As you read here yesterday, redistricting allows incumbents to pack districts with supporters, making it difficult for challengers to mount an effective campaign against them. There's often talk about the bad legislative districts that this process creates.

We got curious about what a good district looks like.

Robert Cheetham is president and CEO of Avencia Inc., the Philadelphia-based computer mapping company that runs Redistricting the Nation, which we mentioned earlier.

Cheetham says that a good district represents a "capsulation of a community of interest," like a set of neighborhoods or an ethnic group.

Gerrymandered districts tend to split communities apart into different pieces and can end up looking a little strange — in fact, a funny-looking district may be a sign of gerrymandering.

One way to judge how "funny" a district looks is to measure its "compactness," or how shaped like a circle a district is. Avencia's website uses (and explains) commonly used measures for compactness.

Cheetham cautions that compactness isn't a perfect measure of gerrymandering. One district in Arizona, which has a non-partisan redistricting process, looks particularly tortured because a Hopi Indian reservation wanted to be in a separate legislative district from a surrounding population of Navajos.

Because of that, he says that a public process is essential to redistricting fairly. He even suggests that community members be allowed to submit their own redistricting plans for consideration.

We asked Avencia to find us an example of a good, compact Pennsylvania district. Analyst Dana Bauer says that the fourth Senate District, representing parts of Montgomery and Philadelphia counties, looks good. The district is highly compact, and follows administrative and natural boundaries, she says. Check out the map and stats.

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