Initial Publication Date: March 20, 2010

Week 6: Following Rivers Through Time

Intro to Symbolization in GIS

Symbolization is an important skill in cartography, or map making. It is the process of choosing an appropriate representation for specific features on a map. We can symbolize point features as dots, squares, triangles, flags, or other shapes and we can symbolize line features using solid, dashed, or other patterns. The symbols we choose should help describe additional information about the features on the map.

In general, we associate large size with greater numerical values and intense color with strong events. For example, when symbolizing earthquakes on a map, using dots that are all of the same color highlights the locations of the individual earthquakes. To emphasize the difference in distribution between high magnitude and low magnitude earthquakes, we can symbolize them using dots of varying sizes, with the largest dots representing the highest magnitude earthquakes. We can use color in a similar manner, symbolizing the higher magnitude earthquakes with intense shades, such as dark red and lower magnitude earthquakes with lighter, pastel shades, such as pink.

In his book, Visual Display of Information, Edward Tufte (1982) argues that graphic representations of data should be chosen with the same rigor we put into descriptive writing. Symbolization in a GIS can be a very powerful analysis technique, helping us to more easily see geographic patterns. However, when applied haphazardly, it can lead to misinformation and misleading interpretation of the underlying patterns.

This map shows earthquakes represented as dots of a uniform color.

This map shows earthquake magnitude, symbolized by size and color.