For states, where no SPCS legislation was found, the online control point lookup application at the NGS website was used to find the control data sheet for a control point near the county centroid. The coordinate system reference on the data sheet was used for the zone assignment. The U.S. Census Bureau's counties.txt file was used to obtain the county names and centroids. The counties.txt file was also used for comparison to verify completeness, in that a reference was made for each county or equivalent entity.
The NGS publication "State Plane Coordinate System of 1983 - NOAA Manual NOS NGS 5" contained a map depicting the U.S. States and Counties along with the State Plane Zone boundaries for the 1983 coordinate systems. The zone assignments on this map were compared with those determined by statute and differences are noted as follows: 1) Pulaski County, AR, is clearly shown to be aggregated within the South Zone, as depicted in the Manual. This county is included with the North Zone based on the Coordinate System legislation ( 15-21-302 ). 2) Valencia County, NM, is clearly shown to be aggregated within the West Zone, as depicted in the Manual. This county included in the Central Zone based on the Coordinate System legislation ( 47-1-49 ).
A similar map for the 1927 Coordinate Systems is no longer available from the USGS or NGS. Portions of an existing map were copied so that the 1927 Zone assignments could be identified for Nebraska and South Carolina.
The UTM Zone assignments shown in the spreadsheet are based on the location of the county centroid.