Illinois Population per square mile, 2010 by County

Value for Illinois (No. of people per square mile): 231.1

Source: U.S. Census Bureau, data file from Geography Division based on the TIGER/Geographic Identification Code Scheme (TIGER/GICS) computer file. Land area updated every 10 years. Geography/TIGER or American FactFinder.

Source: U.S. Census Bureau, Census of Population and Housing. Land area is based on current information in the TIGER® data base, calculated for use with Census 2010.

Definitions:

Land area - an area measurement providing the size, in square meters, of the land portions of geographic entities for which the Census Bureau tabulates and disseminates data.

Area is calculated from the specific boundary recorded for each entity in the Census Bureau’s geographic database (see "MAF/TIGER Database"). The Census Bureau provides area measurement data for both land area and water area. The water area figures include inland, coastal, Great Lakes, and territorial sea water. Inland water consists of any lake, reservoir, pond, or similar body of water that is recorded in the Census Bureau’s geographic database. It also includes any river, creek, canal, stream, or similar feature that is recorded in that database as a two-dimensional feature (rather than as a single line). The portions of the oceans and related large embayments (such as Chesapeake Bay and Puget Sound), the Gulf of Mexico, and the Caribbean Sea that belong to the United States and its territories are classified as coastal and territorial waters; the Great Lakes are treated as a separate water entity. Rivers and bays that empty into these bodies of water are treated as inland water from the point beyond which they are narrower than 1 nautical mile across. Identification of land and inland, coastal, territorial, and Great Lakes waters is for data presentation purposes only and does not necessarily reflect their legal definitions.

Land area measurements are originally recorded as whole square meters (to convert square meters to square kilometers, divide by 1,000,000; to convert square kilometers to square miles, divide by 2.58999; to convert square meters to square miles, divide by 2,589,988).

Persons per square mile - population and housing unit density are computed by dividing the total population or number of housing units within a geographic entity by the land area of that entity measured in square miles or in square kilometers. Density is expressed as "population per square mile (kilometer)" or "housing units per square mile (kilometer)." To determine population per square kilometer, multiply the population per square mile by .3861.

Scope and Methodology:

TIGER is an acronym for the digital (computer-readable) geographic database that automates the mapping and related geographic activities required to support the Census Bureau's census and survey programs. The Census Bureau developed the Topologically Integrated Geographic Encoding and Referencing (TIGER) System to automate the geographic support processes needed to meet the major geographic needs of the 1990 census. Land area was calculated from the specific set of boundaries recorded for the entity (in this case, counties, which were then aggregated to metropolitan totals) in the Census Bureau's geographic database.

Land and water area measurements may disagree with the information displayed on Census Bureau maps and in the MAF/TIGER database because, for area measurement purposes, hydrologic features identified as intermittent water, glacier, or swamp are reported as land area. The water area measurement reported for some geographic entities includes water that is not included in any lower-level geographic entity. Therefore, because water is contained only in a higher-level geographic entity, summing the water measurements for all the component lower-level geographic entities will not yield the water area of that higher-level entity. This occurs, for example, where water is associated with a state but is not within the assigned area of any congressional district.

The accuracy of any area measurement data is limited by the accuracy inherent in 1) the location and shape of the various boundary information in the MAF/TIGER database, 2) the identification, and classification of water bodies coupled with the location and shapes of the shorelines of water bodies in that database, and 3) rounding affecting the last digit in all operations that compute and/or sum the area measurements.

More information:

• Population, Housing Units, Area, and Density: 2010 - United States--States; and Puerto Rico
• Population, Housing Units, Area, and Density: 2010 - County by State; and Puerto Rico
• Population, Housing Units, Area, and Density: 2010 - State--Place (and in selected states) County Subdivision