"Surveys are used as a basis for the land descriptions in deeds. A deed is the legal document describing a certain piece of land... . So if everyone used a different method of surveying for describing that land, no one would be sure who owned what."
Lewis & Clark practiced the art of surveying while at the confluence of the Ohio and Mississippi Rivers (Cairo, Illinois). The Smithsonian has a picture of a compass used by the expedition online here.
The above sign mentions that "because the 3rd Principal Meridian begins at the mouth of the Ohio, astronomical observations at this point were crucial." Here's a map in granite depicting the 3rd Principal Meridian. An explanation of meridians can be found on my blog here.