Thematic maps

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Thematic maps

• J426/J520/V550 Class 7
• February 1, 2011

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Overview

• Graduated color maps
• Using counts versus rates
• Graduated symbol maps
• Proportional symbol maps
• Dot maps

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Thematic maps

• Display data associated with geographic features
• ArcGIS provides several types
• Graduated color
• Graduated symbol
• Proportional symbol
• Dot

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Graduated color maps

• Show some attribute table quantity value
  associated with features
• Classify features by value, dividing features
  into set of classes based on values
• Display feature class on map by using graduated
  colors, color ramp

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Example graduated color map UCR crimes per acre
by beat
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Using counts versus rates

• Counts are quantities of something, like
  population
• Rates are quantities normalized (divided) by
  other quantities
• Generally do not display counts for arbitrary
  areas such as census tracts or police beats
• Differences in counts may reflect differences in
  sizes of areas, not real differences in incidence

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Rates, normalized values

• One count divided by another count to show
  incidence
• Population per area (density)
• Number of crimes per area or population
• Black population divided by total population
  (proportion of population black)
• Births to teenage mothers by total births or by
  total teenage girls
• Need to be clear about idea you are trying to
  communicate

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Mapping number of crimes versus crimes per acre
Number of crimes
Crimes per acre
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Mapping crimes per acre versus crimes per
population
Crimes per acre
Crimes per population
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Graduated symbol maps

• Vary size of symbol, width of line based on some
  quantity
• Population size of cities as point features
• Stream flows or traffic volumes for line features
• Number of crimes in IPD beat area
• Generally will be displaying counts, with size
  varying by count

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Example graduated symbol map UCR crimes by beat
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Proportional symbol maps

• Similar to graduated symbol maps in that symbols
  are varied by size to display quantity
• Features are not classified into a (generally)
  small number of classes
• Rather, size of symbol is made proportional to
  value

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Dot maps

• Illustrate density of quantity being mapped
• Appropriate for count data
• Need to understand and communicate that dot
  density maps display overall patterns of
  distribution
• Locations of dots assigned randomly within areas
• Little use in crime mapping where you have
  locations of individual incidents or crimes

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Dot density map by beat versus point crime
locations
Point crime map
Dot density crime map