Maptitude for Redistricting
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Maptitude for Redistricting Tech Corner

How do I compare election and voter registration data from different elections where VTD boundaries have changed?

Maptitude for Redistricting makes it easy to compare election and voter registration data from different elections where VTD (precinct) boundaries have changed. In fact, this a perfect example of how the robustness of the Maptitude platform lets you perform many of the tasks associated with the redistricting process without the need for special programming. All the functionality is present in Maptitude itself.

Assume that you have election and registration data for an election held in 1998. Since then, you have participated in the Census Phase 2 Program and have modified your VTD boundaries for 2000. You would like to "move" the 1998 data to the new VTDs.

The process requires two steps. First, you disaggregate the 1998 VTD data to the underlying census blocks. Second, you aggregate this block data to the new VTDs.

The following is a step-by-step mini tutorial so that users of Maptitude for Redistricting can try it out and everyone else can see how straightforward it is to do with the software:

To Disaggregate the Old VTD Data to Census Blocks

  1. If there is not already a field in the block data base for VTD (note: the block data on our Redistricting Data CD already contains a VTD field), choose Dataview-Modify Table and add a new character field of length 11 called VTD.
  2. Add a 4-byte integer field for each VTD field that you want to disaggregate (e.g., Registered Democrats, Registered Republicans, and all election results).
  3. Highlight the new VTD field in the block dataview, and choose Edit-Fill-Tag with the VTD field of the VTD layer to fill this field with the appropriate VTD identifier for each block.
  4. Click the Join button, and join the VTD dataview to the block dataview using the common VTD identifier. This is a one-to-many join (one VTD to many blocks).
  5. Highlight the first column that is to contain the disaggregated data (e.g., Registered Democrats). Choose Edit-Fill. This time, fill the column with the formula VTD.Registered Democrats * [Census Block].Age_18+ / VTD.Age_18+ + .5
  6. Repeat step 5 for all fields you wish to disaggregate.

To Aggregate the Disaggregated Data at the Block Level to the New VTDs

  1. Make the New VTDs the Working Layer.
  2. Choose Tools-Overlay and overlay the New VTDs with the Blocks. This operation produces a new table of block data aggregated by new VTD.
  3. Click the Join button, and join the aggregated data table to the New VTD geography layer using the unique New VTD identifier.

That’s it!

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