Visual Variable

Any graphical object, including point, line and area symbols on maps, is drawn using a collection of visual devices that could potentially be drawn in different ways (hence "variable") and chosen by the designer. Although cartographers have been using these devices since the invention of maps, this concept was first developed during the advent of academic cartography in the mid-20th Century and systemized by Jacques Bertin in his 1967 book, Sémiologie Graphique.

In Sémiologie Graphique, Bertin recognized a core set of graphical variables that can be chosen and controlled to create symbols on maps and other graphic design media. This set has since become canonical and has been extended. The visual variables include:

• Size – Represents quantity or magnitude (e.g., larger circles on a population density map indicate higher values).
• Shape – Differentiates categories (e.g., squares for hospitals, circles for schools on a map).
• Color Hue – Shows qualitative differences (e.g., different colors for various land use types).
• Color Value (Lightness/Darkness) – Represents intensity or ranking (e.g., darker shades for higher elevations).
• Texture/Pattern – Distinguishes different surfaces or zones (e.g., hatching for forested areas).
• Orientation – Shows directional differences (e.g., wind flow maps with arrow orientation).
• Position – Determines spatial relationships (e.g., location of cities on a world map).

These variables can be used to establish contrast between different geographic features, build a visual hierarchy, and represent quantitative or qualitative attributes of the features.

Importance of Visual Variables

• Enhance data comprehension and make maps and charts more informative.
• Improve usability by allowing users to distinguish patterns and trends easily.
• Aid in decision-making by effectively communicating complex spatial data.

Conclusion

Visual variables are essential in cartography and data visualization, helping to effectively communicate spatial and statistical information. The choice of visual variables depends on the type of data being represented and the intended message of the map or chart.

• Size (Represents Magnitude or Quantity)

  • Example: A proportional symbol map uses larger circles to represent cities with higher populations and smaller circles for less populated areas.
  • Example: In a bar chart, taller bars indicate higher values, such as GDP per country.
     

• Shape (Distinguishes Categories)

  • Example: A transportation map uses different shapes (e.g., squares for bus stops, triangles for train stations, circles for airports) to differentiate transit types.
  • Example: Election maps use unique shapes to indicate different political party strongholds.
     

• Color Hue (Represents Qualitative Differences)

  • Example: A land-use map assigns different colors (green for forests, yellow for agricultural land, blue for water bodies) to visually separate land types.
  • Example: A subway map uses different colors for each train line to help users navigate easily.
     

• Color Value (Lightness/Darkness) or Color Intensity (Shows Intensity or Ranking)

  • Example: A choropleth map of population density uses darker shades of red to indicate areas with higher population density and lighter shades for less populated areas.
  • Example: A heatmap uses varying intensities of color (e.g., bright red for high crime areas, light yellow for low crime areas).
     

• Texture/Pattern (Distinguishes Surfaces or Zones)

  • Example: A soil type map uses different textures, such as dots for sandy soil, wavy lines for clay, and diagonal stripes for rocky terrain.
  • Example: A zoning map uses hatch patterns to show industrial, residential, or commercial areas.
     

• Orientation (Indicates Direction or Flow)

  • Example: A wind map uses arrows with different orientations to indicate wind direction and speed.
  • Example: A flow map shows migration patterns by using arrows pointing from one region to another.
     

• Position (Shows Spatial Relationships)

  • Example: A topographic map places elevation contour lines in positions that reflect the shape and height of terrain features.
  • Example: A weather radar map positions storm systems over geographic regions to indicate affected areas.