By definition a plat is considered a plan or map showing property boundaries and geographic features. It is also said to be a small plot or area of land.
Plat books make use of the Public Land Survey System (PLSS) to represent land ownership patterns on a county-by-county basis. They are a convenient reference for local governments, private realty, surveying companies, and the general public. Commercial plat books for the State of Michigan are generally prepared by private map publishing companies in conjunction with county governments. However, recently counties have begun to update the maps themselves.
Plat books are prepared from a variety of source data including U.S. Census TIGER (digital line) files, historical local government tax parcel maps, aerial photography, and property descriptions. These records are used to produce a generalized representation of current land ownership patterns, along with geographic features, such as roads, water bodies, and railroads.
Public and private property depicted on these plats is not considered a full or legally accurate representation of ownership interests or extents, but is a generalized picture, or snapshot-in-time, of the last deed filed on a property at the time the County Register of Deeds' records were searched. The maps then remain static until they are updated some time later. A "last deed" depiction represents only the last document filed on the property and does not necessarily reveal or resolve full ownership by title or discrepancies in the record.
The State of Michigan began maintaining a file of all plats in the state in 1873. A copy of all plats prepared after that date were filed with the State. Beginning in 1909 and completed in 1911, a copy of all plats on file with the 83 county Register of Deeds and recorded prior to 1873 were duplicated and the duplicate copies incorporated into the State's files. This system includes all plats in the State of Michigan beginning with the plats created under the 1821 territorial act for recording town plats. Information obtained from Michigan.gov website