a concrete post on the side of the road, or a sidewalk or bridge, take a moment to take a look. Is there a year or quota written on the disc? Which organization installed the brand? In Yellowstone, these signs are an important part of park history—not only for establishing boundaries and infrastructure, but also for documenting changes associated with magmatic and hydrothermal activity over the past century.