VISUALIZATION QUALITY. Visual Representation. MacEachren s Model. See: MacEachren, 1994, Some Truth with Maps AAG Resource Series, Chpt.

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1 VISUALIZATION QUALITY See: MacEachren, 1994, Some Truth with Maps AAG Resource Series, Chpt. 4 Visual Representation Mapping constraints MacEachren s Model 1

2 Symbolizing Data in Areas First step - determine the nature of the phenomena you are mapping E.g. Population, tax rates, number of service workers, average income This is NOT the same as your DATA Phenomena Space: Two dimensions in Phenomena Space Discrete versus Continuous variation Discrete: phenomena occurs at isolated locations; areas of no occurrence between Continuous: phenomena occurs at all places in varying amounts A continuum: phenomenon may fall at either end, or somewhere in between Abrupt versus Smooth variation Abrupt variation: distinctive and substantial breaks possible Smooth variation: continuous variation over space with no breaks A continuum: phenomena may fall at either end, or somewhere in between Data examples in phenomena space: From MacEachren p. 59 2

3 Symbolization Space The next step is to match this to an appropriate symbolization method... Symbolization space: same dimensions as phenomena space Choosing an Appropriate Symbolization of the Data MacEachren fig. 3.25: Gould mapped AIDS data with four different area symbolization techniques Matching symbol space to data space AIDS data - available by county in Pennsylvania Choropleth map represents continuous data with symbolization of abrupt variation But, AIDS phenomena is not evenly distributed in each county There is an important distinction between phenomena and data AIDS as a phenomena is actually less abrupt and more discreet than the data 3

4 What to do? Map the data with a choropleth map -- but that reflects the way the data was collected rather than the phenomena being mapped Consider other methods Graduated circle map: more appropriate than choropleth map, as it is less apt to imply that the phenomena of AIDS is equally distributed in each county. It may imply that there is NO variation at all in each county. Dot map: Dots imply variation through county, but also implies that AIDS is discreet - and as a disease it is not (it is contagious). Isopleth map: implies AIDS is contagious, but also everywhere, which is not the case. Better if you are trying to scare people. But in reality AIDS as a phenomena is not as continuous (it isn't really everywhere) and it is not so highly contagious (like influenza or measles) as to be easy to catch with casual contact. As a disease, AIDS is actually a phenomena that is: between continuous and discreet and between abrupt and smooth Back to symbolization space: most appropriate is the middle technique: the "chorodot" 4

5 AIDS example reveals: the difference between phenomenon itself and the data representing the phenomenon that you have to think about how you symbolize your data you have to be familiar with the phenomenon understand how different area symbolization techniques are less or more appropriate Problem: The best area mapping method may not be easily chosen Truth in Interpretation Visual representation Function: Presentation - avoid misunderstanding Exploration - flexibility, various symbol-referent links Avoiding erroneous interpretation Visual quality is linked to representation of uncertainty A Statistical Analogy Hypothesis testing Null Hypothesis Two Kinds of Error Rejecting the null hypothesis when it is in fact true (TYPE 1 Error) Seeing wrong Failing to reject the null hypothesis when it is false (TYPE 11 Error) Not seeing 5

6 SEEING WRONG Identification of False Patterns or Relationships Scale Dimension Transformation Interpolation DATA TRANSFORMATION Dimension and Interpolation Points transformed from spherical 3D to plane 2D then data interpolated Interpolation on points in spherical space (3D) then transformed to plane (2D) MacEachren, 1994, Fig Map Transformation (Projection) Conformal (Mercator) Equal area (Goodes Interrupted) MacEachren, 1994, Fig

7 Map Stability Spatial Resolution 50 meters 500 meters MacEachren,1994, Fig 4.04; Map Stability Depends on data and symbolization method GIS makes multiple views easy Change data or change symbolization Multiple views provide different insights for exploration notice significant patterns Patterns that remain stable in different views provide confirmatory evidence pattern is likely to exist BUT Seeing Wrong & Not Seeing still a danger INTEGRATION GIS provides us with power to integrate map analysis, statistical analysis and modeling Flexible Shareable Visual thinking aide 7