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Area measurement data provide the size, in square units (metric and nonmetric) 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 twodimensional 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 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 county but is not within the legal boundary of any county subdivision. 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.