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How to create flow maps depicting journey-to-work flows across counties

The following steps describe a procedure to make "desire line" maps using a GIS.

I used Transcad, a CALIPER product, to make maps depicting worker flows into and out of King County, Washington. While Transcad has an inbuilt tool to make flow diagrams, you can write an Avenue or AML script to do the same in ESRI data products.

STEP 1: Data Access and Preparation
  1. Access data from Census Bureau Journey to Work Website: http://www.census.gov/population/www/socdemo/journey.html

Table formats include html, and ASCII

  1. Two files are available for county-county geography. One file is sorted by county residence, and the other is sorted by county of work

    Open the residence HTML file in MS Excel, and find the county of origin you want.

Select King County

  1. Find the destination counties you want

Select King, Pierce, Snohomish, Skagit, and Kitsap

  1. Repeat step 2 for each county that you are interested in.
  1. Create a dbf file.
    • Copy the columns containing State and county FIPS for residence, State and county FIPS for work, and number of workers into a spreadsheet.
    • The new spreadsheet should contain the following columns: State&county FIPS for residence, State&County FIPS for work, and total workers.
    • Combine State and County FIPS codes to create a numeric value as and ID for both residence and work. Name the headers for these IDs as FIPS_R and FIPS_W respectively.
    • Save this file as a D-BASE file.

Combine State and County FIPS codes to create a numeric value as and ID. For example, in MS Excel you can use the "concatenate" function, and then copy and paste the text as numeric values

Saved file to king_origin.dbf

  1. For the counties in steps 2,3, and 4, you can access the geographic files from the ESRI geography network at www.geographynetwork.com or alternately from the CALIPER Data CD-ROM.

    Ensure that this file contains a concatenated field for State and county FIPS. If not, follow step 5d.

Select King, Pierce, Snohomish, Skagit, and Kitsap counties into one layer called Wa_counties.shp

STEP 2: USING TRANSCAD SOFTWARE to make "desire line" maps
  1. Open an empty map (File-Open), and add the geographic file for counties.
Map 1: Contains the layer: Wa_counties.shp
  1. Open the DBASE/Spreadsheet file containing the worker flows
Open (File-Open) the table named "King_origin.dbf."
  1. Click on the Matrix Menu, and then click on Import. This activates the matrix import wizard.
    1. On the next screen, choose the defaults "Import by creating a new Matrix file" and that "the dataview has one record for each cell with a field for each matrix." Click on next.
    2. Choose the default "Name" on the next screen, for the "selection" button, choose all features, for row ID, choose residence FIPS, and column ID choose Workplace FIPS, and select the flow field for values.
    3. TRANSCAD reads the D-BASE or spreadsheet file into a matrix.
b. Choose the default "Name" on the next screen, for the "selection" button, choose all features, for row ID, choose FIPS_R, and column ID choose FIPS_W. Selected "Flow" as values field.

c. Saved file to Wash1.mtx

  1. To make "desire lines"
    1. Make sure the map on which you want to display desire lines is the current window.
    2. Choose Tools-Geographic Analysis-Desire Lines to display the Desire Lines dialog box.
    3. Choose "County" as the input layer "Name", "All records" for "FROM", and "All records" for "TO", and FIPS for the ID field. Click on "Next".
    4. The desire lines should show up on your map.

Named the output layer containing the "desire lines" as "King_origin.dbd".

Step 3: Formatting the Map
  1. You can follow steps 1, and 2 to produce a map for depicting flows TO King County. The only difference is that in step 1-2, you would choose the HTML file sorted by work end.
  1. Ensure that the width of the flows on the two maps are to the same scale.
  1. Adjusting the county polygon centroids.
    1. On the maps, you will notice that Transcad uses areal centroids to draw the flow lines. Many times, you may be interested in moving these lines, so that the flows realistically depict worker flows.
    2. Locate the approximate population centroids for the counties on a paper map.
    3. Click on the "TOOLS-MAP EDITING - TOOLBOX"
    4. Click on the "Modify Tool".
    5. Clicking on the map will now activate the centroids. You can select one centroid by pointing, and move it closer to the population centroid by dragging.
    6. Repeat step e for all the centroids.

Journey to Work Communte Flows: King County, WA 1990-2000

Map of King County, Example Only, no real data.

Map of King County, Example Only, no real data.

Map of King County, Example Only, no real data.

Map of King County, Example Only, no real data.

Journey to Work Communte Flows: Pierce County, WA 1990-2000

Map of Pierce County, Example Only, no real data.

Map of Pierce County, Example Only, no real data.

Map of Pierce County, Example Only, no real data.

Map of Pierce County, Example Only, no real data.

Updated: 05/03/2011
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