Mastering the Government Rectangular Survey System for the Illinois Exam

Last updated: April 2026

Whether you are preparing to sit for your broker license or simply brushing up on your property law fundamentals, understanding legal property descriptions is non-negotiable. For Illinois real estate professionals, the Government Rectangular Survey System (also known as the Public Land Survey System or PLSS) is the bedrock of how land is identified, measured, and legally transferred. A solid grasp of this system is essential for passing the state exam, and we cover it extensively in our Complete Illinois Exam Guide.

This mini-article will break down the mechanics of the rectangular survey system, pinpoint the specific principal meridians that govern Illinois real estate, and walk you through the exact acreage math formulas you will encounter on test day.

Understanding the Framework: Meridians and Base Lines

Created by the Land Ordinance of 1785, the Government Rectangular Survey System was designed to replace the erratic "metes and bounds" system with a logical, grid-based method for identifying land. The system is built on two primary sets of intersecting lines:

Principal Meridians: Imaginary lines running perfectly North and South.
Base Lines: Imaginary lines running perfectly East and West.

Every parcel of land described under this system is referenced to a specific Principal Meridian and its corresponding Base Line.

Township Lines and Range Lines

From the principal meridians and base lines, a grid system is formed:

Range Lines: Run North-South, parallel to the Principal Meridian, spaced 6 miles apart. The columns they form are called Ranges.
Township Lines: Run East-West, parallel to the Base Line, spaced 6 miles apart. The rows they form are called Township Tiers.

Where these 6-mile-wide ranges and 6-mile-tall tiers intersect, they form a 36-square-mile square known as a Township.

The Illinois Connection: Principal Meridians

To demonstrate genuine expertise for the Illinois state exam, you must know how this system applies specifically to the state. Illinois land is surveyed using three distinct Principal Meridians. Understanding which meridian governs a specific county is crucial for title agents, surveyors, and brokers alike.

The Second Principal Meridian: Located in Indiana, but it controls the legal descriptions for a few tracts of land along the eastern border of Illinois.
The Third Principal Meridian: The most significant meridian for Illinois. It begins at the confluence of the Ohio and Mississippi rivers (near Cairo) and runs straight north through the center of the state, governing the majority of Illinois land, including the Chicago metropolitan area.
The Fourth Principal Meridian: Begins near Beardstown and runs north, governing the western and northwestern portions of the state.

Approximate Illinois Land Coverage by Principal Meridian (%)

Deconstructing Townships and Sections

A standard township contains 36 square miles. To make land easily divisible and sellable, each township is divided into 36 Sections.

Here are the absolute must-know facts for your exam:

1 Section = 1 square mile.
1 Square Mile = 640 acres.
1 Acre = 43,560 square feet (Memorize this number: 4 old ladies driving 35 in a 60 mph zone).

The Boustrophedon Numbering System

Sections within a township are numbered 1 through 36 in a very specific "snake-like" pattern (boustrophedon). Section 1 is always in the top right (Northeast) corner. The numbering goes left to Section 6, drops down one row to Section 7, and goes right to Section 12, continuing back and forth until ending at Section 36 in the bottom right (Southeast) corner.

Historical Note: Section 16 is located near the center of the township and was historically reserved for the local school, making it known as the "School Section."

Real Estate Math: Calculating Acreage

The Illinois real estate exam will inevitably test your ability to calculate the acreage of a parcel based on its rectangular survey description. Fortunately, the formula is straightforward.

The Denominator Method

To find the acreage of a legal description, start with 640 (the total acres in one section) and divide by the denominators of the fractions provided in the description. Always work backward (from right to left) when visualizing, but you can divide in any order.

Example Scenario:
Calculate the acreage of the following parcel: The N ½ of the SW ¼ of the SE ¼ of Section 12.

The Math:

Start with 640 acres.
Divide by 4 (for the SE ¼) = 160 acres.
Divide by 4 (for the SW ¼) = 40 acres.
Divide by 2 (for the N ½) = 20 acres.

Understanding these measurements is critical not just for the exam, but for practical application. When a property goes under contract, the exact acreage and legal description will populate the title commitment and closing documents. If you are reviewing closing documents with a client, you can reference our Illinois settlement statement walkthrough and our Illinois escrow process timeline to see where the legal description plays its part.

Regulatory Framework: The Illinois Plat Act

While the Government Rectangular Survey System handles vast tracts of land, what happens in modern subdivisions? Under the Illinois Plat Act (765 ILCS 205/), when a landowner subdivides a parcel of land into two or more parts, any of which is less than 5 acres, they must have the land surveyed and a subdivision plat created.

Once the plat is recorded, the legal description shifts from the pure rectangular survey (e.g., "NW 1/4 of Section 8") to the Lot and Block system (e.g., "Lot 4, Block 2, Oakwood Subdivision, located in the NW 1/4 of Section 8"). The Lot and Block system always rests on top of the foundational Government Rectangular Survey System.

For more practice questions regarding legal descriptions and the Plat Act, be sure to check out our Illinois best study materials and resources.

Frequently Asked Questions (FAQs)

1. How many acres are in a standard section of land?

A standard section of land contains exactly 640 acres, which is equivalent to one square mile.

2. Which Principal Meridian governs the Chicago metropolitan area?

The Third Principal Meridian governs the majority of the state of Illinois, including Chicago and its surrounding suburbs.

3. How do I calculate the acreage of a parcel with an "AND" in the description?

If a legal description contains the word "AND" or a semicolon, you must treat it as two separate parcels. Calculate the acreage for the first half, calculate the acreage for the second half, and then add the two totals together.

4. What are fractional sections in Illinois?

Because the earth is curved, a flat grid cannot fit perfectly over it. Surveyors make corrections for this curvature on the north and west edges of a township. Therefore, Sections 1 through 6, 7, 18, 19, 30, and 31 are often "fractional sections" that contain slightly more or less than 640 acres.

5. What is the difference between a Range and a Tier?

A Range is a vertical column of land 6 miles wide running North-South parallel to the Principal Meridian. A Tier (or Township Line) is a horizontal row of land 6 miles tall running East-West parallel to the Base Line.