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Symbology is the cartographic practice of representing information using symbols (and colours) on a Map. The Legend offers a quick summery of the symbology used for a Map , often presenting important layers of information in groups for easy reference.

For Layers with complicated symbology (perhaps listing icons, or changing colour based on attribute) the symbology will change on a feature by feature basis to reflect the values being communicated by the map.

General Cartography

Maps produced for general use showing a wide range of features. The maps are often produced against a set standard as part of a series.

The definition of the symbology used is provided by an organisation or government body; often with very exacting requirements that can difficult to reproduce exactly!


  • Maps used for emergency response will often dictate the set of symbols used (in keeping with the training of the operators)
  • Maps used for nautical navigation where lives are at risk based on the exact interpretation of the map provided. You will often find datasets containing a disclaimer that their information is not suitable for this purpose.

Thematic Cartography

This is the most common use for a desktop mapping application such as uDig. In this case the map is produced in order communicate information around a specific dataset; with a map of the physical location serving as a background.

  • Choropleth: differences in colour or shading used qualitative differences in an attribute.
    • Useful when the polygons are all roughly the same size; or you run the risk of making larger polygons appear more important.
    • A Dasymetric map is the opposite of this where the features are generated in order all be a similar size.
  • Proportional Symbol: Change point symbol size based on an attribute to illustrate relative values. ** Remember when mapping an attribute to symbol size (i.e. radius) the visual effect is not linear and you may wish to adjust accordingly.
  • Contour: A great way to render raster values such as height or atmospheric pressure where lines are drawn along the edge of value changes revealing shapes in the underling data; and the distance between these lines showing a rapid change in value.
  • Dot: used to represent individual observations or measurements

These maps are often used for purposes such as exploring data or scientific visualisation. Depending on the dataset and information being communicated a number of techniques may be employed.

Related concepts


Cartography - Map Type (wikipedia)

Thematic map (wikipedia)

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