Symbology¶
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!
Examples:
- 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