Andrew Ellicott (January 24, 1754 – August 28, 1820) was an American land surveyor who helped map many of the territories west of the Appalachians, surveyed the boundaries of the District of Columbia, continued and completed Pierre (Peter) Charles L'Enfant's work on the plan for Washington, D.C., and served as a teacher in survey methods for Meriwether Lewis.

Young Andrew was educated at the local Quaker school, where Robert Patterson, who later became a professor and vice provost at the University of Pennsylvania in Philadelphia, was his teacher for some time.

In 1770, his father, together with his uncles Andrew and John, purchased land on the falls of the Patapsco River, upriver and west of Baltimore and the Chesapeake Bay.

When the Revolutionary War broke out in 1775, Andrew enlisted as a commissioned officer in the Elk Ridge Battalion of the newly organized Maryland state militia despite his Quaker upbringing.

In this survey, he worked alongside David Rittenhouse and Bishop James Madison, making first connections with the scientific society of Philadelphia.

[14][15] Ellicott stated in his letters that, although he was refused the original plan, he was familiar with L'Enfant's system and had many notes of the surveys that he had made himself.

[18] Ellicott gave the first version of his own plan to James Thakara and John Valance of Philadelphia, who engraved, printed and published it.

[17] After L'Enfant departed, Ellicott continued the city survey in accordance with his revised plan, several larger and more detailed versions of which were also engraved, published, printed and distributed.

He spent the next two years with this task, plotting a road from Reading, Pennsylvania, to Presqu'Isle, where the port city was to be built, and also supervising the construction of Fort Erie.

Ellicott travelled with a military escort from Pittsburgh via the Ohio and Mississippi rivers and worked together with Spanish commissioners, despite many difficulties, for the next four years.

[20] A Mississippian, possibly a descendant of Isaac Guion, hinted at some of the difficulties Ellicott had in the unfamiliar political and social climate of pre-territorial Mississippi, writing, "Ellicott was a Pennsylvania quaker by birth and education; by some knowledge of natural history, astronomy and botany, he had recommended himself to Mr. Jefferson; but he was vain, weak, ambitious and obstinate, and wholly unfit for a position that demanded something more than civil engineering.

"[21] Another "Ellicott's Line" from this survey, running along latitude parallel 31° North, still defines the border today between the future states of Alabama and Florida.

Ellicott showed considerable diplomatic talent during this joint project to bring it to a successful completion, and he presented his final report with maps to the government in 1800.

His prior negative experiences with the government political administrations may have had something to do with this,[24] but at the age of 49, he also wanted to spend more time with his family and feared that this new position might require him to travel too much.

Ellicott confirmed earlier findings that the border, which was supposed to follow latitude 35°N, was several miles further south than the Georgians claimed.

In 1817, Ellicott was again called upon to participate as astronomer in a field survey, namely a re-survey – agreed upon in the Treaty of Ghent – of the Collins–Valentine line.