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Jeff Blossom: An Approach to Automate Block Cartogram Creation


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NACIS 2016 Presentation

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Jeff Blossom: An Approach to Automate Block Cartogram Creation

  1. 1. An Approach to Automate Block Cartogram Creation Jeff Blossom Center for Geographic Analysis Harvard University NACIS 2016 – Colorado Springs October 21, 2016
  2. 2. Our mission: “To support research and education that relies on geographic information” 1737 Cambridge St., Cambridge, MA
  3. 3. 10 full time staff (9 at this point – we’re recruiting a GIS Specialist) • •
  4. 4. Services: Education • 2 hour instructor led training workshops on various topics, offered Friday’s throughout both semesters: – QGIS – ArcGIS, ArcGIS Online – Google Earth – Spatial Analysis – WorldMap – Google’s Mapping Products – Coming soon….CARTO Also… • 2 full day workshops (cartography workshop!) • A 2 week Institute • Custom training modules for Classes, Centers, research groups, etc. To see a list of courses and register go to Click “Non-credit Training”
  5. 5. Services – Projects/consultation • Geocoding • Visualization • Analysis • Interactive web maps
  6. 6. “Ghost Rides” and Block Cartograms
  7. 7. https://www.e-
  8. 8. project for Prachi Sanghavi, Health Policy researcher. Map purpose: Visualize ghost rides and total ambulance rides Ghost rides: ambulance rides not reported at hospital
  9. 9. Ghost rides: ambulance rides not reported at hospital Size of counties adjusted to total rides project for Prachi Sanghavi
  10. 10. Map by Scott Zillmer
  11. 11. Max total rides per county: 57,266 (Harris county = Houston) Many counties with less than 50 Way too many dots to draw! Time to normalize. Solution: divide by 200, yielding 254 as the maximum number of dots (symbolized as squares) needed per county. Caveat: Counties with less than 200 rides each assigned one square.
  12. 12. Excel macro created one line for each Square needed, so Add XY in ArcMap produces the correct number of coincident points per county.
  13. 13. Disperse markers “expanded” with 6 point spacing
  14. 14. Disperse markers “squares” with 2 point spacing
  15. 15. Workflow for block cartogram creation with ArcMap Dispense Markers • Normalize your statistic if necessary • Convert areas to polygon centroids • Create table with one line per observation, with the correct X,Y of the centroid it refers to. • Cartography toolbox > Disperse markers • Play with the disperse method, point spacing, and symbol size.
  16. 16. Thanks for listening!
  17. 17. Thanks for listening!