Redistricting maps with GIS: (1) Background

Now that state legislatures around the country are elected, they will be preparing for their 10-year task of drawing up new congressional districts. (Some states such as Texas and Georgia also performed mid-decade redistricting, typically for partisan political purposes.)

GIS is useful here in analyzing some potential outcomes. Here is the current situation in terms of districts in Georgia which is due to gain an additional, 14th district.

PVI is a measure of the district’s politics. R+16 for example means that the district is 16 points more Republican than the country as a whole. (Georgia overall is currently R+7.)

As can be seen, Democrats have 5 districts out of the current 13. In the November 2010 election they lost one, GA-08, the middle southern Georgia district here indicated as R+10, which despite its rating as a strong Republican district was held by Jim Marshall, a “Blue Dog” member. (The D+1 districts you see here are also held by Blue Dog members, as is perhaps surprisingly David Scott of the West Atlanta district, which is D+15.)

This geography is not too difficult to explain as this map of percent black in 2010 shows.

Here I’ve assigned each tract (except for one with missing data from Esri) to 4 categories showing percent black. Going on my previous observation that districts more than about a 1/3 black do not vote Republican, you can see Atlanta and that middle third of the state to the south showing up very nicely. Republicans have now carved into that district (GA-08) and you can see the advantages (for the GOP) of the N-S shape diluting the black vote. (Blacks tend to vote around 90-95% for the Democratic party; it was 94% in 2008 and 90% in 2010.)

What you’d like to do here (again, if you’re the GOP since they control the state legislature and the redistricting) is create two districts that are fairly black that contain as many black votes outside Atlanta as you can get. If the GOP take it as a political reality that these will be Dem-leaning seats they might be satisfied with that (after all they’d have 8 solid seats).

Here is a more general picture from Election Data Services (EDS) of likely gains and losses based on demographic numbers provided by Esri.

EDS, Sept 26, 2010.

In a future post I’ll look at some possible configurations for Georgia of its additional seat. We can play out different scenarios: aggressive GOP, generous GOP, neutral GOP.


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