Classification for maps
A choropleth map divides a range of values into classes and gives each class a colour. The class boundaries decide what the map shows as different and what it shows as the same, so they are an analytical choice, not a matter of style. Classifying for a map changes only how data are drawn; turning the data themselves into classes is the reclassify Operator (Conversion and data management).
Used in: Comparable choropleth maps (Task), Distance to habitat (Task).
| Method | How the boundaries are set | Good for | Watch out |
|---|---|---|---|
| Equal interval | the range cut into equal steps | values spread evenly; easy to read | one extreme value leaves most classes empty |
| Quantile (equal count) | the same number of areas in each class | ranking areas | very different values can share a class; boundaries change with the data |
| Natural breaks (Jenks) | boundaries in the gaps of the data | showing clusters in one dataset | boundaries are specific to the dataset, so two maps get different classes |
| Standard deviation | steps of one standard deviation from the mean | showing above and below average | readers must know what the mean is |
| Manual (fixed) | boundaries you set, e.g. round numbers | comparing maps, or meaningful thresholds | you must justify them |
Classes shared between maps
Two or more maps of the same variable use the same class boundaries, so the same colour means the same value on every map.
Choices to record: how the shared boundaries were found, e.g. round numbers covering the range of all maps together, or quantiles of the pooled values. A method computed separately for each map (quantiles or natural breaks per map) makes the maps look alike even where the values differ.
Tools: QGIS Graduated symbology (Tool - QGIS) with Mode set and classes edited by hand, then the same style loaded on the other layers.
Missing and unreliable values
Areas without a value (no points) or with too few points to trust get their own class, shown in a neutral colour and named in the legend, e.g. “fewer than 20 dwellings”.
Tools: QGIS Rule-based or Categorized symbology on an expression; see Comparable choropleth maps (Recipe - QGIS).