Like other historical maps, plat maps give genealogists a detailed snapshot of a community at a given time. They’re often labeled with landowners’ names, pointing to deeds and other land records. Other details might include the square footage for a plot (giving insight into an ancestor’s economic status), the crop grown or number of dwellings constructed on a plot.
Plat maps weren’t uniformly created, making them somewhat more difficult to find than other kinds of maps. The Bureau of Land Management, the federal body in charge of public land surveys, has digitized surveys and plat maps for several states, but the collection’s coverage is spotty. The David Rumsey Map Collection, Historic Mapworks and the Library of Congress collection offer some plat maps. Many county atlases, which you can find at large local libraries and state archives, are based off of plat maps and therefore contain similar information. A Google search for plat map and the name of your ancestor’s town, township, county or state should turn up links to resources.