This article briefly discusses map generalization in general and how it affects maps in a broader context. It also describes a specific example of how map generalization rules affect the default maps and default map styles in MapTiler Cloud.
Cartographic generalization is a complex discipline that has been evolving for more than a hundred years with countless scientific publications and tons of research. This article is merely a brief overview of basic generalization operators.
Cartographic generalization, or map generalization, includes any changes in a map that are made when deriving a smaller-scale map from the larger-scale map or map data. It is a central part of cartographic design. Whether performed manually by a cartographer, or by a computer, or by a set of algorithms, generalization aims to abstract spatial information at a high level of detail into information that can be rendered on a map at a lower level of detail.
The cartographer can customize the content of their maps to create an appropriate and useful map that conveys spatial information while striking the right balance between the purpose of the map and the precise detail of the item being mapped. Well-generated maps are those that highlight key map elements while representing the world as faithfully and recognizable as possible.
Selection is the process of simply removing entire geographic features from the map. There are two types of selection, which are combined in some models, and separated in others. There are 2 basic types of selection: feature selection, and layer selection.
Simplification is the removal of vertices in lines and area boundaries.
For line features (and area boundaries), Smoothing seems similar to simplification, and in the past, was sometimes combined with simplification. The difference is that smoothing is designed to make the overall shape of the line look simpler by removing small details; which may actually require more vertices than the original. Simplification tends to make a curved line look angular, while Smoothing tends to do the opposite.
Merging involves combining neighboring features into a single feature of the same type, at scales where the distinction between them is not important.
Aggregation is the merger of multiple features into a new composite feature, often of increased Dimension (usually points to areas). The new feature is of an ontological type different than the original individuals because it conceptualizes the group.
Typify is a symbology operator that replaces a large set of similar features with a smaller number of representative symbols, resulting in a sparser, cleaner map. For example, an area with dozens of wind power plants might be symbolized with only 3 or 4 wind power plant symbols that do not represent actual wind power plant locations, just the general presence of wind power plants in the area.
This operator reduces the Dimension of a feature, such as the common practice of representing cities (2-dimensional) as points (0-dimensional), and roads (2-dimensional) as lines (1-dimensional).
This operator primarily simplifies the attributes of the features, although a geometric simplification may also result. While Categorization is used for a wide variety of purposes, in this case, the task is to take a large range of values that is too complex to represent on the map of a given scale and reduce it to a few categories that are much simpler to represent, especially if geographic patterns result in large regions of the same category.
Exaggeration is the partial adjustment of geometry or symbology to make some aspect of a feature larger than it really is, in order to make them more visible, recognizable, or higher in the visual hierarchy. For example, a set of tight switchbacks in a road would run together on a small-scale map, so the road is redrawn with the loops larger and further apart than in reality.
Displacement can be employed when two objects are so close to each other that they would overlap at smaller scales, especially when the exaggerate operator has made the two objects larger than they really are.
This is the addition of symbols or other details on a smaller scale map to make a particular feature make more sense, especially when such understanding is important for the map's purpose.
Generalization in MapTiler Cloud map styles
Let me give you an example of how some generalization rules influence the visibility of features in MapTiler Cloud maps. Let's say I use Maptiler basic map and I'm trying to get the minor roads in zoom level 7+ to be visible at all zoom levels (even in those higher than 7, like 4, 5, or 6). I have the min zoom level for "road_network" set to 0, but it seems there are other factors determining when the minor roads appear within the style.JSON file. What are they? below are two pictures that demonstrate the visibility of the road network across different zoom levels.
Zoom level: 6,39
Zoom level: 7,15
How to make the road network visible in higher zoom levels?
MapTiler cartography team defined the default map styles (including the Baic) in a way that complies with the rules of cartographical generalization and that renders the maps easy to read and somehow pleasant to the eye. As a result of that, certain features are by default visible only at certain zoom levels, with little possibility of altering the visibility range (usually making the visibility range shorter).
That is to avoid situations when e.g. a low-level road network would suddenly appear in higher zoom levels and would cover most of the map surface, making the map hard to read.
This is achieved by a set of rules in the style.json on one hand, and by uploading only a limited amount of tiles into the map repository on the other hand. Therefore, the road network cannot be made visible only by moving the sliders in the Advanced map editor, because the map tiles for that layer and for that zoom levels are simply not generated and uploaded to the map's repository.
To make the road network visible (going a bit against against the cartographical rules), you can generate a dataset that contains the map tiles with the road network in the required zoom levels (generate is with MapTiler Desktop) and once the new tileset is ready, upload it to MapTiler Cloud. Follow these steps:
- Download MapTiler Desktop
- Generate a new set of vector tiles (road network)
- Add the generated dataset to your custom MapTiler Cloud map
The map styles in MapTiler maps are designed by professional cartographers while following well-established principles as well as the latest trends in the field. The selection of features for each individual map follows these guidelines and makes the map as good as it possibly can be. In case you still wish to add certain features to zoom levels where these features originally do not appear, you can do it while using a set of other MapTiler cartographical tools, e.g. MapTiler Desktop or the Advanced editor.