Maptitude for Redistricting Tech Corner
How can I use political data (collected by precinct) in my plan?
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.
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
- (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.
- 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.
- 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.
- 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).
- 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.
- Return to Step 4 and use the appropriate formula for all the fields
you wish to disaggregate.
To Aggregate the VTD Data to Counties
- (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.
- 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.
- Choose Dataview-Join, click the Options
tab, and check Allow all types of joins.
- 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.
- 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.
- Click OK in the Join dialog box. Maptitude for Redistricting creates
a one –to-many join (one county to many VTDs).
- 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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