Cartographic abstraction Summary session GEO 381550 October 5

Cartographic abstraction Summary session GEO 381/550 October 5 th, 2004

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

Basics Geographic phenomenon Measurement scale Data distribution

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

Measurement scale of geographic phenomenon Nominal Ordinal Interval/Ra tio Concept Type, category Result of ranking Result of measuring Example Male/female, Mega/large/ Temperature, agricultural medium/sma Mortality rate region ll city Year, land use, elevation, strongly agree/strongly disagree, religion, coffee consumption, national income, occupation

Describing data distribution Central tendency Dispersion Nominal Ordinal Interval/Rat io Mode: most frequently occurring value Variation ratio Median: value exactly in half when ranked Quartile deviation Mean: = Σx / N Standard deviation Σ (x - )2 / N

Histogram and descriptive statistics

Components of cartographic abstraction Selection Classification Simplification Symbolization

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

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

How do you determine class boundary? l Equal interval l l Quantile l l put the same number of values into class Natural break l l put any number of values into class with the same interval marginal change in values Standard deviation l how much deviated from the mean?

Data classification method Equal interval l 1 l 2 l 3 l 4 Quantile l 5 a 1 a 2 a 5 a 4 a 5 σ Natural break Standard deviation

Simplification l Alter geometry such that relevant details are pronounced while irrelevant details are suppressed Line simplification Area dissolution

Criteria for symbolization l Measurement scale visual variables l l l Use ordering visual var. for quantitative scale Use distinguishing visual var. for qualitative scale The behavior of phenomenon map types l l 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)

Appropriate use of visual variables - measurement scale qualitative quantitative point Shape Size line Shape, Hue Size area Hue, Arrangement Value, Texture

Appropriate choice of map types - behavior of phenomenon discrete abrupt smooth 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!