Many different horizontal reference datums exist, but in the United States only three datums are commonly used:
The North American Datum of 1927 (NAD27) uses a starting point at a base station in Meades Ranch, Kansas and the Clarke Ellipsoid to calculate the shape of the Earth.
The North American Datum of 1983 (NAD83) was developed when satellites enabled a better model. Depending on one’s location, coordinates obtained using NAD83 could be hundreds of meters away from coordinates obtained using NAD27.
The World Geodetic System of 1984 (WGS84) is identical to NAD83 for most practical purposes within the United States. WGS84 is the default datum setting for almost all GPS devices. But most USGS topographic maps published through 2006 use NAD27. This conflict in datums can cause big problems for the uninformed GPS user.
The primary purpose of the Historical Topographic Map Collection project is to make older USGS topographic quadrangle maps--those published between 1884 and 2006--available to the public. Scans of those maps are referenced to the spatial reference system of the original published map.
These maps have many uses for historical analysis but the USGS does not endorse using them for any current operational purpose.