PLSS Map

                                        
The PLSS system stands for the "public land survey system" and had been used for determining exactly what land can be spatially identified "for sale" by the use of land parcels. It addition, a lot of this land which is owned by the federal government will not be offered for sale but will be used as National Wildlife Refuges, Wildlife Management Areas, and Nature Conservatories. It was first developed in the nineteenth century to make the separation and dividing of land and "easier" task. As we can see in this map depicted by the illustration, areas of land are divided up into parcels by meridians that run north and south to help separate boundary lines which then designate land areas. 

http://nationalatlas.gov/articles/boundaries/a_plss.html