Before the Global Positioning System became available for use by the general public, geodesists would "connect" geodetic reference points together using a method called triangulation. This method required a "line of sight" between markers. When the monument was located on flat ground, large towers such as this were often erected above them. This allowed geodesists to observe the markers' accurate position over large distances.
Heard of geodesy? These videos from NOAA's National Geodetic Survey offer a deep dive into the science of knowing where you – and everything else is in the world!
The National Science Teaching Association (NSTA) includes this resource in its database. NSTA provides educators and students access to Web-based, educationally appropriate science content that has been formally evaluated by master teachers.