Have you ever wondered why today’s towns are laid out the way they are? The look of a downtown area, with its square blocks, dates back to when the country was first dividing what would eventually become a large portion of the United States. Beginning in the late eighteenth century, these unexplored lands were surveyed under the Public Land Survey System, a rectangular survey system that required the platting of lands according to particular distance and marking them on plat maps. This replaced earlier systems that measured a given piece of land according to its landmarks, allowing for non-square parcels. The original city or village plat began with townships and sections of land, with each section being one-mile square, and each township containing 36 such sections, for an area measuring six miles square.

Historic plat maps are maps, drawn to scale, showing the divisions of the city’s land. These show the individual blocks, and usually the individual lots after the blocks were subdivided, for the purpose of selling land to settlers. This plat map was filed with the local government. Depending on the time of filing and the purpose of the plat map creation, the plat map might may kept at the local courthouse, the General Land Office, the local urban planning board or some other government entity. After filing, each lot of land could be referred to either by reference to its block and lot numbers as stated in the plat map, or to the portion of the section in which it lies, after the Public Land Survey System method. These plat maps may also be known as parcel maps, tax maps, landowner maps, lot and block survey system, or land survey maps.

Plat maps are usually drawn as part of the process of incorporating a town or city, although there are several other reasons why a plat map may be filed. If a particular landowner purchases several adjacent parcels of land and consolidates them into a single parcel for legal purposes, a Plat of Consolidation may be filed. This requires the landowner to have a survey done of the property and a plat map drawn. A Plat of Subdivision is the opposite process: a landowner is dividing land into smaller parcels, generally for the purpose of selling these parcels individually. Again, this process requires a survey and the drawing of a plat map.

When the plat map is filed, it is reviewed and approved by the local government. There may be multiple zoning rules governing the layout of a plat map; for example, in some areas, a plat map must contain a given amount of land set aside for green areas, schools, or other purposes besides residential or commercial use. The plat map must also designate roads and similar rights of way, and ensure that all parcels have access to a public right of way. In other words, there can be no parcel of land created in the middle of other lots with no access to a road or a right-of-way to access a road. This prevents the unethical process of selling landlocked parcels of land that can only be accessed via trespassing across other properties or via helicopter.