GIS Data Guide

What is a GIS map?

A GIS map is a map that carries data, not just pictures of roads. Each layer is a separate dataset tied to real coordinates, so a GIS map can be queried: turn on flood zones and the flood layer draws over the same streets, or click a point and read back its county, ZIP code, elevation and land cover. GIS Map renders those layers in the browser over Esri basemaps, using boundaries and attributes published by the U.S. Census Bureau, FEMA, the USGS and other federal agencies.

What is a FIPS code?

A FIPS code is the numeric identifier the U.S. government assigns to a geographic area. A county FIPS code has five digits: the first two identify the state and the last three identify the county within that state. For example, Guilford County, North Carolina is 37081 — 37 for North Carolina, 081 for Guilford. FIPS codes are what let Census TIGER boundaries, FEMA flood maps and other federal datasets be joined to the same place, and every county page on GIS Map shows its FIPS code.

What is a ZCTA, and how is it different from a ZIP code?

A ZIP Code Tabulation Area (ZCTA) is the Census Bureau’s statistical approximation of a USPS ZIP Code service area. USPS ZIP codes are delivery routes, not polygons, so they cannot be mapped directly; the Census Bureau builds ZCTAs by grouping census blocks that share the same ZIP code. The two mostly agree but not exactly, and some ZIP codes have no ZCTA at all because they are post-office boxes or single-building routes. The ZIP pages on GIS Map map ZCTAs, which is why the boundary is described as an approximation.

What is the difference between a county, a parish, a borough and a census area?

They are all county-equivalents — the level of government or geography below the state. Louisiana calls its 64 divisions parishes, a legacy of French and Spanish administration. Alaska has organized boroughs plus larger unorganized census areas, and its single consolidated municipality is Anchorage. Connecticut replaced its eight counties with nine planning regions for statistical purposes. Everywhere else the unit is a county. GIS Map covers all of them, and the county pages use the local name rather than forcing everything into the word county.

What is the FEMA National Flood Hazard Layer?

The National Flood Hazard Layer (NFHL) is FEMA’s digital database of the effective Flood Insurance Rate Maps. It carries flood zone designations such as zone AE along rivers and coastlines and zone X outside the mapped floodplain, plus floodways and base flood elevations. The flood layer on a GIS Map page draws straight from the NFHL, and flood zones only appear once you zoom in far enough for the data to resolve. It is a screening tool: insurance and permitting decisions require an official flood determination or elevation certificate.

What is the FEMA National Risk Index?

The National Risk Index is FEMA’s baseline measure of natural hazard risk for every county in the United States. It combines three components — expected annual loss from 18 natural hazards, social vulnerability, and community resilience — into an overall risk rating and a set of per-hazard ratings. GIS Map shows the county’s overall rating, expected annual loss, social vulnerability and resilience on city, county and ZIP pages. The current release is version 1.20, published December 2025.

What is an EPA radon zone?

The EPA Map of Radon Zones sorts every county into one of three zones by its predicted average indoor radon level. Zone 1 is the highest potential, at or above 4 pCi/L, which is the level at which the EPA recommends a mitigation system for new construction. Zone 2 is moderate, between 2 and 4 pCi/L, and Zone 3 is low, below 2 pCi/L. The zone is a regional planning tool built for building codes and screening programmes, not a prediction for any individual home — elevated radon can occur in any zone, so the EPA recommends testing every home regardless of its zone.

What does the Air Quality Index mean, and why do some counties have no value?

The Air Quality Index (AQI) is the EPA scale for how polluted the air is on a given day, running from 0 to 500. Values up to 50 are good, up to 100 moderate, up to 150 unhealthy for sensitive groups, up to 200 unhealthy, up to 300 very unhealthy, and above that hazardous. GIS Map shows the median, 90th percentile and peak AQI for the year, plus the number of days at unhealthy for sensitive groups or worse. Only counties that host an EPA monitoring site have these figures — roughly 970 of the 3,143 counties — so a county with no value has no monitor, which is not the same as clean air.

What is the Drought Severity and Coverage Index?

The Drought Severity and Coverage Index (DSCI) is the U.S. Drought Monitor summary of how much of an area is in drought and how severe it is, on a scale from 0 to 500. It is the sum of the cumulative percentages of area in each drought category, from abnormally dry (D0) through exceptional drought (D4), so 500 means the entire area is in exceptional drought and 0 means none of it is in drought. The U.S. Drought Monitor is a weekly product of the National Drought Mitigation Center, the USDA and NOAA, so a county page shows the position on one specific map date rather than a climate average.

