PrintGeneralization Operators
As suggested by the previous OpenStreetMap example, generalization is also process for dealing with conflict and congestion among map symbols. Generalization can also be used as a strategy for creating a more readable and useful map by selectively including a limited set of labels, for example. Though this is a complex and context-dependent problem, some resources are available to help you determine the appropriate level of detail for your maps. The now-defunct website ScaleMaster (Brewer et al. 2007), for example, offered advice to mapmakers on which features ought to be included at different scales, and for different mapping purposes.
We will not go into the details of ScaleMaster in this lesson, but you are encouraged to read more about ScaleMaster through the linked Cartographic Perspectives article if you are interested. The most important takeaway is that different scales require differing levels of detail, and that the appropriate level of detail is mediated by the map’s context (e.g., topographic vs. zoning maps).
Generalization can be broadly categorized as either selection or symbolization. In the context of scale, selection is simple—it refers to the decision of whether to include (or not) a feature at a certain scale, while symbolization refers to alteration of the way a feature is designed in order to make its design more appropriate for the scale at hand. For example, when designing a small-scale map, you might choose to not include cities unless they are high population (selection), and to symbolize these cites as labeled points rather than as areas (symbolization). Generalization traditionally refers to reducing detail in a map as much as is necessary to maintain legibility and usefulness at a specified scale. Generalizing multi-scale web maps (which exist at many rather than one scale) is more challenging, but not fundamentally different—we can think of every possible scale step (or zoom level) of a multi-scale web map as its own map for which an appropriate level of detail must be determined.
As generalization is a fundamental topic in cartography, many cartographers have proposed theoretical frameworks for discussing generalization. For simplicity, in this lesson, we will focus on the set of generalization operators proposed by Roth et al. (2011), as they were developed based on a comprehensive review of previous literature. As we discuss generalization operators, an important distinction should be made between generalization operators and generalization algorithms. Operator refers to a cartographer’s conceptualization of an intended change (e.g., I want to remove some roads to reduce the visual clutter of this road network), while an algorithm is a system followed for implementing this idea (e.g., I will remove all roads with speed limits below 25mph) (Roth et al. 2011). Like Roth et al., we focus on operators rather than algorithms in this lesson as they are more widely applicable to map-making tasks, and not dependent on the use of specific datasets or GIS software tools.
Roth et al. (2011) classify feature generalization operators into three groups: content, geometry, symbol. Content operators directly alter the content of the map, typically by adding or removing features at particular scales. An example would be deciding not to include local roads or trails in a small-scale map as these features would not be visible at small scales. These operators include: add, eliminate, reorder, and reclassify.
Geometry operators describe the ways in which different features' geometry can be altered to create a map that is more legible and aesthetically pleasing. Examples include smoothing a line feature and representing a city as a point rather than an area. Geometry operators include: simplify, aggregate, collapse, merge, displace, exaggerate, and smooth.
Symbol operators alter feature symbology to improve legibility, but do not change the features’ underlying geometry. An example would be simplifying the pattern in an area fill so it still looks good at a smaller scale. Symbol operators include adjust color, enhance, adjust iconicity, adjust pattern, rotate, adjust shape, adjust size, adjust transparency, and typify.
It is not necessary to memorize the above operators, but you should aim to understand the difference between the three groups of operators (i.e., content, geometry, symbol) and think critically about situations in which each might be useful.
Recommended Reading
Brewer, Cynthia A., and Barbara P. Buttenfield. 2007. “Framing Guidelines for Multi-Scale Map Design Using Databases at Multiple Resolutions.” Cartography and Geographic Information Science 34 (1): 3–15. doi: 10.1559/152304007780279078.
Roth, Robert E., Cynthia A. Brewer, and Michael S. Stryker. 2011. “A Typology of Operators for Maintaining Legible Map Designs at Multiple Scales.” Cartographic Perspectives 68 (68): 29–64. doi:10.14714/CP68.7.
Brewer, Cynthia A., and Barbara P. Buttenfield. 2010. “Mastering Map Scale: Balancing Workloads Using Display and Geometry Change in Multi-Scale Mapping.” GeoInformatica 14 (2): 221–239. doi:10.1007/ s10707-009-0083-6.