What Is A Benchmark?
If you have ever used a map, chances are that map was made with the help of benchmarks. Also known as survey markers or survey monuments, benchmarks are objects used to mark key points on the Earth’s surface. Throughout history they have been used for navigation, to indicate land borders, and for mapmaking across the globe.
Historically, anything from rock towers and cliff carvings to clay pots has been used as a benchmark. More modern benchmarks are usually metal disks set in stone, on top of concrete pillars, or otherwise fixed to the ground.
Before GPS-based surveying, benchmarks could be used to map areas using triangulation. The benchmarks would be set in groups, with the main one (also known as a triangulation station) in the middle. The other benchmarks would be arranged in a circle at a set distance around the first one. This setup could be used to find the angles and distances between different landscape features using trigonometry. (source)
Today, there are two major types of benchmarks: vertical control points and horizontal control points. Both are metal disks embedded into a surface, and they look very similar. The key difference is that horizontal control points have longitude and latitude coordinates engraved on them, while vertical control points also contain elevation above sea level. (more info on coordinate systems here)
You might be wondering how benchmarks could still be useful when modern surveying is dominated by GPS-based methods. The truth is, there are lots of ways benchmarks are still used by surveyors today. For example, to save costs, surveyors may choose to run their own RTK base station. To do this, the surveyor must tie into an existing benchmark by collecting a data point on it. Sometimes weather, tree cover, or cost can prevent the use of GPS equipment. In these cases, benchmarks are still used as a known location to place a different survey device, like a total station. (source)
How To Find Benchmarks
There are hundreds of thousands of benchmarks recorded over the last two centuries in the US alone. Benchmarks are everywhere, but how do you find out how many are in a specific area and where? In the USA, benchmarks can be found using a database called the National Geodetic Survey Data Explorer. Each benchmark is given a unique identification code and recorded along with its latitude, longitude, and elevation if included. Other countries have similar databases that can be used to locate benchmarks. (source)