Surveyor and mathematician.
Moved from Pennsylvania to Ellicott's Mills, Maryland, where his father and uncles had founded a colony.
Joined the Maryland militia (1778); rose to the rank of major.
Published a series of almanacs under the title The United States Almanack. Surveyed the boundary between Virginia and Pennsylvania, west of the unfinished Mason and Dixon Line (1784). Served a term in the Maryland legislature (1786). Member of the American Philosophical Society.
Appointed by the Executive Council of Philadelphia to run Pennsylvania’s western (1785) and northern (1786) boundaries, and to survey the islands in the Ohio and Allegheny rivers (1788).
Moved to Philadelphia (1789). Made the earliest topographical study of the Niagara River. Commissioned to survey Washington, D.C. Ran the boundary between the United States and Florida (1796). Secretary to the Pennsylvania Land Office. Appointed Professor of Mathematics at West Point (1813).
Born in Bucks County, Pennsylvania. Married Sarah Brown in 1775; nine children.