Domain 3: Cartography and Visualization
Domain 3 covers cartographic design and visualization: matching map type to data, classifying and symbolizing values, choosing projections, and laying out readable maps. The exam asks whether a design communicates the data truthfully and whether the map's scale and generalization support its purpose.
Map type and data level
Choropleth maps suit normalized rates or ratios, not raw counts. Proportional symbols suit totals; dot density suits density patterns; isarithmic maps suit continuous fields. A map's scale is a ratio: 1:24,000 is large-scale and shows more detail than 1:1,000,000. Generalization-simplification, smoothing, aggregation, displacement, and exaggeration-is necessary at small scales but must be documented.
Classification and color
Common classification methods are equal interval (equal value ranges), quantile (equal counts), natural breaks/Jenks (minimizes within-class variance), and standard deviation (distance from the mean). Sequential palettes encode ordered values with lightness or hue; diverging palettes suit values around a meaningful midpoint; qualitative palettes suit nominal categories. Use no more than five to seven classes for most thematic maps, and avoid rainbow schemes that imply false order.
Worked example
A county poverty-rate map has a right-skewed distribution with a few very high values. Equal interval would place most counties in the lowest class; quantile would show relative rank but hide absolute differences; natural breaks would group similar rates and expose the high outliers. Because the data are rates, a choropleth is appropriate; a sequential palette from light to dark fits. If the map instead showed poverty counts, large-population counties would dominate, so the analyst should normalize by population or use a different symbol type.
Traps
- Mapping counts in a choropleth.
- Using a diverging palette without a meaningful midpoint.
- Choosing too many classes or a rainbow scheme.
- Forgetting to update the scale bar after resizing a layout.
- Using a conformal projection for a small-scale area map where area comparison matters.