How county land area and internal points are measured

The Gazetteer fields

The county Gazetteer record layout defines ALAND as land area in square meters, AWATER as water area in square meters, and ALAND_SQMI and AWATER_SQMI as the same values in square miles. INTPTLAT and INTPTLONG hold the latitude and longitude of the internal point in decimal degrees. This site uses ALAND_SQMI for every land area figure and the internal point for every county location.

Land and water add up to the total area inside the county line. Counties on coasts and large lakes can have large water areas, so total area rankings differ from land rankings. Every ranking on this site uses land only.

What counts as land

The Census glossary warns that land and water figures may not match what a map shows, because features marked as intermittent water, glacier or swamp are reported as land. So a county with large marshes or seasonal lakes keeps that area as land in the Gazetteer. Density figures built on land area inherit this choice.

What the internal point is

The Census Bureau computes one internal point per area. For a regular shape it is usually the centroid. When the centroid falls outside the area, as it can for crescent or ring shaped counties, the point moves to the nearest spot inside the boundary, preferably on land. That makes it a good label position and a fair stand-in for measuring distances between counties, but it is not the county seat or the population center. The center of population guide shows how far the two can differ.

The largest and smallest counties by land

The ten largest county-equivalents hold 12.8% of all US land in this data, and most are in Alaska. Outside Alaska the largest is San Bernardino County, CA at 20,068 square miles. The smallest units are independent cities; the smallest regular county is Kalawao County, HI at 12.0 square miles.

State totals are sums

State land areas on this site are the sum of their counties' ALAND values, 3,533,469 square miles for the 50 states and DC. Every part of a state belongs to exactly one county-equivalent, so the county sum covers the whole state. See states ranked by land area and compare shapes with the county size comparison tool.

Square miles and square kilometers

The Gazetteer gives both units, and one square mile is 2.589988 square kilometers. Yukon-Koyukuk's 145,769 square miles are 377,540 square kilometers. Small units are easier to picture in hectares: Falls Church city, VA covers 2.05 square miles, or about 531 hectares. Every area on this site is shown in square miles, the unit most US readers use.

Where land area feeds other numbers

Land area is the denominator of every density figure on this site, so a county with a lot of land counted as marsh or intermittent water will look sparser than one with the same people on dry land. It also sets each county's weight in the land centers used by the center of population guide, and it drives the largest and smallest county rankings.