Coordinate reference system in 1880 US Census

The “xgps” and “ygps” variables in the 1880 US Census are said to represent the longitude and latitude of geocoded households. But I’ve been unable to find the Coordinate Reference System corresponding to these variables. Which CRS are the variables “xgps” and “ygps” expressed in?

Thanks for your help!

According to the Urban Transition Historical GIS project technical documentation, geographic coordinates were created based on US Census Tiger Files (2000-2005). The metadata for these Tiger Files indicate that they use the North American Datum 1983 (NAD83) Geographic Coordinate System for the 48 contiguous states, the District of Columbia, Alaska, Puerto Rico, and the Virgin Islands. Regional datums were used for Hawaii and the Pacific Island Areas.

Thanks, Grace!