Cartographic abstraction

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Cartographic abstraction

• Summary session
• GEO381/550
• October 5th, 2004

2
Outlines

• Basics
• Geographic phenomenon
• Describing data distribution
• Components of cartographic abstraction
• Data classification
• Quantitative classification methods
• Simplification
• Map symbolization
• Visual variables by measurement scale
• Map types by the behavior of geographic phenomenon

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Basics

• Geographic phenomenon
• Measurement scale
• Data distribution

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Geographic phenomenon

• Location, Scale
• Spatial dimension
• Continuous vs. discrete
• Q. number, Mars, human organ
• Q. Tornado path, elevation
• Q. Temperature, cold/hot, population, population
  density

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Measurement scale of geographic phenomenon
Nominal Ordinal Interval/Ratio
Concept Type, category Result of ranking Result of measuring
Example Male/female, agricultural region Mega/large/medium/small city Temperature, Mortality rate
Year, land use, elevation, strongly
agree/strongly disagree, religion, coffee
consumption, national income, occupation
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Describing data distribution
Nominal Ordinal Interval/Ratio
Central tendency Mode most frequently occurring value Median value exactly in half when ranked Mean ? Sx / N
Dispersion Variation ratio Quartile deviation Standard deviation S (x-?)2 / N
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Histogram and descriptive statistics
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Components of cartographic abstraction

• Selection
• Classification
• Simplification
• Symbolization

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• Selection preliminary steps
• Classification
• Simplification data processing
• Symbolization choosing symbols

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Classification

• Group values into class such that geographic
  pattern can be better revealed

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How do you determine class boundary?

• Equal interval
• put any number of values into class with the same
  interval
• Quantile
• put the same number of values into class
• Natural break
• marginal change in values
• Standard deviation
• how much deviated from the mean?

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Data classification method
Equal interval
Quantile
l1
l2
l3
l4
l5
a1
a2
a5
a4
a5
s
?
Natural break
Standard deviation
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Simplification

• Alter geometry such that relevant details are
  pronounced while irrelevant details are
  suppressed

Line simplification
Area dissolution
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Criteria for symbolization

• Measurement scale ? visual variables
• Use ordering visual var. for quantitative scale
• Use distinguishing visual var. for qualitative
  scale
• The behavior of phenomenon ? map types
• Observed in a discrete/continuous scale in a
  abrupt/smooth frequency
• Maps sometimes reflect the way data collected
  rather than phenomenon. (e.g. crime is reported
  in the unit of jurisdiction)

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Appropriate use of visual variables- measurement
scale -
qualitative quantitative
point Shape Size
line Shape, Hue Size
area Hue, Arrangement Value, Texture
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Appropriate choice of map types - behavior of
phenomenon -
abrupt smooth
discrete Graduated symbol map Dot density map
Chorodot
continuous Choropleth map Isopleth map
Because of the discrepancy between phenomenon and
data, we need to process data by manipulating
spatial scale. Handling GIS data well is an
essential skill for advanced map-making!