7.3 The 4 semiological errors

This section aims to present the most common mistakes encountered in cartography, so that you can avoid repeating them.

TABLE OF CONTENTS

• Proportional circles vs. solid colors
• Color gradient vs. distinct color palette
• Gradient in a single color vs. Gradient between two colors
• Limited geometric symbols vs. complex icons and symbols

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Proportional circles vs. solid colors

This is one of the most common mistakes in mapping. While this representation is graphically appealing, it is still false and distracts from the message of the map. Many media and other map producers make this mistake.

A mistake because:

• You lose the order relationship between the data (a circle can be twice as big as another one, a color cannot be “twice as dark”)
• Countries with a large surface area stand out more visually (example: Russia in the example below)
• We are trying to represent a data that has nothing to do with the area of a country, but rather speaks of individual elements (pilgrims) agglomerated.

NOT TO BE DONE

Legend : Number of pilgrims

Source : Mapping the Muslim World and its many “MapFail”, Sylvain Genevois, 2018

TO BE DONE

Legend :

• Proportional circles: Number of muslims
• Color ramp: Part of muslims in the total population

Source: “The World of Islam. Where do Muslims live?” France culture, 10/12/2015

Color gradient vs. distinct color palette

A representation that “feels right” because it seems logical that a “low” rate would be represented differently than a “high” rate.

A mistake because :

• By using a differentiating color visual variable you lose the ordinal relationship between entities. It is a visual value variable (gradient of the same color) that should be used.
• The visual variable of differentiating color is used to clearly differentiate between distinct entities.

NOT TO BE DONE

Title: Area committed to organic farming (2016)

Legend: Percentage of the area committed to organic farming in relation to the useful agricultural area (UAA)

• Between 10% and 22,7%
• Between 6% and 10%
• Between 3% and 6%
• Between 0% and 3%

Source: Immersion, Surface agricole biologique en France

TO BE DONE

Title: Share of organic agricultural land by department in 2018

Source: Immersion, L’agriculture bio en France (2020)

OR

Legend :

Share of organic UAA in 2017 (%)

Areas committed to organic farming in 2017

• Green and Orange : 20 000
• Orange: Areas in conversion
• Green: Certified organic areas

Source: Agriculture biologique: quelle région est la plus bio? Futura Science, 2020

Gradient in a single color vs. Gradient between two colors

This is a mistake that is often found because our brains are used to prioritizing certain colors, especially green to red, or blue to red. We must remember that if our values are always positive, we must stay in the same color that we will degrade into several shades.

A gradient between two colors can be used when it is necessary to show a gradation that can go from negative to positive. As for temperatures, it makes sense to distinguish negative values (in shades of blue for example) and positive values (in shades of red).

A mistake because:

• It is a mistake because the map will send a message of divergence, of opposition between certain values, when we are simply trying to represent a hierarchy between values.
• In this way, the color itself directly indicates information about the trend (positive/negative or increasing/decreasing)

NOT TO BE DONE

Legend: Ratio between the share of primary residences in metropolitan France represented by a department in 2015 / in 1968 (%)

Source: V. Dejoux et al, “Panorama de l’évolution des conditions de logement en France depuis la fin des années 1960”, Espace populations sociétés 2019

TO BE DONE

Title: UAA evolution trend between 2014 and 2018

Source: Parcellaire agricole : Carte par commune des tendances générales d’évolution de la SAU de 2000 à 2018, corrèze.gouv.fr

Limited geometric symbols vs. complex icons and symbols

Multiplying symbols (and data) on a map is a common desire, to have a comprehensive map, but you have to be quick to pay attention to the overload of the map and the readability of the map. Using too many symbols (especially geometric ones) makes it difficult to read and therefore understand the map.

The eye can easily distinguish 4 to 5 different symbols, beyond that, it is difficult to quickly distinguish the elements.

An error because :

• This is a less “serious” error, because it does not return false information on the map, but it complicates the map and limits its impact.
• Sometimes you are forced to represent several symbols, so you must be careful about overlapping points and overloading the map.

NOT TO BE DONE

Source: internal to CartONG

TO BE DONE

Title: Some meetings of the organized civil society (1996-2016)

Legend:

• WTO Ministerial Conference
• G8 Summits since 1998
• G20 Summits since 2008
• G8 and G20 members
• Other G20 members
• World social forum
• Thematic or regional social forum

Source: WTO, The historical meetings of organized civil society in the world.