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Maptitude for Redistricting Tech Corner

Home > Maptitude for Redistricting > Tech Corner > Disaggregating Data

I have precinct geography (and data) that does not follow block boundaries. How can I disaggregate the precinct data and split the blocks to reflect accurate information?

We have a Polygon Overlay add-in that computes the percentage overlap between two layers you specify. The output is a new geographic file that lists the percentage of overlap between each area in Layer 1 with each area in Layer 2. You can download the add in from this web site. When you install the add in, it will automatically be placed in the Tools-Add Ins dialog box.

Create the Block Pieces Layer
We will use Census Blocks as Layer 1 and Precincts as Layer 2 in the following examples. The new geographic file that is created is block pieces (the pieces are created by precinct boundaries splitting the blocks). Type Block Pieces as the layer name and label name (in the Polygon Overlay dialog box), and name the dbase file and geographic file accordingly.

The Block Pieces geographic file contains one record for every block piece, and the following six fields:

Field Comments Example
ID The unique ID for each new area The unique ID for each Block Piece
Area The area for each new feature The area for each Block Piece
AREA_1 The ID for each area in Layer 1 The ID for each Census Block
PERCENT_1 The percent of each feature in Layer 1 that lies within the feature from Layer 2 The percent of each Census Block that lies within the Precinct listed in AREA_2
AREA_2 The ID for each area in Layer 2 The ID for each Precinct
PERCENT_2 The percent of each feature in Layer 2 that lies within the feature from Layer 2 The percent of each Precinct that lies within the Census Block listed in AREA_1

Append Demographics from the Census Block Layer
The next step is to append voting age population. Appending voting age population to the new block pieces file requires disaggregating the block data to the block pieces. You should:

  1. Choose Dataview-Modify Table and add a new 4-byte integer field for either the Census Block Voting Age Population field (named 18+_Pop) – you could also use Population, and click OK to close the Modify Table dialog box. Maptitude for Redistricting adds the new field to the Block Pieces table.
  2. Ensure that both the Block Pieces layer is open and the Census Block layer is open.
  3. Select Join from the Dataview menu.
  4. Click on the Options tab of the Join dialog box and check Allow all types of joins.
  5. Click on the Settings tab of the Join dialog box and select the following values:

    Joining From Table: Block Pieces
    Field: Area_1
    To Table: Census Block
    Field: ID

  6. The joined view will open.
  7. Click the heading for the first column that is to contain the disaggregated data (e.g., 18+_Pop), choose Edit-Fill, click the Formula radio button, create the formula:

    [Census Block].18+_Pop * PERCENT_1 + .5
  8. Repeat for all fields you want to disaggregate.
  9. Drop the join by using Dataview-Drop Join and selecting the joined view.

Disaggregate Precinct Data
Now that the Block Pieces layer contains the census block population, you can disaggregate the Precinct data to the Block Pieces. To disaggregate the Precinct data, you should make Block Pieces the Working Layer and:

  1. Choose Dataview-Modify Table and add a new 4-byte integer field for each Precinct 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 Block Pieces table.
  2. Choose Dataview-Join and select the following value:

    Joining From Table: Block Pieces
    Field: AREA_2
    To Table: Precincts
    Field: ID

  3. Maptitude for Redistricting creates a many to one join (many block pieces for one Precinct).
  4. 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:

    Precinct.[Registered Democrats] * Block Pieces.18+_Pop / Precinct.18+_Pop + .5
  5. Click OK. Maptitude for Redistricting fills the field with the formula. When you are done disaggregating, closed the joined view.

Recreate the Census Block Layer
The final step of this procedure is to append the disaggregated precinct data from the Block Pieces layer to the Census Block layer. To do this, you should go to Dataview-Join and select the following value:

Joining From Table: Census Block
Field: ID
To Table: Block Pieces
Field: Area_1

The join will aggregate the disaggregated precinct data [by census block piece] to the census blocks. To make the join permanent, choose Redistricting-Plan Utilities-Copy Joined Fields.

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