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Proportional Symbol Maps   Choropleth Maps   Cartograms   Isarithm Maps  

• Data can be, absolute (raw) or derived, of interval or ratio measurement level, and must be aggregated via artificial collection units.
• There is no data generalization, i.e., no classificaiton of data into separate classes.
• A cartogram is most effective if large areas (chorograms) have small values, and small areas (chorograms) have large values. In other words, if data values correlate negatively with the area of size of the chorogram, the cartogram is a likely candidate for a mapping method.
• Note: If data value correlates positively with size of the enumeration unit, then don't use a cartogram.

Two Basic Forms

  Contiguous: The internal numeration units are adjacent to each other.

  Non-Contiguous: Does not preserve the boundary relationshiops between the internal enumeration units. the enumeration units are placed in "more-or-less" correct locations relative to their neighbors.

Communicating with Cartograms

  Advantages of using a cartogram:

• To shock the reader with unexpected spatial peculiarities.
• To develop clarity in a map that might otherwise by cluttered with unnecessary detail.
• To show distributions that would, if mapped by conventional means, be obscured by wide variation in the sizes of the enumeration areas.

  Disadvantages of using a cartogram:

• Some map readers really do not like the look of cartograms-they percieve them as being too far from reality.
• Map readers may not be able to figure out what message that the cartogram is trying to convey-usually due to misunderstanding the logic of the mapping method.
• It may be difficult to make a mental connection between highly distorted enumeration units and the "real-world" counter parts.

• 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.