A commissioned officer in the Maryland militia, Major Andrew Ellicott was a highly accomplished surveyor who, along with Pennsylvania's David Rittenhouse, extended the Mason-Dixon line westward to its originally intended terminus at the southwest corner of Pennsylvania in 1784. English surveyors Charles Mason and Jeremiah Dixon had been forced to halt their work at the 244-mile point in 1767 due to the threat of hostile Native Americans. The following year, Ellicott was hired to revisit this survey in order to establish and mark 100 miles of the western boundary of Pennsylvania, a line that came to be known as "Ellicott's Line".In 1789, Virginia and Maryland had joined together in donating territory to establish a new federal capital city on the banks of the Potomac River. At the suggestion of President George Washington, Secretary of State Thomas Jefferson asked Ellicott to perform the first survey of the District of Columbia.
Ellicott and his assistant, Benjamin Banneker, began work in the spring of 1791. The following year Washington asked Ellicott to finish Pierre Charles l'Enfant's plan for the city. L'Enfant was a military engineer appointed by President George Washington to plan the new nation's capital city in March 1791. He had been dismissed from the project because his perfectionism made him difficult to work with, however, his dedication to perfection shaped a plan that has stood the test of time. Ellicott found it necessary to make some changes to L'Enfant's plan. He changed the alignment of Massachusetts Avenue, eliminated five short radial avenues, added two short radial avenues southeast and southwest of the Capitol, and named the city streets. In less than one month Ellicott had a plan ready for the engravers. A few months later Ellicott, like l'Enfant, found himself at odds with the Commissioners and resigned from the project.