The Keith Line was the result of the East Jersey Surveyor-General's attempt to settle disputes "resulting from the 1676 Quintipartite Deed, which created the two territories". George Keith, born in Scotland in 1638, was a Quaker missionary who emigrated to America in 1684 with his wife, possibly because of years of "persecution and repeated imprisonments" in Scotland and England for preaching "about the 'errors' in the teachings of mainstream Christian churches". It is actually for his writings and for leading a schism in the Quaker movement that he is best known, rather than his activities as a surveyor.
It was the Governor of East Jersey, Robert Barclay who hired Keith as Surveyor General with instructions to survey the line between East and West Jersey which became known as the Keith Line. Barclay and Keith, both Quakers, had been close associates in Scotland. According to the article How a man named Keith took a long walk and defined N.J. forever,
Keith sought to complete the first surveyed boundary between the two provinces, which had been in dispute.... Keith carved a line from what is now Little Egg Harbor in Ocean County
to the south branch of the Raritan River in Warren County. He would
have gone farther, but by that point, many had started to claim that
Keith was woefully off-course.
The article adds:
Keith's endeavor, more or less, had been a failure (the line was
replaced by what came to be known as the Lawrence Line in the 1700s).
But despite this, the line he cut through the heart of what is now New
Jersey still stands today.
After this venture into surveying, Keith does not appear to have pursued this career path any further. He held various other jobs, including one as a teacher, but spent much of the rest of his life in dispute with other Quakers. He eventually became an Anglican and died in poverty in England in 1716.