to define the western border of Pennsylvania with the Ohio Country. This "Ellicott Line" (running north–south at longitude meridian 80°31'12" W) later became the principal meridian for the surveys of the future Northwest Territory of the United States. His work in Pennsylvania intensified his ties with Rittenhouse and other members of the American Philosophical Society, to which he had been elected a member in 1785, and led to encounters with Benjamin Franklin and Simeon De Witt. When he was subsequently appointed to lead other surveys in Pennsylvania, the family moved again in 1789 to Philadelphia. By recommendation of Franklin, Ellicott got a position with the newly established government under the Constitution and was tasked by first President George Washington to survey the lands between Lake Erie and Pennsylvania to determine the border between Western New York and U.S. federal territory, resulting in the Erie Triangle. This survey, during which he also made the first topographical study of the Niagara River including the Niagara Falls, gained Ellicott a reputation for superb accuracy in surveys. From 1791 to 1792, at the request of Secretary of State Thomas Jefferson, Ellicott worked under the direction of the three commissioners that President George Washington had appointed, surveying the boundaries of the federal Territory of Columbia, which would become the District of Columbia in 1801, containing the Federal City also then becoming known as "Washington City". He was assisted in this survey first by the free African-American astronomer Benjamin Banneker and then by Ellicott's brothers, Joseph Ellicott and Benjamin Ellicott. Ellicott's team put into place forty boundary stones approximately 1 mile (2 km) apart from each other that marked the borders of the Territory of Columbia of (see: Boundary Markers of the Original District of Columbia). Most of these stones remain in their original positions. As engravings on many of the stones still show, Ellicott's team placed those that marked the southwestern /southeastern border with Virginia in 1791, and those that marked the northwestern / northeastern border with Maryland in 1792. On January 1, 1793, Ellicott submitted to the three commissioners "a report of his first map of the four lines of experiment, showing a half mile on each side, including the district of territory, with a survey of the different waters within the territory". The Library of Congress has attributed to 1793 Ellicott's earliest map of the Territory of Columbia that the Library holds within