Cartographic symbol scaling lesson (and tool)

The Problem

Too often values are displayed incorrectly as symbols. While bar symbols and the values they represent increase one dimensionally, icons, circles and volumes, such as spheres, depict values exponentially larger than their change in height. These errors occur when individuals oblivious to this phenomenon scale multidimensional symbols based on their heights and as a result display appearances of values that are orders of magnitude too large.

Graphic explaining visually the problem between scaling symbols linearly rather than based on area or volume

Selecting the correct symbol type

The goal of a symbol is to communicate a value, either nominal (existence), ordinal (rank), interval (difference) or rational (absolute value). These symbols should only be used for rational values. Whether we should use a linear, area or volume symbol depends on the relative range of the values (A map with the values 1, 2, 3, 4 will appear the same as one with 1,000, 2,000, 3,000, 4,000).

If the values have a small relative difference then bar symbols may be best. Area symbols are better for moderate value ranges and volumetric symbols for larger relative value ranges.

The question of which symbol type to choose simply comes down to noticeable difference. You wish your different values to appear noticeably different. If you use spheres to display values with little variation it will be rather difficult to tell them apart. Alternatively if you have a very wide range of values and use the bar symbols, the small value symbols will not even be visible.


The resource provides a tool to demonstrate how customizable values should be scaled correctly, and exported.

Tool example output

Created by Cyrille Médard de Chardon
on 2024-05-24