Each GIS layer is like one plastic overlay. Where before GIS, you had to look at different paper maps, each probably at a different scale and potentially different projection, now using GIS, you can overlay all these different maps easily to find the relationships betrween the different features you are researching. Computerized Geographic Informations Systems are useful in analyzing data about land uses, migration and settlement patterns, and trails and roadways. Aerial photographs, historic maps, and current maps can be compared using GIS techniques to determine the nature and extent of land use changes through time. A GIS-based cartographic database can be both continuous and scale free, creating a standard scale for maps and photos having different scales.
Pattern recognition is something most people excel at.
There is a vast difference between seeing data in a table of rows and columns and seeing it presented in the form of a map. GIS gives the historian the layout and drawing tools to present facts with clear, compelling documents.