A symbol may show a count, percentage, flow or equal value. The legend tells you which.
Read the legend before comparing map symbols. A dot map may use one dot to represent a fixed count. If 1 dot means 5 visitors, four dots mean 20 and eight mean 40. A dot does not necessarily mark a visitor's exact home.
A proportional-symbol map uses symbol size to show quantity. If circle area represents visitor numbers, the circle for 40 visitors has twice the area of the circle for 20. It does not have twice the radius.
A choropleth map shades areas using the legend's intervals, often for a rate or percentage. If 20% of households in Area A visit weekly and 40% in Area B do so, B has the higher share. You still need each area's household total to know which has more visiting households. Check dates and classification intervals when comparing maps.
A flow-line map shows movement. Arrows show where a flow starts and ends; line width may show how much moves. An isoline joins places with equal values, such as a temperature of 30 C. Closely spaced isolines show a larger change over a shorter distance.
A sketch map simplifies the positions and relationships relevant to your question. Use points, lines and areas, add a key, and state when it is not to scale.
Describe the arrangement as well as the values. Look for clusters, lines or other geometric shapes, and repetition such as evenly spaced symbols. Give a location, such as a cluster in the north.
Step by step
Read the legend
Find out whether symbols show counts, percentages, movement or equal values.
Describe the pattern
Use locations, directions and values to describe clusters, flows or changes across the area.
Check what is unknown
A percentage does not give a count without its total. A symbol for a group may not show each member's exact location.
Worked example: Close lines mean a steeper change
If 30 C and 32 C isolines lie 100 m apart in one area and 400 m apart elsewhere, temperature changes more quickly with distance in the first area. The lines show equal-temperature locations, not roads or the direction of the wind.
Watch out for this
Doubling a proportional circle's radius correctly doubles the value.
When area represents value, doubling the radius makes area four times larger. Read the key and compare the mapped areas or labelled values.
Check your understanding
Area A shows 20% and Area B shows 40% on a choropleth map. What can you conclude?
- B's share is twice as high. We need each area's total to compare counts.
- B must have exactly twice as many visiting households.
- Every household in B behaves in the same way.