30 terms
Glossary
What this covers
The words used across the tools, guides and county pages, from coordinate formats to Census data programs. Each entry has a short definition, a value computed from this site's data, and a link to the page that uses it most.
- Antipode
- The point on the exact opposite side of the Earth. Negate the latitude and add or subtract 180° from the longitude.
- e.g. The antipode of Denver's capitol is 39.74° S, 75.01° E.
- Antipode finder
- Bearing
- The direction from one point to another in degrees clockwise from true north. On a great circle it changes along the way.
- e.g. The initial bearing from Denver to Boston is 73.0°.
- How to calculate bearing
- Census area
- A county-equivalent in Alaska delineated for statistics by the State of Alaska and the Census Bureau, alongside organized boroughs.
- e.g. Alaska has 30 boroughs and census areas in the 2024 Gazetteer.
- Parishes, boroughs and planning regions
- Center of population
- The point where a map would balance if every resident were an equal weight. The Census Bureau publishes one for each census.
- e.g. The Census Bureau puts the 2020 US center at 37.415725° N, 92.346525° W, near Hartville, Missouri.
- Center of population by state
- County-equivalent
- Any unit the Census Bureau treats as a county: counties, Louisiana parishes, Alaska boroughs and census areas, independent cities, Connecticut planning regions and DC.
- e.g. The 50 states and DC have 3,144 county-equivalents in the 2024 Gazetteer.
- How many counties are in the US
- Decimal degrees
- Latitude and longitude written as signed decimal numbers, with south and west negative. The format used by web maps and data files.
- e.g. Denver's capitol is 39.739, -104.985 in decimal degrees.
- Convert DMS to decimal degrees
- Degrees, minutes, seconds (DMS)
- Angles split into 60 minutes per degree and 60 seconds per minute, with a hemisphere letter instead of a sign.
- e.g. Denver's capitol latitude is 39°44'20.40"N.
- GPS coordinate converter
- Easting and northing
- UTM grid coordinates in meters: easting from a false origin 500,000 m west of the zone's central meridian, northing from the equator.
- e.g. Denver's capitol is easting 501,285 m, northing 4,398,790 m in zone 13.
- UTM zones explained
- Ellipsoid
- A flattened sphere used as the mathematical shape of the Earth. WGS84 has an equatorial radius of 6,378,137 m and a flattening of 1/298.257223563.
- e.g. On WGS84 a degree of latitude runs from 110.574 km at the equator to 111.694 km near the pole.
- How many miles in a degree
- Equal-area projection
- A flat map projection that keeps relative sizes true at the cost of shape. This site's static US maps use Albers equal-area projections.
- e.g. On an equal-area map, Alaska's 571,242 square miles of land cover 2.2 times the paper of Texas.
- Mercator projection distortion
- FIPS code
- A numeric code for a state (2 digits) or county (5 digits: state plus county). Census now calls these ANSI codes or GEOIDs.
- e.g. Cook County, Illinois is 17031.
- FIPS codes explained
- Gazetteer
- Census files listing every geographic area of a type with its codes, land and water area, and internal point.
- e.g. The 2024 county Gazetteer gives Travis County, Texas 994.3 square miles of land.
- How county land area is measured
- Great circle
- The intersection of the globe with a plane through its center. The shortest path between two points on a sphere follows one.
- e.g. Sacramento to Boston along a great circle is 2,625 miles.
- Great circle route map
- Haversine formula
- The standard formula for great circle distance from two latitudes and longitudes on a sphere.
- e.g. It gives 1,765.0 miles from Denver to Boston with a radius of 6,371.0088 km.
- How distance is calculated
- Independent city
- A city outside any county, treated as a county-equivalent. They exist in Virginia, Maryland, Missouri and Nevada.
- e.g. There are 41 independent cities in the 2024 data.
- What is an independent city
- Internal point
- A Census-computed coordinate inside an area, usually its centroid, moved inside the boundary when the centroid falls outside.
- e.g. Cook County's internal point is 41.8943° N, 87.6455° W.
- How county land area is measured
- Land area
- Area inside a boundary minus water, as reported in the Gazetteer. Glaciers, swamps and intermittent water count as land.
- e.g. The 50 states and DC have 3,533,469 square miles of land in this data.
