Geographic Districting

Please write down the ID the instructor/TA assigned to you here ___________________________ (Print or copy to Word to fill out if you work on a digital version of this exercise.)

Estimated time:

Task

Time (minutes)

Introduction and preparation

15

Manual interactive districting

15

Automatic districting

10

Finishing

10

Total

50

Introduction

You will use GIS tools to do geographic districting exercise in this lab. The objective is to let you understand the geographic districting problem and teach you how to use GIS tools to find solutions to the problem. The sequence of lab exercise includes filling one pre-exercise and one post-exercise survey and using one manual interactive and one automatic GIS tool to carry out the districting task. This will be a timed lab, please follow instructor or TA’s instructions to finish this lab.

Instructions

You will need the following data and tools to finish this lab. Please do the followings.

  1. Start ArcMap and click on “Tools -> Extensions”. Check the Districting extension and close the extensions dialog window. If you don’t find the districting extension, that means it wasn’t installed on that computer. Please notify the instructor/TA and switch to another computer. (Note: The districting extension can be downloaded from the ESRI website http://support.esri.com/index.cfm?fa=downloads.samplesUtilities.viewSample&PID=15&MetaID=622. Users need to have administrator privilege on the computer to install the extension. They also need to have full read/right privilege on the installed folder (e.g., C:\Program Files\AiDistGen\) to run the tool. Users logged in not as a system administrator might also encounter unexpected errors. Usually one can just click OK to dismiss the message window and continue to finish the exercise.)
  2. If the districting toolbar doesn’t appear in the ArcMap menu, select “View -> Toolbars -> Districting” to add the toolbar to the menu. Ask the instructor if you cannot find the districting toolbar.
  3. You will need the files listed below for this exercise. On Portland State University campus, these files are also available in I:\Students\Instructors\Geoffrey_Duh\Optimization\Districting folder. You can also download the compressed file (districting.zip) here. You will need to decompress it to your working folder (e.g., c:\temp\your_name). Notify the instructor/TA if you cannot copy these files to your working folder.

distrinting.mxt (This is an ArcMap template containing the automatic districting tool.)

blockgrp shapefiles (These are the files associated with 47 census block groups in the NE Portland neighborhood.

 

Step I. Pre-exercise survey

Now, go to http://survey.oit.pdx.edu/ss/wsb.dll/jduh/districtingA.htm and complete the questionnaire.

 

Step II. Using ESRI districting extension

  1. Now you will use ESRI’s districting tool to solve a districting problem. The districting problem for this exercise is to group contiguous census block groups into three districts (or zones) that have the same population. Remember, the goal of solving this problem is to create districts that have as similar population sizes as possible. You don’t need to worry about the shapes of these districts as long as all the polygons of the same districts are contiguous (i.e., have shared boundaries).
  2. To use the ESRI districting tool, you need to create a plan to record the districting result. First, from the Districting menu, point to Districting Admin and click Create New Plan. Click Create a new plan workspace. Browse to your working folder and enter the Plan workspace name (e.g., “workspace”) and click Save. Enter the Plan name (e.g., “plan”) and click Next to continue the Districting Wizard.
  3. Check “Source geography has attribute data” option box and click OK to dismiss the message window. Under Source Geography, click Add New. From the Add Source Geography dialog box, click the geographic data source type drop-down arrow and select “shapefiles” as the geographic source type. Click the browse button and browse to select the “blockgrp.shp” shapefile in your working folder. Click Add.
  4. Enter the source geography name (e.g., Blockgroup) and select “FIPS” from the drop-down arrow for the key field name. Then, enter a source geography description (e.g., “Census blockgroup”). Make sure the universe field and optimal-value field are both set to POP90. Click OK to close the Add Source Geography window and return to the Create Plan dialog window.
  5. Click Next to continue. Set the number of districts to 3 and click Next. You should see 14977 as the value for the optimal target value. This number is the population for each district when all districts have the same population. It is derived by dividing the total population of the study area by the number of districts (i.e., 3). If the number on your screen is different, please notify the instructor/TA.
  6. Click Next and click Finish to complete the set up. Now STOP and wait for instructions.

 

STOP +++++++++++++++++++++++++++++++++++++++++++++++++STOP


  1. Now instructor/TA will demonstrate how to use the districting toolbar. Here is a brief summary of the tools.

1)      Select “Start Editing” from the Districting pull-down menu to begin the districting process.

2)      Select “Preference…” from the Districting pull-down menu and check “Assign changes immediately after selection” option button. Click OK to dismiss the preference dialog window.

