GIS software packages provide a wide variety of predefined colour palettes and options for user-created colour schemes in mapping. However, such easy access to the full colour spectrum can introduce problems that undermine the core purpose of the map in communicating information. Perhaps it's time to standardise colour palettes.
The use of colour in mapping provides a powerful and efficient way to convey spatial information. Colour schemes allow map-makers to communicate details such as differentiating between features, categorising features into themes, and showing changes in data values across space to highlight hotspots and critical values.
GIS software packages provide a wide variety of predefined colour palettes and options for user-created colour schemes in mapping. This offers a large degree of flexibility to map-makers in how geographic information can be communicated using colour. However, such easy access to the full colour spectrum can introduce problems that undermine the core purpose of the map in communicating information.
Colour vision deficiencies affect how people see colours and can lead to ambiguity in how colours are perceived. For example, red-green colour vision deficiency (protanopia or deuteranopia) affects 1 in 12 men and 1 in 200 women, and makes it difficult for people to distinguish between reds, oranges, yellows, browns, and greens (source). This has direct consequences for map-making as certain colour combinations will not allow some people to correctly interpret maps.
Figure 1: Colour wheel for normal and simulated deuteranopic vision.
Source: Ed Hawkins, Climate Lab Book
Colourful palettes are a popular choice in data visualisation. The spectral colour palette is often used in meteorological maps, elevation maps, and heat maps. In addition to being problematic for people with colour vision deficiencies, the use of such colour schemes has the potential to prevent the correct interpretation of maps. Their use can infer unintended significance to particular values, thereby introducing confusion about the geographic distribution of data values.
Figure 2: An example of the use of a spectral colGlobal Carbon Dioxide Transport from AIRS Data, July 2009.
Similarly, a diverging colour scheme (e.g. green to yellow to red) can be used to emphasise two values separated by a critical midpoint value. The use of this scheme is only appropriate when the map is intended to highlight extreme values separated by a significant midpoint value. The use of a diverging colour scheme in any other instance is likely to lead the map user to incorrectly attribute some significance to the midpoint value.
These examples highlight some of the issues that need to be considered when using colour in the GIS map-making process.
At ABPmer, we are looking to standardise our GIS palettes to avoid the pitfalls associated with the use of colour both in terms of colour vision deficiencies and the potential for colour to introduce ambiguity.
It remains to be seen if the software developers recognise this issue themselves and update products accordingly.
Prepared by Aidan Walsh, GIS Analyst.