NAD27

NAD27, the North American Datum of 1927, is a geodetic datum that served as the reference frame for surveying and mapping across North America for much of the 20th century. It is based on the Clarke 1866 ellipsoid and a fixed origin defined by the Meades Ranch Triangulation Network, anchored in Kansas. The Clarke 1866 ellipsoid has a semi-major axis of 6,378,206.4 meters and a reciprocal flattening of 294.9786982. NAD27 is a regional, non-geocentric datum rather than a global, earth-centered reference system.

Coordinates in NAD27 were commonly expressed in latitude and longitude on the ellipsoid and in projected systems such as the State Plane Coordinate Systems, typically using US survey feet. The Meades Ranch anchor provided a consistent North American reference frame for mapmaking from the early 20th century onward. With the advent of NAD83 in the late 20th century, NAD27 was progressively superseded as a primary datum, offering a geocentric framework and improved alignment with modern satellite data.

Transformations between NAD27 and NAD83 exist to support data exchange, including grid-based methods such as NADCON and various era-specific transformation tools. These conversions account for regional shifts that can amount to tens to hundreds of meters, depending on location.

In summary, NAD27 was the standard North American datum from 1927 until its replacement by NAD83, rooted in the Clarke 1866 ellipsoid and the Meades Ranch Triangulation, and it remains relevant for legacy maps and historical geospatial datasets.