Reference

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

Glossary

Alt text
A short written description of an image, read aloud by screen readers and shown when an image cannot load. For a map, it should describe what is shown and the main pattern.
Attribute table
The table of data attached to a vector layer; each row is a feature (a point, line or polygon) and each column is an attribute.
Basemap
A background map, such as OpenStreetMap or a grey canvas, that gives the data geographic context.
Cartogram
A map in which areas are resized according to a data value instead of their land area.
Categorical (qualitative) color scheme
Distinct hues of similar lightness, used for categories that have no order.
Choropleth map
A map that shades areas (such as states or counties) according to a data value. Best for rates, not raw counts.
Classification
Grouping numeric values into a small number of classes, each shown with one color. Common methods include equal interval, quantile, natural breaks (Jenks), standard deviation, pretty breaks and geometric interval.
Color vision deficiency (CVD)
Reduced ability to distinguish certain colors, most often reds and greens. Often called colorblindness.
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:5070 (USA Contiguous Albers Equal Area).
Diverging color scheme
Two hues that meet at a neutral middle, used for data with a meaningful midpoint such as zero or an average.
Dot density map
A map that places one dot for a fixed number of things (people, cases) inside each area.
Equal area projection
A projection that preserves the relative size of areas. Recommended for thematic maps of rates and densities.
Explanatory visualization
A figure designed to communicate one message to others.
Exploratory visualization
A figure made quickly to understand your own data.
Figure-ground
The visual separation of the main subject (figure) from its background (ground).
Legend
The key that explains a map’s colors, symbols and units.
Map scale
The ratio between distance on a map and distance on the ground, such as 1:24,000. Large-scale maps show small areas in detail; small-scale maps show large areas.
Normalization
Dividing a count by a measure of size (population, area, total) to produce a rate that can be compared fairly between places.
North arrow
A map element showing the direction of north.
Print layout
In QGIS, the window where a map is arranged on a page with its title, legend, scale bar, north arrow and other elements for export.
Projection
A mathematical method for representing the curved Earth on a flat map. Every projection distorts area, shape, distance or direction.
Proportional symbol map
A map that sizes symbols, usually circles, according to a value at each location.
Scale bar
A map element that shows distances on the map.
Sequential color scheme
Colors that run from light to dark in one hue (or a smooth multi-hue ramp), used for ordered data from low to high.
Small multiples
A series of small charts or maps with the same axes or scales, one per group or time period, for easy comparison.
Symbology
The rules that define how features are drawn: colors, symbols, sizes and classes. In QGIS, set in Layer Properties > Symbology.
Visual hierarchy
The order in which a reader notices elements of a map or chart, controlled by size, contrast, color and position.
Visual variables
The properties of a symbol that can change to represent data: position, size, lightness, hue, shape, orientation and texture.

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