Contiguity
For a map to be considered valid in DRA 2020, each district must be contiguous, meaning that all precincts (or blocks) in that district are connected. In other words, a district can’t be discontiguous like this:
Only contiguous maps are considered are included in the map picker for the “Compare Map” feature in Analytics view and for Notable Maps on a state’s home page. (See Notable Maps for more details & exceptions.)
There are two different kinds of contiguity and two different contexts for assessing them.
Rook vs. Queen Contiguity
A district is “rook contiguous” when all the shapes it is made of touch at more than a single point, while a district is only “queen contiguous” if some shapes only touch at a single point.
Because it’s generally considered poor form to draw districts that are only queen contiguous, we check for & report on rook contiguity in DRA 2020.
Some states allow queen contiguity at least some times though. For example, the current State House maps for both PA & VA are only queen contiguous (and we report that they may not be contiguous).
Operational Contiguity
There is a literal the map is not the territory issue with respect to determining whether districts are contiguous or not. Specifically, the shapefile for a state represents that state (ironically, the map in this analogy) but it isn’t the state (again ironically, the territory in this analogy). We call the difference between what is considered connected in real life and what is connected in a state’s shapefile “operational contiguity.”
For most states most of the time, there is a good correspondence between the two: districts that look contiguous are contiguous. Sometimes the two diverge though.
The most common example of this involves islands or other significant bodies of water.
Sometimes island shapes are drawn such that they connect to the nearest mainland shapes, but sometimes they are not, as shown above. In real life, this island is considered connected to the nearest mainland area.
The other way operational contiguity issues arise is what we call split jurisdictions — what some people call “island territories” — where a single legal entity is comprised of two or more areas that aren’t physically adjacent. This happens sometimes, when municipalities evolve and disjoint areas are annexed or left over from annexation.
An extreme example of this is Columbus, OH, shown in blue above with yellow Fairfield surrounding and bits and pieces intermixed. Wisconsin also has many smaller but split jurisdictions.
Contiguity for 2020 Shapes
This is how we handle operational contiguity for 2020 shapes:
- These five states only have one or two congressional districts and significant water-related contiguity issues — AK, HI, ME, NH, and RI. We still report whether districts are contiguous in the Statistics and Analytics views, but we don’t enforce contiguity when populating the map picker in “Compare Maps” or for the Notable 2020 Maps. It’s up to you to make sure maps drawn for these states really are considered contiguous in real life.
- We take a similar approach for state legislative maps for WI where the many split jurisdictions make it difficult to draw contiguous maps.
- In contrast, we have modified the raw connectivity graphs for CA, MD, and NY, so that their official maps show as contiguous.
We have tried to make sure that the precinct (and block) shapes let you draw maps that are considered contiguous in real life. Some states, like Washington, have rules about connectivity — districts must must be connected by transportation routes, e.g. mountain passes, bridges, or ferries¹ — that can be hard to implement. If we report a map as “probably not contiguous” but you think it would be considered contiguous in real life, please let us know (feedback@davesredistricting.org).
