The map symbol must match the measure; a rate and a total answer different questions.
A dot map assigns a fixed quantity to each dot. Clusters show concentrations, but dot placement may be approximate rather than each person's exact home. A choropleth shades bounded areas by a rate or ratio, such as the percentage reporting a barrier. Raw totals can confuse population size with prevalence.
A proportional-symbol map scales symbol area to a total. Doubling a circle's radius makes its area four times larger, so equal radius steps would misrepresent the values. A flow-line map uses direction and line width to represent movement between places; it does not necessarily show the exact route taken.
Isolines join equal values on a continuous surface, such as temperature. They interpolate between observations, so sparse measurements create uncertainty. Contours are isolines of height. Read the interval and units before interpreting gradients.
Every map needs a meaningful title, key, source/date, scale and orientation where relevant. Class boundaries can change a choropleth's apparent pattern. Large districts dominate visually even if few people live there. Describe concentrations, contrasts, connections and exceptions rather than naming colours alone.
Step by step
Name the measure
Distinguish counts, proportions, flows and continuous values.
Read the key
Check quantity per dot, class breaks, symbol areas or line widths.
State a spatial pattern
Locate the pattern, quantify it where possible and note a limitation.
Worked example: Choose a display for the question
Map the percentage of households without cooling as a choropleth to compare prevalence. Map the number of those households with proportional symbols to help estimate service demand. Keep the denominator and year consistent.
Watch out for this
A darker area always contains more people.
Read the key. A darker shade may show a higher percentage even when the area has fewer people in total.
Check your understanding
Which map best shows the direction and volume of commuter movement between districts?
- A temperature isoline map.
- A choropleth of district land area.
- A flow-line map with a width key.