Reference

Last updated on 2026-10-20 | Edit this page

Glossary

Annotation code
A special value such as -666666666 that the Census API writes instead of an estimate (or margin of error) that does not exist. It must be turned into a missing value before any calculation.
Areal weighting
Splitting a polygon’s count (such as population) among the pieces it is cut into, in proportion to area. It assumes the count is spread evenly across the polygon.
Batch processing
Running one QGIS tool or model several times with different settings, one row per run.
Buffer
A zone at a fixed distance around features. Distances need a CRS in meters or feet.
Clip
Cutting a layer to the shape of another layer, keeping only the first layer’s attributes.
Codebook (data dictionary)
The document that says what each column and each code in a dataset means.
Coefficient of variation (CV)
The standard error of an estimate as a percentage of the estimate: (MOE / 1.645) / estimate x 100. A common rule of thumb: under 12% high reliability, 12 to 40% medium, over 40% low.
Coordinate Reference System (CRS)
The system that links coordinates to places on Earth. EPSG:4326 is latitude and longitude; EPSG:26916 (UTM zone 16N) uses meters and suits Indiana.
Dasymetric estimation
Areal weighting refined with other data, such as land cover, so that counts go where people actually live.
Define versus reproject
Defining a CRS states what the coordinates already are (no numbers change); reprojecting converts them to another CRS (every number changes).
Dissolve
Merging features into one, or into one per value of a column, and combining their attributes.
Function
Named, reusable code with parameters (inputs) and a return value (output).
GEOID
The Census Bureau’s identifier for a geographic area. A tract GEOID has 11 characters: state (2), county (3) and tract (6). It must be stored as text so leading zeros are kept.
Intersect
Cutting two layers against each other and keeping the overlapping pieces, with the attributes of both.
Invalid geometry
A shape that breaks the rules of its type, such as a polygon whose edges cross itself. Overlays can fail or give wrong areas.
Map algebra
Calculations on rasters cell by cell, such as elevation < 170.
Margin of error (MOE)
For ACS estimates, the range around the estimate that contains the true value with 90% confidence.
Model (QGIS)
A chain of Processing tools saved as a new tool, with inputs and outputs, built in the Model Designer.
Raster
A grid of cells, each holding a value, such as elevation or a land cover code.
Reclassification
Mapping old raster values to new ones with a table, for example 16 land cover codes into 5 groups.
Refactor Fields
A QGIS tool that creates a new layer with fields renamed, retyped or computed from expressions.
Zonal statistics
Summaries of the raster cells inside each polygon, such as the mean elevation of each tract.

Library and tool documentation