37 lines
5.6 KiB
Markdown
37 lines
5.6 KiB
Markdown
---
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title: "Cartographic generalization"
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chunk: 4/5
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source: "https://en.wikipedia.org/wiki/Cartographic_generalization"
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category: "reference"
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tags: "science, encyclopedia"
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date_saved: "2026-05-05T15:17:39.374723+00:00"
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instance: "kb-cron"
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---
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=== Collapse ===
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Also called Symbolize
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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). Frequently, a Map symbol is applied to the resultant geometry to give a general indication of its original extent, such as point diameter to represent city population or line thickness to represent the number of lanes in a road. Imhof (1937) discusses these particular generalizations at length. This operator frequently mimics a similar cognitive generalization practice. For example, unambiguously discussing the distance between two cities implies a point conceptualization of a city, and using phrases like "up the road" or "along the road" or even street addresses implies a line conceptualization of a road.
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=== Reclassify ===
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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 is much simpler to represent, especially if geographic patterns result in large regions of the same category. An example would be to take a land cover layer with 120 categories, and group them into 5 categories (urban, agriculture, forest, water, desert), which would make a spatially simpler map. For discrete fields (also known as categorical coverages or area-class maps) represented as vector polygons, such as land cover, climate type, soil type, city zoning, or surface geology, reclassification often results in adjacent polygons with the same category, necessitating a subsequent dissolve operation to merge them.
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=== Exaggerate ===
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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. A symbology example would be drawing highways as thick lines in a small-scale map that would be miles wide if measured according to the scale. Exaggeration often necessitates a subsequent displacement operation because the exaggerated feature overlaps the actual location of nearby features, necessitating their adjustment.
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=== Displace ===
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Also called conflict resolution
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Displacement can be employed when two objects are so close to each other that they would overlap at smaller scales, especially when an exaggerate operator has made the two objects larger than they really are. A common place where this would occur is the cities Brazzaville and Kinshasa on either side of the Congo river in Africa. They are both the capital city of their country and on overview maps they would be displayed with a slightly larger symbol than other cities. Depending on the scale of the map the symbols would overlap. By displacing both of them away from the river (and away from their true location) the symbol overlap can be avoided. Another common case is when a road and a railroad run parallel to each other. Keates (1973) was one of the first to use the modern terms for exaggeration and displacement and discuss their close relationship, but they were recognized as early as Imhof (1937)
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=== Enhance ===
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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 the map purpose. A common example is the addition of a bridge symbol to emphasize that a road crossing is not at grade, but an overpass. At a large scale, such a symbol may not be necessary because of the different symbology and the increased space to show the actual relationship. This addition may seem counter-intuitive if one only thinks of generalization as the removal of detail. This is one of the least commonly listed operators.
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== GIS and automated generalization ==
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As GIS developed from about the late 1960s onward, the need for automatic, algorithmic generalization techniques became clear. Ideally, agencies responsible for collecting and maintaining spatial data should try to keep only one canonical representation of a given feature, at the highest possible level of detail. That way there is only one record to update when that feature changes in the real world. From this large-scale data, it should ideally be possible, through automated generalization, to produce maps and other data products at any scale required. The alternative is to maintain separate databases each at the scale required for a given set of mapping projects, each of which requires attention when something changes in the real world.
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Several broad approaches to generalization were developed around this time:
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The representation-oriented view focuses on the representation of data on different scales, which is related to the field of Multi-Representation Databases (MRDB).
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The process-oriented view focuses on the process of generalization.
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The ladder-approach is a stepwise generalization, in which each derived dataset is based on the other database of the next larger scale.
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The star-approach is the derived data on all scales is based on a single (large-scale) data base. |