Density mapping

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Density mapping

• J426 Class 24
• April 9, 2009

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Overview

• Limitations of point data
• Crime density mapping
• Choices in density mapping

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Limitations of point pata

• Point maps of crime locations are very useful but
  can have limitations in understanding patterns of
  crime
• Judgments about patterns subjective
• Problems in perception of patterns
• Problem of hidden points

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Aggravated assaults in 2003
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Problems in perception of patterns

• Which areas have the highest occurrence (crimes
  per area) of aggravated assaults?
• Can certainly identify areas that are higher and
  lower
• More difficult to say which areas are the
  highest, given the number of points

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Problems of hidden points

• Can have multiple crimes, points at same location
• 131 locations on map have 2 or more aggravated
  assaults (maximum of 5)
• On smaller scale maps, nearby points can overlap
• 727 out of 1929 aggravated assaults within 250
  feet of another, producing significant overlap at
  this scale (estimate dot size around 500 feet)

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Locations of multiple aggravated assaults
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Locations of aggravated assaults within 250 feet
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Zoom in on section of previous map
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Crime density maps

• Attempt to portray the overall distribution of
  crimes in the area
• Idea is to estimate the density of crimes per
  unit area at each point on the surface
• Created and displayed using raster (grid cells)
• Estimate crime density for center of each grid
  cell

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Aggravated assaults in 2003 points
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Crime density map for aggravated assaults
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Crime density map with aggravated assault points
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Creating crime density maps

• Use ArcGIS Spatial Analyst extension
• Creates raster grid
• For center of each grid, counts the number of
  points within specified search radius
• Calculates the densitycrimes per area of circle
• Assigns that value to grid cells
• These are values displayed in graduated color
  thematic map

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Choice of search radius

• Creating density map involves specification of
  search radius
• Larger radius looks at crimes in larger area
• Results in smoother maps emphasizing broad
  pattern
• Smaller radius looks at crimes in smaller area
• Results in maps emphasizing more localized
  variations in occurrence of crimes

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Aggravated assaults search radius 2,500 feet
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Aggravated assaults search radius 5,000 feet
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Aggravated assaults search radius 10,000 feet
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Choice of density type

• Manner in which density calculated
• Simple density just counts the total number of
  points within the search radius
• Kernel density gives more weight to crimes closer
  to grid cell center than to those farther away
• Choice of density type
• Kernel usually better for visual display
• Simple sometimes better when using density
  surface for certain types of analysis

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Aggravated assaults simple density radius 5,000
feet
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Aggravated assaults kernel density radius 5,000
feet