Scale and Generalization
The meanings of scale
The word scale has a few different uses related to mapping.
Geographic scale is “scale” in the sense you may be used to in everyday life, referring to the general scope or extent of things. In this sense, “large scale” for example means something affecting or covering a large area, like a country or even the whole planet.
Cartographic scale may sound backwards at first blush. In the cartographic sense, “small scale” essentially means more “zoomed out” than large scale. Scale on a map is defined mathematically, often expressed as a representative fraction. For example, many USGS topographic maps have a scale of 1:24,000. This means that one inch on the map represents 24,000 inches in the real world. If you “zoomed in” an inch would represent less real-world distance, say 10,000 inches; thus the fraction (1/10,000) actually becomes a larger number and the maps scale is said to be larger. Remember it this way: an area appears larger on a large scale map, and smaller on a small scale map.
Data has scale too, in that it was collected or digitized at some resolution, which has implications for the map scale at which it can be displayed. Ideally a map should not have a larger scale than its data. For example Census data collected at the block level works well on a large scale map, but state-level data can’t be extrapolated down to block level and displayed as such. Scale or resolution also applies to how detailed the actual vector or raster geometry is.
At its core, cartography is about abstraction. We don’t show data in its raw form; we clarify it in a variety of ways, often by removing things. It simply isn’t possible to show every tiny detail! Data and graphics should be generalized appropriately to the map scale: basically, a large scale a map can (and often should) have more detail than at small scale.
Common generalization tasks include:
Selection: choosing which objects to include on the map
Simplification: reduce the number of vertices in an object
Smoothing: reduce sharp angles to smoother curves
Aggregation: group points into areas
Amalgamation: group areas into larger areas
Collapse: reduce a detailed object to a point symbol
Merge: grouping of line features
Refinement: select only portions of an object to display
Exaggeration: amplify a part of an object (for clarity)
Enhancement: add detail that visually elevates an object
Displacement: separate objects (for clarity)
Multi-scale map design
Generalization is a hugely important task in modern mapping, as many web maps cover a wide scale range and thus many different levels of generalization. Increasingly, some of the work is done for you behind the scenes, algorithmically. For example, consider Mapbox’s OpenStreetMap-based vector tiles, which deliver data pre-simplified to levels appropriate for various scales:
Notice how coastline, roads, labels, etc. become more detailed as the map is zoomed in. Part of that is because the data itself is simplified. In this case that’s done automatically, but in other scenarios you may need to do it yourself, using a tool like Mapshaper to create several different versions of your data, simplified to different levels.
The other half of the equation, of course, is the cartographer’s design choices. Most map design tools, including GIS software, allow you to specify style rules according to scale. Generally speaking, in addition to objects becoming more numerous and detailed at larger scales, points, lines, and labels should become larger at larger scale.