3)      Click on the Statistic Window  icons to open the statistics window.

4)      Click the drop-down arrow after “District:” on the Districting toolbar to select the district (i.e., D1, D2, or D3) to which the selected polygon will be assigned. Select D1. Click the Select Features  icon and begin to assign features to districts. Make sure individual districts must be formed by contiguous block group polygons (i.e., the polygons must share edge segments). Select as many block groups for D1 as you think will help to make an even distribution. Then change the district to D2 using the District dropdown menu on the Districting toolbar. Select what seems like a fair share of block groups to District 2. Then assign block groups to District 3 in the same way. You can also click and drag across block groups to select them, although you can’t unselect them, so be careful. You can change the district of a block group by selecting the new district value from the District pulldown menu on the Districting toolbar and clicking on that block group with the Districting Select Features tool.

5)      When done, click the Plan Report icon to show a report of the districting plan. Transcribe the POP90 values to the table below.

  1. You will spend 10 minutes to find one solution by using a trial and error method to reassign block groups to districts 1, 2 and 3. Your goal is to create districts with equal numbers of people, which will minimize Sum Absolute Diff. in the table. After the instructor notifies you that the time is up, please write the finial population for each of your districts and the difference values (i.e., Pop90_dev) between district population and the target population (i.e., 14977) in the table below. Then, sum the absolute values of pop90_dev and write the result under Sum Absolute Diff. Record the results from your final attempt in the table below.

 

District

Pop90

Pop90_dev

1

 

 

2

 

 

3

 

 

 

Target Popu.

Sum Absolute Diff.

 

14977

 

 

  1. After you complete the table above, click the Chart Window and Statistics Windows icons to close these windows. Select File, then Exit. Select Click Yes to quit the editing session and click Yes to save the editing result in an ArcMap document (.mxd). You can open this mxd file to view the districting plan later. Navigate to optimization.mxt using Windows Explorer, and double click it to open it. After the file is open, please check if there is a Districting toolbar in the ArcMap menu bar. The toolbar has only one button labeled “Districting”. Notify the instructor if you cannot find it. Now STOP and wait for instructions. 

STOP +++++++++++++++++++++++++++++++++++++++++++++++++STOP

 

Step III. Using the Districting Optimization tool

  1. Now instructor/TA will demonstrate how to use the automatic districting tool to do the same districting exercise you accomplished in Step II. Here is a brief summary of the tools.

1)      Click the Districting tool button to open the Automatic Districting dialog window. In the dialog window, click the browse button to select the “blockgrp.shp” file in the c:\temp folder.

2)      After the blockgrp.shp map is displayed, set POP90 as the attribute field, change the # of zones to 3, make sure the “Use Contiguity Constraint” checkbox is checked (this will force districts to be contiguous). Accept all other defaults, and then click the Initialize button. Click Yes to overwrite the values in the districtid field. You can specify a different random seed number to generate different initial plans.

3)      Click Run to start the automatic districting process. Click Stop to stop the process. After you click Stop, you will have to wait a little while to let the tool refresh the districting result in ArcMap. Be patient! A window saying “Optimization stopped” will appear. Click OK.

4)      The default maximum number of trials is 5000. That means the optimization process will stop after 5000 trials. The best solution in these 5000 trials will be preserved and displayed in ArcMap when optimization stops. It takes less than 20 seconds to evaluate 5000 trials on the computers in the lab. You can initialize the optimization process with different random seeds (e.g., 5, 20, 389,…) and see which initial map results in the best solution. Just write down the random seed numbers and the value on the last row of the OFV (Objective Function Value) column in the report panel. The smaller the OFV value, the better the solution. You can always use the same seed number to regenerate an identical solution you got last time.

  1. You will spend 5 minutes to find one solution. After the instructor notifies you that the time is up, please write the random seed number you used to initialize the optimization process, the finial population (Pop90), and the difference values (i.e., Pop90_dev) between district population and the target population in the table below. Then, sum the absolute values of difference values and write the result under Sum Absolute Diff. Record the best result from your attempts in the table below.

 

Random seed number used:                .

District

Pop90

Pop90_dev

 1

 

 

2

 

 

3

 

 

 

Target Popu.

Sum Absolute Diff.

 

14977

 

 

  1. After you complete the table above, close the districting tool, exit ArcMap, and proceed to Step IV.

 

Step IV. Post-exercise survey

Go to http://survey.oit.pdx.edu/ss/wsb.dll/jduh/districtingB.htm and complete the questionnaire.

Please return this lab worksheet to the instructor before you leave the lab. Thanks!