There are two forms of State Plane 1927: State Plane 1927 and State Plane 1927 Exact. State Plane 1927 is the first State Plane system developed for the United States. It is based on tables and coefficients for each zone in each state. The State Plane 1927 Exact is a revised version of the original State Plane 1927 system. It uses pure projection equations to calculate points for conversion.