Cartography Guide

A short, friendly guide to basic principles of map design

Scale and Generalization

The meanings of scale

The word scale has a few different uses related to mapping.

Generalization

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.