Cartography Word of the Day: Plat

They work “behind the scenes” to provide precise property plans

The map that is our Word-of-The Day this week, plat, is a map you won’t see without having to go to the local city or planning office, but it’s one that works behind the scenes to determine who owns what, and where everything from your fences to your streets go.

The Wikipedia defines “plat” thusly:

That is the basis, and it’s simple enough. To get an idea of just what it does for us, though, it’s best to get an actual look at one. The one we’re using to illustrate is a public document, a plat of a subdivision in moderrn Corvallis, Oregon, called “25th Street Terrace” (the entire online file is a TIFF which can be downloaded by clicking here; the entire online library of Corvallis plats are here.) We highly suggest doing so, as displaying such deatailed maps in this medium is fairly frought with loss issues. The illustration will give one a good idea of what it is they are looking at, however.

A portion of a plat map of the “25th Street Terrace” subdivision in Corvallis, Oregon (courtest Benton Country GIS)(Click to get a much better view)

As can be seen, the plat map offers a wealth of information about where property lines are, where rights-of-ways go, and how to find them, as well as legal authority. The plat map essentially forms the bridge between the Public Lands Surveys and the ground at the local level; the PLSS refers to the area, and the plat map refers to the PLSS, so that lot descriptions can be simpler because they only have to refer to the plat map document.

A plat map is significant in one other way: when legally incorporating a city government, one must be provided as part of the record, so some initial surveying is required. Due to shifts in authority over time the original plat filings of various cities will actually be maintained in places sometimes hundreds of miles away from the actual site. The plat documents for San Francisco, California, for example, are actually maintaned in Oregon City, Oregon; at the time of the filing, Oregon City was in fact the only incorporated city in the western United States, and had the only governmental recording authority.