Which spheroid was the first United States datum based on?

The Clarke 1866 Spheroid was the first United States datum based on a spheroid model. Introduced in the mid-19th century, it was significant for its use in mapping and navigation within the United States, particularly for enabling more accurate representations of the Earth's surface. This spheroid was developed by the American mathematician and geodesist Alexander Keith Clarke, and it provided a mathematical model to describe the shape and size of the Earth.

The adoption of the Clarke 1866 Spheroid helped advance the field of geodesy in the United States, establishing a standardized frame of reference for various mapping projects and surveys. Its influence continued until later spheroids, such as the GRS80 and the International 1924 Spheroid, were developed, though the Clarke 1866 remains foundational in the history of geospatial science in the U.S.