SemanticGIS

Comparable choropleth maps (Task)

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Comparable choropleth maps — Task

Implementation: Comparable choropleth maps (Recipe - QGIS).

Comparable choropleth maps is the Task of mapping two or more groups (two periods, two tenures, two years) so that a reader can compare them: the same zones, the same classes, the same colours, and the same treatment of zones without a reliable value. Without that, two maps that look alike can show different values, and two that look different can show the same.

Used in examples: Dwelling size by construction period in Copenhagen (Example).

Task Schema (tool-agnostic)

  1. Start from aggregated zones — one layer per group, from aggregating points to zones, with the same zones and the same minimum count.
  2. Look at the values of all maps together — their combined range and distribution.
  3. Choose one set of classes for all maps (decision point; Operator: classes shared between maps) — e.g. round boundaries that cover the combined range.
  4. Give zones without a reliable value their own class (Operator: missing and unreliable values) — a neutral colour, named in the legend.
  5. Apply the same style to every map — same colours, same order, same legend text.
  6. Compose each map — title naming the group, legend, scale bar, north arrow, source; same extent and scale on every map.

Output: two or more maps that can be laid side by side and read with one legend.

Epistemic Choices to record

  • Class method: round manual boundaries make maps comparable and readable; quantiles or natural breaks computed per map do not.
  • Number of classes: five to seven is about what a reader can tell apart.
  • Colours: one sequential colour scheme for a quantity that only goes up; a diverging scheme only if there is a meaningful middle (e.g. the city average).
  • What the colour does not show: the number of points behind each zone. A zone with 25 dwellings and one with 2,500 get the same weight on the map; consider printing counts, or a second map of counts.
  • Large, empty zones dominate: a big zone with few dwellings draws the eye more than a small dense one with many.

Graph View

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