Redistricting’s partisan impacts: a GIS analysis

Our team at the Center for Urban Research is collaborating with The New York World to analyze the impacts of redistricting in New York State.  The latest effort was featured today on the front page of the Times Union; it focuses on how the majority parties in the State Senate and Assembly would likely retain — and strengthen — their control of both houses through the redrawn district lines.

Briefly, we found that the new boundaries for state Senate and Assembly districts proposed by LATFOR would increase the number of seats held by the majority parties in both chambers.  We based the analysis on 2010 election data available from LATFOR’s website.  The goal was to determine the results of state legislative elections held within the new districts if voters cast their ballots in the exact same way as they did in 2010, the most recent election year for State Senate and Assembly.

  • In the State Senate, the Republican Party’s 32-to-30 majority would expand to 34-to-29 if each voter cast his or her ballot in support of the same party as in the 2010 elections.
  • In the State Assembly, the 98-to-50 advantage the Democrats enjoyed following 2010’s elections would also increase, to 102-to-48.

The project was a good example of the power of GIS.  The analysis didn’t necessarily need a map to display the results (though Michael Keller at the NY World put together a nice one). But the analysis effectively wouldn’t have been possible without GIS.

Converting Polygons to Points

We analyzed election results at the level of voter tabulation districts, or VTDs, which are several blocks in size and typically cast no more than a few hundred votes in state legislative elections.  We mapped the VTDs onto the new lines proposed by LATFOR, then added up the votes of all VTDs that fell within a proposed district to determine its outcome.

In order to allocate the VTD-level vote counts to LATFOR’s proposed districts, CUR matched VTDs spatially with the current and proposed legislative district using ESRI’s ArcGIS Desktop software. The current and proposed Senate and Assembly districts are coterminous with Census blocks (in fact, the districts are “built” using Census blocks).  Unfortunately, neither LATFOR nor the state’s Board of Elections provides election results at the block level.  The Board of Elections records data by election district, which sometimes are smaller than VTDs, but for this project we did not have access to the election district data.

The challenge was that where the VTDs were larger than Census blocks in some places, the VTD boundaries crisscrossed the district lines (see example at right from Queens; click to enlarge).  In order to assign Senate and Assembly district IDs to each VTD, CUR converted the VTD boundaries to centroids (the geographic center-point of each VTD).  We used the lat/lon centroid values provided by the Census Bureau’s TIGER data.  Then we used a spatial join using ArcGIS to add legislative district identifiers to each VTD based on the legislative district its centroid was inside.  See the image below for the locations of the VTD centroids in this area of Queens.

In the instances where VTDs crisscross legislative districts, this technique will allocate all of a VTD’s votes to a single legislative district rather than splitting them across multiple districts.  This will over- and underestimate vote totals in some districts. But the process avoids the cumbersome effort involved in the alternative: splitting VTD vote counts.  The splitting process uses one of two methods:

  • using block-level population to “spread” the VTD votes across the VTD (multiplying the VTD vote count by the percentage of the VTD population occupied by each block and assigning the result to each block), or
  • weighting the VTD vote count based on the area of the portion of the VTD in each district.

Either of these approaches will result in fractions of people being assigned to one legislative district or another.  In fact, LATFOR appears to have used some sort of weighting method to assign election district vote counts to VTDs, since some of LATFOR’s VTD vote totals included fractions.

The centroid-approach and the weighted population / area approach both make assumptions about how to allocate vote counts.  But we tested the centroid process with current legislative districts and found that our VTD-allocated vote totals either exactly matched the results from the Board of Elections or were within a few hundred votes (which did not change the 2010 outcome).

Whether we used the centroid-approach and the weighted population / area technique, it otherwise would’ve been difficult if not impossible to determine how to allocate the VTD-level vote counts to legislative districts without GIS.  There are almost 15,000 VTDs across New York State, and there are (currently) 62 Senate districts and 150 Assembly districts.  With GIS, the process was relatively straightforward and efficient.

Aggregating by District

At the VTD-level, LATFOR provides the total number of votes cast by party in each election, not by candidate.  One challenge that we confronted was assigning the votes cast for fusion candidates who were backed by a major party but also received support on smaller parties’ ballot lines.  For example, many Democratic candidates received significant numbers of votes on the Working Families Party ballot line, and many Republicans got substantial support on the Conservative Party line.  Cross-party endorsements were even more variable for the Independence Party: in some districts, the Democrat received support on the Independence Party line; in others, its endorsement went to the Republican.

We decided that the most accurate way to re-map the election results was to assign the votes for each VTD based on the actual vote patterns for the Senate or Assembly district that contained that VTD in 2010.  In other words, if the Democratic candidate in an Assembly district ran on the Democratic, Conservative, and Independence lines, we assigned the Democratic, Conservative, and Independence votes in all the VTDs in that district to the Democratic candidate.  When we allocated the VTDs to the proposed Senate and Assembly districts, we added up the votes based on these patterns.  This ensured that the local voting patterns from 2010 were allocated accurately to the proposed districts.

The Results: Maps vs. Plain Old Numbers

The result is that we were able to calculate the number of proposed districts that, all other things being equal, would have had a Democratic winner in the Assembly and a Republican winner in the State Senate.  The important finding is that both parties would have increased their majority — which is especially interesting in the Senate, where the Republicans currently only have a 1-seat majority.  In Albany, the majority in each house is extremely powerful, so holding on to (or improving) those margins is all-important.

Of course, as the New York World/Times Union article points out,

To be sure, no district votes the exactly the same way in consecutive elections: the quality of candidates, changes in the population and the national political climate (which in 2010 favored Republicans) all play important roles. But voting behavior in previous elections offers the best available indication as to how a district is likely to perform.

The map that the New York World published along with the article uses red/blue color-shading to visualize the impact of the voting patterns on the proposed districts.  In the state Senate, the analysis shows the majority party increasing the number of seats by two.  On the map, that result is almost lost in the sea of red districts (most of the Republican seats are in upstate New York and Long Island, where the districts cover much larger areas than the more densely populated and largely Democratic districts in New York City).  The real power of our finding is the change in number: from 32 to 34.  In some ways, that says it all.

Nonetheless, the map (along with CUR’s interactive map comparing current and proposed district boundaries) provides a strong graphic and interactive element to the story, and provides context as you move your mouse over the districts to see the vote totals change from one to the next.

Watch for more analysis as LATFOR publishes its proposed Congressional district lines, and when the final Senate and Assembly districts are drawn.

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