4. Distortion

The maximum ellipsoid to grid distortion for SPC 27 was 1/10,000. This limited individual zone extents to around 160 miles necessitating multiple zones in most states. At the time of their original design, 1/10,000 was considered sufficient as most surveys (done with tape and transit) were below that standard.

By time of NAD 83 adoption, typical surveying accuracy easily exceeded 1/10,000 and because distortion is systematic, some states combined multiple SPC 27 zones into fewer SPC 83 zones. Although 1/10,000 was still a nominal design standard for SPC 83, it became less of an issue.