Types of Scale

When you read the phrases “large scale” or “small scale,” do you know what they mean? Sometimes “large scale” describes a large area, and sometimes it describes a small area, depending on if the author was thinking about process scale or cartographic scale. This is a problem for communication. In this post, I will describe the different types of scale used in geography, which will hopefully encourage others to be specific when they are discussing scale.

This section of a soil map from 1902 of Abbeville County, South Carolina, illustrates the interrelationship between attribute scale, analysis scale, and cartographic scale on paper maps. The soil mapper was limited in how much detail could be included on that sheet of paper. If more delineations had been added, more variations in the feature space would have been identified, and the legend would have had to expand. Similarly, if the attribute scale included more details in the feature space, then the map units would need to be subdivided to show where those differences in attribute classes were.

First, we have to recognize that the world “scale” has evolved. Its origin reaches back to Old French or Germanic terms for cup or shell. The use of similar-looking objects to form the two sides of a weighing instrument likely explains the naming of that instrument as a scale. The verb “scale” primarily comes from the Latin “scala”, which describes a ladder or flight of stairs. From these various pathways in time, we are left with some central concept of measuring something in proportionality. As vague as that may sound, it does somewhat account for the wide range of uses we have for the word. Still today, our evolving understanding of spatial concepts continues to create more stems on the geographical branch of scale’s etymology tree. Therefore, even the traditional use of the word in geography now warrants clarification.

The most fundamental uses of “scale” in geography are the cartographic scale and the attribute scale. The cartographic scale (aka map scale) is the ratio between the size of objects in the real world and their representation on the map, making it a measure of proportionality in the most central idea of space, geographic space. The attribute scale (aka thematic scale) is equally important to cartographers because it is how the feature space is divided/measured for representation in the map legend.

As geography expanded from the design aspects of map-making to more spatial analysis, geographers began to recognize connections between the cartographic scale of a map and the spatial patterns observed in that map. Because the map was only a representation of reality, they needed a term to distinguish between the spatial characteristics of the reality and the spatial characteristics of the map pattern. Although mostly a philosophical concept, geographers describe the extent over which the pattern is seen operating as phenomenon scale (aka process scale). For example, in climatology the influence of the jet stream can be seen at global scales, while influences of topography produce micro-climates observable at local scales. Similarly, international politics operate at a global scale, while interstate commerce mostly operates at a regional scale.

The last type of scale to be discussed here needed to be recognized as a separate form of scale after we gained more flexibility in how we map. Although past cartographers had some control over the sizes of delineations in their maps, this was largely constrained by what could legibly fit on the piece of paper at the cartographic scale being used. Now with geographic information systems (GIS), that restriction or link between cartographic scale and delineation sizes has been removed. Geographers can now analyze space using whatever size or shape of spatial units that they please. Hence, we now also recognize analysis scale as separate from cartographic scale. We can view or produce maps at whichever cartographic scale suits our needs, and independently from that we can adjust the analysis scale as we search for patterns in the real world’s phenomenon scale.

So what about the original question of the difference between “large scale” and “small scale”? Well, usually larger means larger in size, and for most uses of the word “scale” that is true. However, the wrench in the works is the common expression of the cartographic scale in terms of a representative fraction. Because smaller fractions mean a greater difference between the size of objects in reality and the size of their representation on the map, a map with a smaller representative fraction can cover a larger area of reality. For example, a global map would probably have a small cartographic scale (e.g., 1:800,000), and a city map would probably have a large cartographic scale (e.g., 1:20,000). That is why it is a problem to only state the size of the scale. To avoid confusion, one should either specify the type of scale or use terms that don’t have multiple meanings, such as “extent” or “regional” versus “local.”

The cartographic scale of this city map is large because its representative fraction is large (originally about 1:4,000).
The cartographic scale of this global map is small because its representative fraction is small (originally 1:35,000,000).

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