sug243 - cartography (generalization)

GeneralisationGeneralisation

Generalisation and Symbolisation 2

GeneralizationGeneralization =Cartographic generalization The selection and simplified representation of detail

appropriate to the scale and/or purpose of the map (ICA 1973)

Generalization is the process of reducing the information content of maps due to scale change, map purpose, intended audience, and/or technical constraints. (Slocum et al)

To fit portrayal of selected features to the map scale and to the requirements of effective communication (Robinson et al 1995)

Generalization is "the reduction of detail or simplification of reality“.

Reduction of the complexity of a map to ensure its fitness for use.

maps cannot retain 'all detail', and if they could, they would be too complex to understand.


GeneralizationGeneralization Is an umbrella term for several processes, all

intended to remove unnecessary detail. Because maps cannot show everything,

mapmaker must select which features to show and which to omit.

Features need to be simplified to be legible at smaller scale.

Data need to be classified, divided into smaller groups of similar values.

Finally symbol must be chosen to represent features on the map.


Purpose of Purpose of GeneralizationGeneralization the reduction in scope, amount, type and

cartographic portrayal of mapped or encoded data maintenance of graphical clarity at the target

scale Basic principle of map generalization is to

emphasis salient object AND omit less important ones

Ensure readability AND preserve geographical meaning of the map as faithfully as possible.


GeneralisationGeneralisation and and symbolisationsymbolisation The elements of generalisation The controls of generalisation Classification, simplification and exaggeration

manipulations Symbolising geographical features


Selection and Selection and GeneralisationGeneralisation Selection - to limit our concern to those classes of

information that will serve the map’s purpose Which elements remain depends on the purpose

of the map : topographic, thematic etc How many elements remain depends mostly on

map scale: if map scale is halved, then the total map area size is reduced by 4, and the number of elements: points, lines and areas are reduced accordingly


Generalisation ConceptsGeneralisation Concepts Classification - order, scale and group features by

their attributes Simplification - determine important

characteristics of feature attributes and eliminate unwanted detail

Exaggeration - enhance or emphasise important characteristics of the attributes

Symbolisation - use graphic marks to encode the information for visualisation and place them into a map

Induction-making inferences from interrelationships among features on map.


The Elements of The Elements of GeneralisationGeneralisation Classification

Qualitative attributes Quantitative attributes

Simplification Exaggeration

E.g. 20m street on 1:100,000 map scale would be only 0.2mm wide

Symbolisation


Effect of Scale on Effect of Scale on Visual Perception Visual Perception

There is a practical level of detail asssociated with any scale. Traditionally on printed maps, one could identify a 'minimal resolvable unit' of about 0.5 mm. This translates for common scales to these amounts in metres:

1:20,000 10 1:50,000 25 1:25,000 125 1:1,000,000 500

That is, if your map or display is for example at 1:50,000 it is not reasonable to be able to depict a feature or detail smaller than about 25 metres in size. Data captured at one scale are not transferrable to widely different scales. Data captured at a larger scale are too detailed for smaller scales, and data captured at a small scale are too generalised for larger scales.


ClassificationClassification

Classification of a point pattern. After clustering the points, the cartographer selects a position within each cluster and places a dot to “typify” the cluster. The “typical” position need not coincide with the position of any of the original data points.

From Robinson, et al., 1995


SimplificationSimplification

Simplification by point elimination. In the illustrated clusters of points, one original point is selected to represent each cluster of original points on the generalised map.



Simplification Simplification (Cont.)(Cont.)

Reducing map scales leads to the consequent simplifications.



ExaggerationExaggeration

Two representations of Great Britain and Ireland. (A) simplified to fit the scale and is suitable for a reference map intended to give the impression of detailed precision. (B) diagrammatric generalisation suitable as a base on which to display thematic data.



Generalization-Generalization- Point features Point features


Generalization-Generalization-linear featureslinear features


Generalization – Generalization – Areal featuresAreal features


The Controls of The Controls of GeneralisationGeneralisation Map purpose and conditions of use Map scale Quality and quantity of data Graphic limits

Physical limits Physiological and psychological limits


Classification ManipulationsClassification Manipulations

Point feature methods Collapsing Typification

Line feature typification Aggregation of areas Aggregation of volumes - e.g.

Classification for choropleth maps


CollapsingCollapsing

Illustrations of the collapsing process in cartographic generalisation. Each feature represented in the left diagrams has lost at least one dimension in its portrayal in the right diagram.

Cited in Robinson, et al., 1995


Representations illustrating typification by classification of point, line and area features.


TypificationTypification


Aggregation Aggregation of Areasof Areas

The original data area mapped at a scale of 1:X. (A) represents a smaller-scale agglomeration of the original data. (B) represents the further aggregation of areas for an even smaller-scale representation.



Elimination Point elimination Area elimination

Smoothing Filtering

Simplification Simplification ManipulationsManipulations


EliminationElimination

Simplification accompanied by scale reduction. Since the scale is successively reduced from (A) to (E), an increasing number of points in the outline must be eliminated.

After Robinson, et al., 1995


Elimination Elimination (Cont.)(Cont.)

Simplification applied at a constant scale. The four maps (A through D) represent increasing simplifications of the coastline and hydrography.



Point EliminationPoint Elimination

Simplification of the outline by point elimination. The points indicated on the map to the left were retained on the map to the right where they were connected with straight-line segments. All points not selected on the map to the left were eliminated in producing the map on the right.



Line Simplification ProcessLine Simplification Process

Successive stages in line simplification process: (1) The initial line. (2) Point C, having the greatest perpendicular distance to line AB in (1) is selected for retention. Lines AC and CB are drawn. (3) The elimination of points between points A and C, because no perpendicular exceeds the threshold, and retention of point D, because its perpendicular distance to line CB does exceed the threshold.



Area EliminationArea Elimination

Simplification by feature elimination. Areas on the left map are either shown in their entirety or completely eliminated in the feature-simplified map on the right.



Area Elimination Area Elimination (Cont.)(Cont.)

Simplification by area elimination. Example algorithm using size and proximity to determine which features to eliminate.


sug243 - cartography (generalization)

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