At the point where U.S. Highway 51 and the Jefferson-Marion County road meet, the Third Principal Meridian intersects its east-west base line, a cardinal point established by a Federal surveyor on February 1, 1815. At least 60 percent of the land in Illinois is measured from and identified by these two coordinates. In the original thirteen states, land had been measured by metes and bounds using known landmarks and compass points, but that system proved inaccurate and impermanent. As a result, the Jefferson Committee on Public Lands devised the rectangular method in 1784, and it became law the following year with the Land Ordinance of 1785 for government lands not yet surveyed northwest of the Ohio River. The ordinance directed surveyors to divide the territory into townships of six miles square by north-south lines crossed at right angles by east-west lines. Each new survey had to be tied to a principal meridian and its base line. The First Principal Meridian governed land mostly in the Ohio country, the Second mostly in Indiana, and the Third, running due north from the confluence of the Ohio and Mississippi rivers, in Illinois only. Surveyors' townships are numbered north and south of a base line and placed in ranges numbered east and west of a meridian. Unless a township is fractional, it is further divided into 36 sections, each measuring one mile by one mile, and a full section contains 640 acres.