Unit 9
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Air Lot
A legal description of a three-dimensional space above the surface of the earth, often used for condominiums, air rights, or subsurface rights.
A legal description for a penthouse condo may describe an air lot beginning 200 feet above ground level and extending upward.
Base Line
An imaginary east-west line used in the rectangular survey system as a reference for measuring township tiers. Established by government survey.
The 4th Principal Meridian base line runs through central Illinois and is used to number townships north and south.
Benchmark
A permanent marker placed by surveyors to indicate a known elevation. Used as a reference point for measuring elevations in surveys and construction.
A brass disk set in concrete at a known elevation serves as a benchmark for a new subdivision survey.
Correction Line
An adjustment line in the rectangular survey system, usually every 24 miles, to compensate for the curvature of the earth and keep sections approximately square.
Correction lines in the government survey system prevent sections from becoming distorted as surveys move north from the base line.
Datum
A reference point or surface used to measure elevations. In surveying, often mean sea level or a local benchmark.
All elevations in a survey are measured from a datum of 500 feet above mean sea level.
Fractional Section
A section in the rectangular survey system that is less than 640 acres due to natural boundaries such as rivers or lakes, or because it is on the edge of a correction line.
A section along the Mississippi River may be a fractional section containing only 480 acres instead of the standard 640.
Government Check
A 24-mile square area in the rectangular survey system bounded by correction lines and guide meridians. Contains 16 townships.
Government checks help surveyors organize large areas and apply correction lines every 24 miles.
Government Lot
A parcel of land in a fractional section that is numbered and described separately because it is irregular in shape due to water boundaries or other natural features.
Along a lake, irregular parcels are numbered as government lots rather than standard quarter-sections.
Legal Description
A precise written description of a parcel of land that is legally sufficient to locate and identify the property without ambiguity. Used in deeds, mortgages, and court documents.
A legal description using metes and bounds or the rectangular survey system allows a surveyor to locate the exact boundaries of a property.
Lot-and-Block System
A method of legal description used in platted subdivisions. Identifies property by lot number, block number, and the name of the subdivision plat.
A home is legally described as 'Lot 12, Block 3, Oak Grove Subdivision, according to the plat recorded in Book 45, Page 22.'
Metes-and-Bounds
A method of legal description that describes a parcel by its boundaries, starting at a point of beginning and following compass directions and distances around the perimeter back to the starting point.
A rural property is described by metes and bounds: 'Beginning at a stone monument at the intersection of County Road 5 and the old fence line, thence North 89° 15' East 1,245.6 feet...'
Monument
A fixed physical object used as a reference point in a survey or legal description. Can be natural (tree, rock) or artificial (iron pin, concrete marker).
The legal description references an iron pin monument set at the southwest corner of the property as the point of beginning.
Plat Map
A detailed map of a subdivision showing lot boundaries, streets, easements, and other features. When recorded, it becomes the basis for lot-and-block legal descriptions.
The developer records the final plat map with the county recorder, creating the official lot and block numbers for all parcels in the subdivision.
Point of Beginning (POB)
The starting point in a metes-and-bounds legal description. The description must close by returning to this exact point.
The metes-and-bounds description begins: 'Commencing at the point of beginning, being the northwest corner of Section 14...'
Principal Meridian
A north-south line used in the rectangular survey system as a reference for measuring ranges. Established by government survey.
The 3rd Principal Meridian runs through eastern Illinois and is used to number ranges east and west of the line.
Range
A vertical column of townships in the rectangular survey system, numbered east or west of a principal meridian.
A property located in Range 2 East of the 4th Principal Meridian is two columns of townships east of that meridian.
Rectangular Survey System
Also called the government survey system. A method of describing land using a grid of townships, ranges, sections, and fractions. Used in most of the United States, including Illinois.
Most rural land in Illinois is described using the rectangular survey system: Township 12 North, Range 3 East, Section 24.
Section
A one-square-mile parcel (640 acres) within a township in the rectangular survey system. There are 36 sections in a township.
A farmer owns the Southeast Quarter of Section 16, containing 160 acres.
Survey
The process of locating and measuring the boundaries of a parcel of land and preparing a map or plat showing the results. Performed by a licensed surveyor.
Before closing on raw land, the buyer orders a boundary survey to confirm the exact acreage and location of corners.
Township
A six-mile-square area in the rectangular survey system containing 36 sections. Townships are numbered north or south of a base line.
The property is located in Township 20 North of the base line.
Township Line
An east-west line that forms the north or south boundary of a township in the rectangular survey system.
Township lines run parallel to the base line and are spaced six miles apart.
Township Tier
A horizontal row of townships running east-west, numbered north or south of the base line in the rectangular survey system.
Township Tier 5 North includes all townships five rows north of the base line.