How Can I Use Political Data (Collected by Precinct) in My Plan?

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Often you have political data at the VTD level, and you want to use it in a plan. Since a field identified as a plan summary field must be in all layers of the geographic hierarchy, it would be necessary to disaggregate or aggregate the data to each layer in the hierarchy. You would disaggregate the VTD data to lower levels of geography such as census blocks and aggregate it to higher levels of geography such as counties. In addition, the data may have been collected using old VTD boundaries, but you want to estimate the results based on new VTD boundaries.

Maptitude for Redistricting contains all the tools required to do this. You can use the Disaggregation/Aggregation tool in Redistricting>Plan Utilities>Disaggregate/Aggregation to disaggregate or aggregate data from a geographic layer or table. However, there is another way of attaching political data to maps.

It is best explained using the example of comparing 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 Maptitude for Redistricting lets you perform many of the tasks associated with the redistricting process without the need for special programming. All the capabilities are present in Maptitude for Redistricting 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 these block data to the new VTDs.

To Disaggregate the VTD Data to Census Blocks

  1. (Note: the census block layer on the Redistricting Data CD already containsa VTD field; if you are using this layer, skip to step 3.) If there is not already a field in the census block table for the VTDs, open the dataview for the census block layer, choose Dataview>Modify Table, add a new character field of length 11 called VTD.
  2. If you had to create a VTD field, click the heading for the new VTD field, choose Edit>Fill, click the Tag radio button, choose the VTD layer from the Using Layer drop-down list, choose the VTD field from the Tag With drop-down list, and click OK. Maptitude for Redistricting fills the VTD field with the appropriate VTD identifier for each census block.
  3. Choose Dataview>Modify Table, add a new 4-byte integer field for each VTD field that you want to disaggregate (e.g., Registered Democrats, Registered Republicans, and all election results), and click OK to close the Modify Table dialog box. Maptitude for Redistricting adds the new fields to the census block table.
  4. Choose Dataview>Join, choose Census Block from the Joining from Table drop-down list, choose VTD from the Joining from Field drop-down list, choose Voting District from the To Table drop-down list, and choose VTD from the To Field drop-down list. Maptitude for Redistricting creates a one-to-many join (one VTD to many census blocks).
  5. Click the heading for the first column that is to contain the disaggregated data (e.g., Registered Democrats), choose Edit>Fill, click the Formula radio button, create the formula:

    VTD.[Registered Democrats] * Block.[Voting Age Population] / VTD.[Voting Age Population] + .5

    and click OK. Maptitude for Redistricting fills the field with the formula.

  6. Return to Step 4 and use the appropriate formula for all the fields you wish to disaggregate.

To Aggregate the VTD Data to Counties

  1. (Note: the VTD layer on the Redistricting Data CD already contains a County field; if you are using this layer, skip to step 3.) If there is not already a County field in the VTD table, open the dataview for the VTD layer, choose Dataview>Modify Table, add a new character field of length 5 called County, and click OK to close the Modify Table dialog box. Maptitude for Redistricting adds the new fields to the VTD table.
  2. If you had to create a County field, click the heading for the new County field, choose Edit>Fill, click the Tag radio button, choose the County layer from the Using Layer drop-down list, choose the County field from the Tag With drop-down list, and click OK. Maptitude for Redistricting fills the County field with the appropriate County identifier for each VTD.
  3. Choose Dataview>Join, click the Options tab, and check Allow all types of joins.
  4. Click the Settings tab, choose County from the Joining from Table drop-down list, choose County from the Joining from Fields drop-down list, choose Voting District from the To Table drop-down list, and choose County from the To Field drop-down list.
  5. Click the Options tab, and next to the One to Many Join radio button, click aggregations. Click the Clear button to reset the list and check Add for each of the fields you want to aggregate (e.g., Registered Democrats, Registered Republicans, and all election results). Click OK to close the Aggregation dialog box.
  6. Click OK in the Join dialog box. Maptitude for Redistricting creates a one –to-many join (one county to many VTDs).
  7. If you want to make the aggregated fields a permanent part of the county layer, choose Dataview>Fields, and drop the fields in the join that you don’t want to copy. Then, choose Redistricting>Plan Utilities>Copy Joined Fields.
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