Where do property records and parcel maps come from?

Parcel boundaries, deeds and assessed values are kept by local government, not by any federal agency, and the office that holds them has a different name in different states. In Texas, property is appraised by a county appraisal district, usually called a CAD. In Kentucky, the property valuation administrator, or PVA, handles assessments. Elsewhere it is typically the county assessor, auditor, appraiser or recorder. GIS Map does not publish parcel or deed records and does not name a specific county system, because it holds no data on which vendor or portal a given county uses.

How does GIS Map date and source its data?

Every place page ends with a data-sources section that names the publishing agency, the release and a link to the agency’s own page for each dataset used. Demographic figures are the U.S. Census Bureau American Community Survey 2020-2024 5-year estimates; population change uses the Census Bureau Population Estimates Program; climate figures are the NOAA 1991-2020 U.S. Climate Normals; hazard ratings are the FEMA National Risk Index; radon zones are the EPA Map of Radon Zones; air quality is the EPA annual Air Quality Index summary; drought is the U.S. Drought Monitor; boundaries are Census TIGER. Figures are reproduced as published, without adjustment.

How should GIS Map be cited?

Cite the page and, where a number matters, the agency behind it. For example: GIS Map, “Guilford County, North Carolina GIS Map”, https://gismap.land/north-carolina/guilford, citing the U.S. Census Bureau American Community Survey 2020-2024 5-year estimates. The same citation line appears in the data-sources block at the bottom of every place page.

Every dataset GIS Map publishes, with its release

These are the primary sources behind the numbers on this site. Each links to the publishing agency's own page, so a figure can be checked at the source.

• American Community Survey 5-year estimates — U.S. Census Bureau, 2020-2024. Used for population, income, housing, education and poverty figures.
• Gazetteer Files — U.S. Census Bureau, 2024. Used for place coordinates, land area and the county/place/ZIP crosswalk.
• TIGER/Line Shapefiles — U.S. Census Bureau, 2024. Used for county, ZIP Code Tabulation Area and school district boundaries.
• Population Estimates Program — U.S. Census Bureau, Vintage 2024. Used for population change since the 2010 and 2020 censuses.
• National Flood Hazard Layer — Federal Emergency Management Agency, current effective Flood Insurance Rate Maps. Used for flood zones, floodways and base flood elevations.
• National Risk Index — Federal Emergency Management Agency, v1.20, December 2025. Used for county natural hazard risk, expected annual loss, social vulnerability and resilience.
• 3D Elevation Program (3DEP) — U.S. Geological Survey, current. Used for elevation contours, shaded relief and point elevations.
• National Land Cover Database — Multi-Resolution Land Characteristics Consortium, NLCD 2021. Used for land cover classes such as developed, forest, crop and water.
• U.S. Climate Normals — NOAA National Centers for Environmental Information, 1991-2020. Used for monthly average highs, lows, precipitation and snowfall.
• U.S. General Soil Map (STATSGO2) — USDA Natural Resources Conservation Service, STATSGO2. Used for dominant soil map unit, drainage class and hydrologic group.
• Inventory of Owned and Leased Properties — U.S. General Services Administration, current. Used for federally owned and leased buildings and their rentable square footage.
• Map of Radon Zones — U.S. Environmental Protection Agency, September 2025. Used for the county radon zone (1, 2 or 3) and its predicted indoor radon potential.
• Air Quality Index annual summary by county — U.S. Environmental Protection Agency, 2024. Used for median, 90th percentile and peak AQI, and days at unhealthy levels, for counties with an EPA monitor.
• U.S. Drought Monitor — National Drought Mitigation Center, USDA and NOAA, weekly, current map. Used for the share of county area in each drought category and the Drought Severity and Coverage Index.
• ArcGIS basemaps — Esri, current. Used for streets, satellite, topographic and dark gray basemap tiles.
• Wikipedia city summaries — Wikimedia Foundation, current. Used for the short city overview text.
• Wikimedia Commons photographs — Wikimedia Foundation, current. Used for city photographs, with attribution on each image.
• GeoNames — GeoNames, current. Used for ZIP code centroids and time zone assignment.

Where to go next

• Browse all states and the cities and counties inside each one.
• City and county rankings by income, home value, growth, climate and hazard risk.
• About GIS Map for the full feature list.