The term "historic data" has two meanings, and both are immensely useful in CRM. The first meaning refers to data related to historic conditions. For example, knowing the location of river paleochannels or paleo-toolstone sources (derived from geological data) can aid in predicting cultural resource presence, or can explain its absence.
This type of historic data can be created for the purpose of evaluating a historic resource's significance and National Register Eligibility. For example, the creation of a dataset of historic land ownership parcels and subdivisions from the 1860s through the 1960s may aid in researching a site's history and understanding its affiliations with specific historic people or events.
The second meaning of "historic data" refers to data sources which are themselves old, obsolete, and historic. Such datasets include Government Land Office (GLO) maps, aerial imagery, Sanborn Fire Insurance maps, engineering or planning drawings, mining and mineral claim maps, real estate maps, and the like. All of these are useful in CRM, but in order to be useful, the data must be georeferenced. (Georeferencing is a term for the process of taking an image, and assigning it to geographic space--essentially "telling it" where in the world it belongs.) After georeferencing, the data can be used in a variety of capacites, including: designing field inventory surveys, selecting subsurface testing locations, or devising data recovery methods. These uses are related to cultural resource predictive modeling, which is further explained with text and visual examples here. The image below is an example of the use of a georeferenced Sanborn Fire Insurance map with the project footprint and Area of Potential Effect overlaid. This data was used in tandem with existing pavement and building data to select areas suitable for subsurface testing. (The intent of the testing was to determine the presence of substantial intact archaeological deposits related to the historic Hotel Cunningham. The testing encountered a brick foundation wall of the hotel, as well as a few artifacts, but none of the expected hotel-related artifacts were present, which was expected given the recent demolition history on the property.)
Incidentally, historic data analysis is not limited to the cultural resource discipline. Historic aerial photographs spanning several decades may be analysed to determine rates of deforestation, succession ecology patterns, or fire recovery for specific locations.