He was then engaged to survey some property in western Pennsylvania which has been purchased by a group of Dutch investors, who had formed the Holland Land Company. He also extended the New York - Pennsylvania border westward. When the company purchased a huge tract of western New York (that became known as The Holland Purchase), Ellicott was hired in 1797 and was sent to perform the monumental task of surveying it. Ellicott spent two years (1798–1800) living outdoors in summer and winter, laying out the townships of the new land in order to complete the Great Survey of the land in October 1800.