These are actually survey markers, or more commonly benchmarks, installed by the National Geodetic Survey.
Geodesy, or geodetics, is the study of the representation of earth.
You’ve probably seen surveyors before, though you may not have realized. Often you will see them standing on the sides of roads, with contraptions sitting atop what are essentially very tall yellow tripods. These tools are actually highly precise and highly accurate tools used for mensuration – literally the study of measurements (typically geometric measurements, but there are all types of mensuration, really).
So when you put those two things together, the National Geodetic Survey is essentially a governmental organization that collects very precise distances, angles, and areas at various points on the earth’s surface (at least within the US), and it uses those measurements to create 2D and 3D representations of the earth (again, within the US).
These measurements and representations are used to create topographic maps, plan infrastructure, make all sorts of maps, and generally make life easier for everyone everyday. A lot of work goes into making life in the 21st century more navigable.
Benchmarks are used to ensure that measurements are taken from the same location. You’ll notice that in the top photo, there’s a triangle on the benchmark. This indicates that the benchmark is used for triangulation, and you might find other “reference” benchmarks around it that act as reference points. Often, they have arrows pointing to the triangulation benchmark, and they can come in very handy if something happens to the more important benchmark (e.g., vandalism or removal, which are illegal and carry a heavy fine).
You’ll find benchmarks at the tops of many mountains because, as you can imagine, being the highest point around can come in very handy for mapping elevation. You don’t always see them, though, and I try to take photos of them when I can, because I’m a mapping nerd. People also photograph them as proof that they summited. I have been to the top of Observation Point in Zion National Park TWICE now, and I’ve forgotten to photograph the benchmark each time! But you can see it peaking out in the lower right-hand corner of this photo:
Anyway, you can find benchmarks all over the place. Literally anywhere in the US. They aren’t always these cast, embedded metal discs, though. Sometimes they are obelisks or cairns. They’re fun to look for.
If you’ve ever heard of geocaching (a sort of treasure hunting that you do, using geographic coordinates to find the cache), there’s a similar game on the geocaching website for people who like to hunt benchmarks. If you put in your postal code (or the nearest postal code to the area in which you’d like to go benchmark hunting), it will list of all Geodetic Survey database items for that area with geographic coordinates.
If that’s too much work for you, here’s a website that will return a KML file (point file) displayed in a Google Earth plugin. Here are all of the benchmarks for the Salt Lake City area as an example:
I selected Dale Peak, the benchmark I visited yesterday (and seen in the first photo of this post), but I also see a bunch of benchmarks by my house in the valley! Because civil engineering.
Anyway, benchmark hunting is fun. Geocaching is fun. Max might be into an active, real life “treasure hunt” of sorts! Usually the cache is a small, weather proof container, and you leave a note and a little trinket with which to replace the one that you take.
If you don’t have a handheld GPS, you can use your cell phone! Apps probably cost money (like the Geocaching.com-endorsed app), but you could also use the free Google Earth app and files saved to your Google account through custom maps in Google Maps. Or just plan ahead and wing it. Because adventure!