Office of Planning, Environment, & Realty (HEP)
Creating Statewide County-to-County Commuter Flow Maps Using ArcView 8.X
Todd Steiss and Jonathan Garner
The following step-by-step process utilizes recently released county-to-county commuter flow files from the 2000 Census and the power of GIS to develop commuter flow maps at the statewide level.
County-to-county commuter flow maps for small groups of counties can be easily generated without the aid of GIS. A metropolitan area of six counties would only require 30 county-to-county commuter flow lines, 6 intra-county commute figures, and some method to represent commuter flows into and out of the metropolitan area. Even a metropolitan area of this size may require several hours to develop a map representing commuter flows. A statewide commuter flow map may involve over a hundred counties with thousands of county-to-county commuter flow combinations. At this level of complexity, a more innovated approach using GIS is warranted.
The steps below were successfully performed for several case studies. The process assumes some reasonable knowledge of ArcView 8.X. Once the basic process is understood, several variations in data sets and level of geography can be used such as multiple state analyses, the use of Traffic Analysis Zones rather than counties, or creating commuter flow maps by means of transportation.
Step 1: Create County Boundary File
This step requires a shapefile that has all the county boundaries that will be used to generate the county-to-county flow map. ESRI provides a "Counties" shapefile for the entire US with your copy of ArcView 8.X. Another source is the ESRI website: http://arcdata.esri.com/data/tiger2000/tiger_download.cfm. If this source is used, the individual county shapefiles by state need to be downloaded and then merged into one shapefile. Whatever source is used, the county boundary shapefile should have fields with the Census designated ID codes for the state and counties either separated or combined. It is recommended as a minimum that for a statewide map, all the counties for the subject state and all adjoining states be used.
Step 2: Create County Center Point File
There are several ArcView scripts available on the ESRI website to create a polygon to point shapefile. Suggest using "ET GeoWizards" by Ianko Tchoukanski. This script allows the user to do many geoprocessing features including convert "Polygon to Point." Select your county boundary shapefile as the input layer and name the output shapefile "County_Point". Select "Center points" to create the point shapefile. You will need to add X and Y coordinates to the "County_Point" shapefile. Suggest using "Calculate XY" by Jon Barlett to do this task.
Step 3: Create County Flow Table
Download the "Work County" excel file for the subject state from the Census website: http://www.census.gov/population/www/cen2000/commuting.html. Delete the title rows (first four rows) and the rows with footnotes at the bottom of the table. Rename the column headers in Dbase format (short and simple). Example:
Save the file in DBF 4 (dBase IV) format and rename the file "ST_2WRKCO", where "ST" is the state abbreviation. The state abbreviation is moved to the front of the file name to avoid an ArcView 8.X error. If the name of a shapefile begins with a number, contains a space or special character, an error will occur during the file joining process.
Step 4: Add and Populate Fields to the County-to-County Flow Table
Open the "ST_2WRKCO" table in Arc Map.
Pair_C (Text, 12), Pair_N (Short Integer, 12), Res_stco (Text, 5), Wrk_stco (Text, 5), x_coord (Double, 10, 6), y_coord (Double, 10, 6).
There is a leading zero in the Wrk_St code that should be removed. Do a calculation on the Wrk_St field using: Mid ([WRK_ST],2 )
Then populate the new fields doing field calculations as follows:
Pair_C: "9"+ [RES_ST] + [RES_CO] + [WRK_ST] + [WRK_CO]
Res_stco: [RES_ST] + [RES_CO]
Wrk_stco: [WRK_ST] + [WRK_CO]
Join the "County_Point" attribute table to the "ST_2WRKCO" table. Use the Res_stco field from the "ST_2WRKCO" table and the state/county code from the "County_Point" attribute table. If a combined state/county code field does not exist in the "County_Point" attribute table, this will need to be added to complete the joining process. After joining the "County_Point" attribute table to the "ST_2WRKCO" table, populate the x_coord and y_coord fields with the X and Y values from the "County_Point" attribute table.
Field Calculator may prompt that there is an invalid row because there is no x or y coordinate to copy over. Click "Yes" to continue and the x_coord and y_coord values will be 0 in areas not represented by the "County_Point" shapefile. These records represent outlying areas beyond the subject and adjoining states and can be left as zero X, Y coordinates (0,0), assigned to a default coordinate just outside the subject and adjacent statesExample: -73, 38 for North Carolina outliers), or assigned coordinates based on geographic location (north, south, east, west, or by state).
Remove the join and clear selection if one was made. Export the "ST_2WRKCO" table as "ST_Res.dbf", where "ST" is the state abbreviation. Add the "ST_Res.dbf" table to the current map.
Join the "County_Point" attribute table to the "ST_2WRKCO" table again. This time use the Wrk_stco field from the "ST_2WRKCO" table and the state/county code field from the "County_Point" attribute table. Populate the x_coord and y_coord fields with the X and Y values from the "County_Point" attribute table. This will overwrite the previously stored x_coord and y_coord values.