- States ranked by land area
- Latitude
- The angle north or south of the equator, from 0° to 90°. Lines of equal latitude are parallels.
- e.g. Juneau is the northernmost state capital at 58.30° N.
- Latitude and longitude explained
- Longitude
- The angle east or west of the prime meridian, from 0° to 180°. Lines of equal longitude are meridians and meet at the poles.
- e.g. A degree of longitude at Denver's latitude is 53.3 miles.
- Miles per degree calculator
- Mercator projection
- A projection that keeps angles true and draws constant compass headings as straight lines, stretching areas by 1/cos² of latitude.
- e.g. At 60° north Mercator draws areas 4.0 times their size at the equator.
- Mercator projection distortion
- Parish
- Louisiana's name for its primary legal divisions, treated as counties in Census data.
- e.g. Louisiana has 64 parishes.
- Louisiana parishes
- Planning region
- Connecticut's county-equivalent since 2022, based on the state's nine councils of governments.
- e.g. Connecticut has 9 planning regions, coded 09110 to 09190.
- Connecticut planning regions
- Population density
- Residents divided by land area.
- e.g. The 50 states and DC average 96.3 people per square mile; Cook County has 5,485.
- How population density is calculated
- Rhumb line
- A path that crosses every meridian at the same angle, so it can be followed on one compass heading. Straight on a Mercator map, longer than a great circle.
- e.g. Sacramento to Boston along a rhumb line is 2,662 miles.
- Great circle vs rhumb line
- SAIPE
- Small Area Income and Poverty Estimates, the Census program that models county income and poverty from survey data, population estimates and administrative records.
- e.g. SAIPE 2023 puts Travis County, Texas at $97,544 median household income.
- County median household income
- Solar noon
- The moment the sun crosses the local meridian and reaches its highest point of the day. It shifts 4 minutes for each degree of longitude.
- e.g. On June 21, 2026 solar noon at Denver's capitol is 1:01 PM local time.
- How sunrise and sunset are calculated
- Statute mile
- The mile used on land in the US, exactly 1,609.344 meters or 5,280 international feet, per NIST.
- e.g. Boston to Providence is 65.4 km, or 40.6 statute miles.
- Crow flies distance calculator
- UTM zone
- One of 60 strips of 6° of longitude, numbered eastward from 180°, each with its own transverse Mercator grid in meters.
- e.g. Zone = floor((longitude + 180) ÷ 6) + 1, so Denver at -104.985 is in zone 13.
- UTM to latitude and longitude
- Vintage
- A yearly release of Census population estimates. Each vintage revises every year back to the last census and replaces earlier vintages.
- e.g. Vintage 2024 puts the 50 states and DC at 340,110,988 residents on July 1, 2024.
- How Census population estimates work
- ZIP Code Tabulation Area (ZCTA)
- A Census area built from census blocks to approximate a USPS ZIP Code. Each block is assigned the ZIP Code most common among its addresses.
- e.g. ZIP radius tools on this site use ZCTA internal points from the 2024 Gazetteer.
- ZIP codes in a radius
Questions people ask
What is the difference between a county and a county-equivalent?
A county-equivalent is any unit the Census Bureau treats as a county, including Louisiana parishes, Alaska boroughs and census areas, independent cities, Connecticut planning regions and DC.
What is the difference between a great circle and a rhumb line?
A great circle is the shortest path on a sphere. A rhumb line keeps one compass heading and is longer, except along a meridian or the equator.
Is a ZIP code the same as a ZCTA?
No. A ZIP Code is a USPS delivery code. A ZCTA is a Census area built from census blocks to approximate it for mapping and statistics.
Sources and data as of
Data as of: Census Vintage 2024 population estimates, 2024 Gazetteer land area and internal points, SAIPE 2023 median household income.
- 01Census Bureau geography glossary (last verified 2026-09-30)
- 02Census Bureau ZIP Code Tabulation Areas (last verified 2026-09-30)
- 03NIST: revised unit conversion factors (international foot and mile) (last verified 2026-09-30)
- 04USGS FAQ: what does the term UTM mean? (last verified 2026-09-30)
- 05Census 2024 Gazetteer, counties
How every number is computed: methodology. Spotted a wrong figure? Report an error.