Types
of Maps
Proportional Symbol Maps Choropleth Maps Cartograms Isarithm Maps
Proportional Symbol Maps
Conceptual basis for Proportional Symbol Mapping: The cartographer selects a symbol form, and varies its size from place to place, in proportion to the quantities the symbol represents. The map reader forms a picture of the quantitative distribution by examining the pattern of the different sized symbols.
When to select this mapping method:
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When data occur at points;
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When data can be aggregated at points within areas;
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When data are absolute values, not derived. Any magnitude can be shown this way, except densities/ratios/rates, which should be shown with a choropleth representation.
Circles are most popular proportional symbol because:
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Their form is compact and easy to construct
- Circle scaling is less difficult than with other symbol forms (with the exception of the square) because the square root of the radius can be used in the scaling procedure.
- Circles are more visually stable than other symbol forms, resulting in images that cause little eye irritation (perceptually).
Scaling of Proportional Symbols:
- Concerns 'scaling' or determining the size of smbols so that the map reader gets an 'accurrate' perception o fthe amounts associated with different symbol sizes.
Methods for Scaling Circles:
- Absolute Scaling: The direct proportional scaling of magnitudes of the symbol's area:
- Apparent-magnitude scaling: This method is perceptually based and compensates for underestimation of symbol values, during the scaling process, symbol sizes are inflated to compensate for underestimation.
- Range-graded scaling: The data are divided into groups, using classification procedures common to choropleth mapping. The design goal is for symbol size discrimination, rather than magnitude estimation. The cartographer chooses symbol sizes for adjacent classes so that the map reader can easily distinguish between circle sizes, and therefore, categories.