Remove the join. Export the "ST_2WRKCO" table to "ST_Wrk.dbf", where "ST" is the state abbreviation. Add the "ST_Wrk.dbf" table to the current map.
Step 5: Creating County-to-County and Intra-County Commute Files
Add the "ST_Wrk.dbf" and "ST_Res.dbf" files as an X and Y event theme. The "ST_Wrk" event theme should have points representing all county center points in the subject state. The "ST_Res" event theme should have points representing all center points where there is a residential county origin within the subject state and adjacent states. The default points(s) should also appear. The location of the default point(s) will depend on how it was defined in Step 4.
Use the Geoprocessing Wizard to merge the "ST_Wrk" and "ST_Res" event themes into a new "ST_flow" shapefile, where "ST" is the state abbreviation. This shapefile will have points representing all origin and destination combinations listed in the "ST_2WRKCO" table. If the table is sorted by the Pair_N (or Pair_C) column, there will be two records for each Pair_N (or Pair_C) id code but with different x and y coordinates. The exception is where the origin and destination is in the same county, in which case the x and y coordinates are the same.
Use the "ST_Wrk" event theme and "Select by Attribute" where field Res_stco =Wrk_stco. This will select only the records where the county of residence and county of work are the same. Export this as a new shapefile called "St_Internal", where "ST" is the state abbreviation.
Remove the "ST_Wrk" and "ST_Res" event themes.
Using the "ET GeoWizards" by Ianko Tchoukanski again, you will convert "Point to Polyline." Select your "ST_Flow" shapefile and save the output shapefile as "ST_Flow_Line". Specify your id field as Pair_N.
The results should be a shapefile with lines connecting all the points representing all the origin and destination combinations listed in the "ST_2WRKCO" table. This may result in having several hundred lines all over the map. The next step will screen out and hide most of these lines.
Step 6: Create County-to-County Flow Theme
Join the “ST_Flow” attribute table to the “ST_Flow_Line” attribute table. Use the ET_ID field from the “ST_Flow_Line” attribute table and the Pair_C field from the “ST_Flow” attribute table.
Create a Graduated Symbol theme with the "ST_Flow_Line" shapefile using the ST_flow.CNT field. Since every possible combination is shown, it is difficult to identify any distinctive pattern of commuter flow. It is best to redefine the value classes in which the lowest class symbol is given a 0 line thickness. The value classes will vary by state and preference. In the example for the state of North Carolina 5 classes were used. The symbol size range was set from 0 to 4. The Ranges were: 1-500; 501-1000; 1001-5000; 5001-10000; and 10001-(max number for an external county).
Step 7: Create Intra-County Commute Theme
Use the "ST_Internal" shapefile to create symbols representing the internal commute patterns. Use the CNT field to create a Graduated Symbol theme. The value classes will vary by state and preference. In the example for the state of North Carolina 5 classes were used. The symbol size range was set from 4 to 18. The Ranges were set to: 1-10000; 10001-50000; 50001-100000; 100001-200000; and 200001-(max number for an internal county).
Step 8: Summary of Results
The dots represent the internal county commuting and the lines represent the county-to-county flow. Large dots usually indicate that the county has a large employment base and small dots represent counties with a small employment base. Although the county-to-county lines do not explicitly show the direction of flow, it can be generally assumed that the major flow represented by the visible line is in the direction from small dots to large dots.
Any hidden lines representing the minor "reverse" flow would typically be from large dots to small dots. There are techniques to offset the lines between counties and use arrows to show both directions of flow, however, this would add significant complexity to a county-to-county commuter flow map at the state level.
Other general conclusions can be inferred by the map results. Counties with small dots that have major flow lines to large dots often represent bedroom communities to a major employment base. Small dots in isolation or that are connected to only a few other small dots tend to represent rural communities with a small residential and employment base. The commuting patterns in some metropolitan areas can be considerably complex. In these cases, more detailed mapping maybe required (ie directional flow lines).
Step 9: Optional Directional Flow Lines
Using the default template, the county with the larger commuter flow volume to a neighbor county will overshadow the smaller volume moving in the reverse direction. This occurs because the two connecting flow lines between county center points overlap. Adjusting the template with an offset and an arrow can make the map clearer.
Click on the template and open the symbol properties. Change the line type to "Cartographic Line Symbol". Select the "Line Properties" tab and adjust the offset. The example uses 3. Select the right-facing arrow type to delineate the direction of the commuter flow. Changing the color and transparency of the lines or points may also add to the display quality.
Adding directional lines adds more details for analytical interpretation but may produce a map that can be rather complex, particularly at the statewide level. This option may be best reserved for more detailed analysis for a smaller group of counties.