The Point of Origin is located in Campus Martius Park in downtown Detroit. The park’s website explains:
In 1788, Campus Martius served as a drill ground for militia training. Campus Martius means “military ground” and was named after the Campus Martius at Marietta, Ohio, a 180-foot stockade.
After the fire of 1805, Judge Woodward was appointed to oversee the plan to lay out the streets, squares and lots with the assistance of the best surveyors from Canada. They placed their instruments and astronomical devices on the summit of a huge stone. He viewed the planets and meteors in order to determine “true North”. Today we still call this point the “Point of Origin” which is located in center of Campus Martius at the junction of Woodward and Monroe. It is from this point that the City of Detroit’s Coordinate system was created.
FYI, this photo appeared on Rolling Past 5000 on Michigan in Pictures, but only in a bit part as one of the zeros!