The Elements of Geodesy: The Horizontal Datum

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 monuments. When the monument was located on flat ground, large towers such as this were often erected above them. This allowed geodesists to observe the monuments' accurate position over large distances.

Geodetic survey tower