An initial point is defined by the intersection of a principal meridian and a base line.
For example, the Fifth principal meridian in the new Louisiana Purchase was established in 1815, with its southern end based on the confluence of the Arkansas and Mississippi Rivers, and extended northward from that point.
The eastern end of the base line was chosen as the confluence of the St. Francis and Mississippi Rivers and extended westward.
The initial point would be established where the meridian and the base line crossed, which turned out to be in the middle of a swamp in eastern Arkansas.
[3] Many of the initial points in the United States have been listed on the National Register of Historic Places.