1. Mapping Quantities: Choropleth Maps Gary Christopherson
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Mapping Quantities: Choropleth Maps Gary Christopherson

2. Review – Preview Review Preview Foundational issues Symbolization
Cartographic processes, geodesy, data models, etc.
Symbolization
Symbol types
Visual Variables
Color
Data Classification
Mapping Quantities
Choropleth Maps
Dot Density Maps
Graduated Symbols
Etc.
Design Issues
Layout
Text
Web Mapping
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3. Mapping Quantities
Many, if not most maps are about quantities of things tied to points lines or polygons
A variety of ways to illustrate quantities
This is the start of a series of lectures looking at different ways to map quantities
Most of these start with data classification
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4. Flow Map
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5. Flow Map
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6. Graduated/Proportional Symbol Map
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7. Graduated/Proportional Symbol Map
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8. Cartograms
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9. Continuous Surface Map
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10. Isarithmic Map
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11. Isarithmic Map
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12. Dot Density Maps
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13. Choropleth Map
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14. Choropleth Map
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15. Choropleth Map
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16. Choropleth Map
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17. Choropleth Map
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18. Definition 1: Choropleth
From Greek:
Choros = place
Plethein = to fill
A method of cartographic representation which employs distinctive color or shading applied to areas … usually statistical or administrative areas. (International Cartographic Association)
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19. Choropleth Mapping What you do Determine symbolization
Mapping area features based on quantity – provides another layer of information beyond location
What you need:
Geometry – polygons
Quantifiable attribute/s
(Almost always )normalized data (for example, data expressed as a rate or density)
Classified data
What the GIS does:
Stores and displays geometry and attributes
What you do
Determine symbolization
Normalize data
Classify Data
Graduated color
Identify patterns in the data
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20. When to Use Choropleth Maps
Theme in well defined enumeration units
Data must work as ratios or proportions
Not necessary to portray actual values in each enumeration unit
Simplified data assumptions must be acceptable to map maker and map reader
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21. 3D Histogram or Stepped Statistical Surface
Choropleth Maps
3D Histogram or Stepped Statistical Surface
Choropleth
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22. Definition 2: Enumeration Units
The spatial extent, or area in which data were collected (enumerated). For example, US census data is collected in a variety of spatial units, including blocks, block groups, and tracts.
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23. Geometry = Enumeration Units
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24. Geometry = Enumeration Units
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25. Definition 3: Data Normalization
The process of dividing one numeric attribute value by another to minimize differences in values based on the size of areas or the number of features in each area. For example, normalizing (dividing) total population by total area yields population per unit area, or density
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26. Data Normalization
Helpful to view data in tables or graphs – this helps the cartographer understand his data
It is often necessary to calculate rates or density measures
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27. Definition 4: Data Classification
The process of sorting or arranging entities into groups or categories; on a map, the process of representing members of a group by the same symbol, usually defined in a legend.
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28. Classification Choropleth maps use data that have been classified
Number of classes usually between 5 and 10, more likely 5 than 10
Classification methods vary depending on data and on the story you are telling
ArcGIS includes a number of different classification schemes
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29. Same Data, Different Classification Schemes
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30. Preliminary Considerations in Choropleth Mapping
Geographic Phenomena
Number and Kind of Enumeration Units
Map Scale
Data Processing
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31. Geographic Phenomena What is it that is being mapped?
What are the appropriate geographic data for the questions being asked and answered?
Are there data that can serve as proxy for the question?
How does the phenomena behave?
Agriculture in the US
States? Counties? Etc
e.g. crop acres for agriculture
Spatial, aspatial, in relation with other geographic phenomenon
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32. Number and Kind of Enumeration Units
Number of units are related to scale
More enumeration units = higher resolution, or greater detail
Type and Size are determined by scale and questions to be answered
Often determined by data available
It would be nice to have these data at a higher resolution, but they don’t exist.
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33. Number and Kind of Enumeration Units
Number of units are related to scale
More enumeration units = higher resolution, or greater detail
Type and Size are determined by scale and questions to be answered
Often determined by data available
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34. Map Scale Appropriate scale is determined by:
Necessity – scale should be sufficient to accommodate symbol recognition
Available space – the size of the sheet. Too small and scale problems will occur; too large and you have extra space but no extra data
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35. Appropriate Data Enumeration units must be areal
Attribute data must be quantitative
Attribute data should be normalized e.g. rates or density
rates e.g. percent Hispanic
density e.g. population per square mile
Assumes data is spread uniformly throughout enumeration unit
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36. Data Classification and Legends
We covered data classification schemes previously
List enumeration units
Modify legend values if necessary
Best to have multiples of 25, or 50, or 100
Try to change class values as little as possible
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