What are NAD27 and NAD83?
The literal answer to this question is that NAD27 and NAD83 are two datums in common use in North America (NAD is short for North American Datum). The more responsive answer is that most people use the terms to refer to two sets of projections used in the United States and Canada. The NAD27 Coordinate System is intimately linked to the spheroid (Clarke 1866) used in its projection computations, and the associated datum (NAD27) used to generate those lat/long values that are to be converted into grid coordinates. That is to say, only lat/long points calculated using the NAD27 datum should be converted to x/y’s using the NAD27 Projection System. But, lat/long’s that are calculated in the NAD27 system are valid only in reference to the NAD27 datum, and do not tie to lat/long’s referenced to any other datum (i.e. everywhere outside of North America). It is now possible, with satellite data, to create a spheroid/datum that is valid throughout the entire World. That datum is WGS84, which is
