Reference

Last updated on 2026-10-17 | Edit this page

Glossary

American Community Survey (ACS)
A yearly U.S. Census Bureau survey of about 3.5 million addresses that produces estimates on social, economic and housing topics. Published as 1-year and 5-year estimates, each with a margin of error.
Annotation code
A special negative value, such as -666666666, that the Census API returns instead of an estimate when a value cannot be calculated. Replace it with “missing” before calculating.
API (Application Programming Interface)
A way for a program to request data from a server with a structured web address, instead of a person clicking through a website.
Block group
A census geography inside a tract, usually with 600 to 3,000 people. The smallest area with ACS 5-year estimates.
Cartographic boundary file
A simplified Census Bureau boundary file, clipped to the shoreline, designed for thematic maps. Its file names start with cb_.
Census tract
A small, fairly permanent census area of about 1,200 to 8,000 people (4,000 on average), used to compare neighborhoods over time.
Coefficient of variation (CV)
A measure of how reliable an estimate is: the standard error as a percentage of the estimate. For ACS data, CV = (MOE / 1.645) / estimate x 100.
Coordinate reference system (CRS)
The system that links map coordinates to places on Earth, identified by an EPSG code such as EPSG:4326 (latitude and longitude) or EPSG:26916 (UTM zone 16N, in meters).
CORONA
A U.S. reconnaissance satellite program (1960 to 1972) whose more than 860,000 images were declassified in 1995.
Decennial Census
The U.S. count of every person, taken every ten years. It has no margin of error but asks only a few questions.
Digitizing
Creating vector features by tracing them from a map or image.
FAIR principles
Guidelines for making data Findable, Accessible, Interoperable and Reusable.
FIPS code
A Federal Information Processing Standards code identifying a state (2 digits) or county (3 digits).
Geocoding
Converting a text location, such as an address, into coordinates. Reverse geocoding goes from coordinates back to an address.
GEOID
A census geography’s identifier, built by joining FIPS and other codes, for example 18157 for Tippecanoe County, Indiana. Used to join census tables to boundaries. Keep it as text.
Georeferencing
Assigning real-world coordinates to a scanned map or image so it lines up with other layers.
GNSS (Global Navigation Satellite System)
Satellite positioning systems, including GPS, Galileo, GLONASS and BeiDou, used by phones and receivers to determine location.
Group quarters
Places where people live in a group, such as university residence halls, nursing homes and prisons. Their residents are not counted as households.
Margin of error (MOE)
The range around an ACS estimate within which the true value lies with 90% confidence.
Match rate
The share of addresses a geocoder places successfully. Always check the addresses that did not match.
Metadata
Documentation describing a dataset: who made it, when, how, where it covers, its attributes, its coordinate system and its license.
OpenStreetMap (OSM)
A free, volunteer-built map of the world. Features are described by key=value tags such as shop=supermarket.
QField
A free, open source app for collecting field data on phones and tablets, using projects prepared in QGIS.
Raster data
Data stored as a grid of cells, each with a value, such as a satellite image or elevation model.
Rate limit
The maximum number of requests a web service accepts in a period, for example one per second for Nominatim.
Resolution
For imagery: the ground size of one pixel (spatial), the number of wavelength bands (spectral), or how often an area is imaged (temporal).
Schema
The planned list of fields in a dataset, with their types and allowed values.
Sentinel-2
European Union Copernicus satellites providing free 13-band imagery at 10 to 60 m resolution about every 5 days.
TIGER/Line
The Census Bureau’s full-detail geographic files of boundaries and features such as roads. Their file names start with tl_.
Vector data
Data stored as points, lines or polygons, each with attributes.
ZCTA (ZIP Code Tabulation Area)
A census approximation of a ZIP code’s area, used to publish census data by ZIP code.

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