North American Datum of 1927
The North American Datum of 1927 (NAD27) is based on the Clarke Ellipsoid of 1866; it was created by manual surveying of the entire continent. The geodetic "center" of NAD27 is a base station at Meades Ranch in Kansas.
| Ellipsoid | Semimajor axis† | Semiminor axis† | Inverse flattening†† |
| Clarke 1866 | 6,378,206.4 m | 6,356,583.8 m | 294.978698214 |
†By definition. ††Calculated.
These are the defining dimensions for NAD27, but Clarke actually defined his 1866 spheroid as a = 20,926,062 British feet, b = 20,855,121 British feet. The conversion to meters uses Clarke's 1865 inch-meter ratio of 39.370432. In a practical sense, the length of a foot or meter at the time could not be benchmarked to better than about 0.02 mm, so the meter equivalents used now can be considered exact to well beyond the uncertainties in the lengths and conversion values of the time.
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