Reference

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

Glossary

Attribute join
Combining a table with a layer by matching a shared ID column, such as a FIPS code. In pandas and GeoPandas, merge().
Choropleth map
A map that shades each area by a value. Use rates or densities rather than raw counts.
Coordinate reference system (CRS)
The system that says what coordinates mean, identified by an EPSG code such as EPSG:4326 (latitude and longitude, in degrees) or EPSG:26916 (UTM zone 16N, in meters).
CSV
Comma-separated values: a plain text table, one row per line, with columns separated by commas.
DataFrame
A pandas table with labeled columns, each with one data type.
Dictionary
A Python collection of key: value pairs, written in curly brackets.
EPSG code
A number that identifies a coordinate reference system, from a registry first kept by the European Petroleum Survey Group.
f-string
A Python string with an f before the quote, in which { } is replaced by a value: f"{name} County".
FIPS code
A Federal Information Processing Standards code identifying a U.S. state (2 digits) or county (5 digits, state plus county). Store it as text.
Function
A named, reusable piece of code that takes inputs (parameters) and can return a result.
GeoDataFrame
A GeoPandas DataFrame with a geometry column holding a shape for each row.
GeoJSON
A text format for vector data, common on the web. Usually in EPSG:4326.
Index
The position of an item in a list or string. Python counts from 0.
Library
A package of ready-made code loaded with import, such as pandas or GeoPandas.
List
A Python collection of values in order, written in square brackets.
Projected CRS
A coordinate reference system on a flat map, with units such as meters. Needed to measure distances and areas.
QGIS
A free, open source desktop Geographic Information System.
Spatial join
Combining two layers by location, for example finding which polygon each point falls inside. In GeoPandas, gpd.sjoin().
String
Text in Python, written inside quotes.
Variable
A name that points to a value, created with =.
Vector data
Data stored as points, lines or polygons, each with a row of attributes.

Further reading