SUG243 - Cartography (Generalization)
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Transcript of 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.
Generalisation and Symbolisation 3
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.
Generalisation and Symbolisation 4
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.
Generalisation and Symbolisation 5
GeneralisationGeneralisation and and symbolisationsymbolisation The elements of generalisation The controls of generalisation Classification, simplification and exaggeration
manipulations Symbolising geographical features
Generalisation and Symbolisation 6
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 and Symbolisation 7
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.
Generalisation and Symbolisation 8
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
Generalisation and Symbolisation 9
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.
Generalisation and Symbolisation 10
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
Generalisation and Symbolisation 11
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.
From Robinson, et al., 1995
Generalisation and Symbolisation 12
Simplification Simplification (Cont.)(Cont.)
Reducing map scales leads to the consequent simplifications.
From Robinson, et al., 1995
Generalisation and Symbolisation 13
Simplification Simplification (Cont.)(Cont.)
Reducing map scales leads to the consequent simplifications.
From Robinson, et al., 1995
Generalisation and Symbolisation 14
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.
From Robinson, et al., 1995
Generalisation and Symbolisation 15
Generalization-Generalization- Point features Point features
Generalisation and Symbolisation 16
Generalization-Generalization-linear featureslinear features
Generalisation and Symbolisation 17
Generalization – Generalization – Areal featuresAreal features
Generalisation and Symbolisation 18
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
Generalisation and Symbolisation 19
Classification ManipulationsClassification Manipulations
Point feature methods Collapsing Typification
Line feature typification Aggregation of areas Aggregation of volumes - e.g.
Classification for choropleth maps
Generalisation and Symbolisation 20
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
Generalisation and Symbolisation 21
Representations illustrating typification by classification of point, line and area features.
Cited in Robinson, et al., 1995
TypificationTypification
Generalisation and Symbolisation 22
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.
From Robinson, et al., 1995
Generalisation and Symbolisation 23
Elimination Point elimination Area elimination
Smoothing Filtering
Simplification Simplification ManipulationsManipulations
Generalisation and Symbolisation 24
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
Generalisation and Symbolisation 25
Elimination Elimination (Cont.)(Cont.)
Simplification applied at a constant scale. The four maps (A through D) represent increasing simplifications of the coastline and hydrography.
From Robinson, et al., 1995
Generalisation and Symbolisation 26
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.
Cited in Robinson, et al., 1995
Generalisation and Symbolisation 27
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.
Cited in Robinson, et al., 1995
Generalisation and Symbolisation 28
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.
Cited in Robinson, et al., 1995
Generalisation and Symbolisation 29
Area Elimination Area Elimination (Cont.)(Cont.)
Simplification by area elimination. Example algorithm using size and proximity to determine which features to eliminate.
Cited in Robinson, et al., 1995