Fourth Principal Meridian, Illinois

This meridian was established in 1815, initially to survey a relatively small area, lands in the northwest corner of Illinois, mostly between the Illinois River and the Mississippi, that had recently been acquired from the Indians. The Initial Point was originally established on the banks of the Illinois River, near Beardstown, but erosion, followed by engineering of the river channel, erased any physical evidence of it. The site has never been re-monumented.

2335 The Initial Point is shown on this USGS topo map of the Beardstown, Illinois area. The red dashed east/west line starting on the left margin of the map is the base line (it extends westward, eventually to its point of origin on the Mississippi River.) It is the line between Township 1 North, and  Township 1 South, as indicated by the “T1N” and “T1S”. The north/south line emerging from the top margin and extending to the middle of the map is the Fourth Principal Meridian, and the line between Range 1 West and Range 1 East is indicated by the “R1W” and R1E” on either side of the line. The intersection of these two lines is the Initial Point. Also note the irregularity of the square-mile sections where the already-established survey grid for the rest of Illinois, emerging from the southeast, meets the newer survey north and west from the Initial Point. This unconformity is common where surveys of large areas meet, as imperfections and variations are inevitable when a mathematically consistent grid of one square mile sections is applied over a wide area, since small errors in measurement are compounded, and since longitudinal lines converge towards the poles, so corrections have to be made to the grid periodically.

2333 The northwestern corner of Illinois (in yellow) was surveyed by the Fourth Prinicipal Meridian, while the rest of the state (in red) was surveyed ten years earlier by the Third Principal Meridian. The Fourth Principal Meridian continues north into Wisconsin, and was used for the survey of that region as well.