Finishing Maps
For a map to be included in comparisons with other similar maps in DRA 2020 (see Comparing Maps), it must meet the general requirements for valid maps.¹ Before sharing a map too broadly, it’s also a good idea to make sure that you’ve met both basic general requirements as well as any state-specific ones.
Note: The tools described below have evolved since we first wrote this post. The icon on the command bar is now tools instead of a magnifying glass.
And there are now tools to reassign stray precincts and combine split ones.
The broad strokes of everything below are still true.
General Requirements
Redistricting maps must typically satisfy four constraints. They must be:
- Complete
- Contiguous
- Free of holes, and have
- Roughly equal population
Checking Complete-ness
Valid maps are complete, meaning all precincts are assigned to districts and no districts are empty.
The Requirements section in Analytics view (shown above) indicates whether a map is complete. You can also infer this from the table in the Statistics view: When the dummy unassigned district — the first row labelled ‘Un’ — is empty and every district has a population greater than zero, then the map is complete.
If all precincts aren’t assigned yet — it’s easy to miss some, especially on large maps! — you can use the Find tool to find these precincts & assign them to districts. Click on the dropdown menu next to the magnifying glass and choose the “Unassigned Precincts” option.
The Find tool lets you iterate through the list of unassigned precincts, zooming to each in turn. You can zoom out, if you need to to see things better.
If you want to assign the unassigned precincts to districts as you iterate through them:
- Select / change the district in the District Selector, and
- Click on the paint brush to get into paint/assign mode
- Then click on the unassigned district to assign it to that district
As you assign each precinct to a district, the tool will automatically zoom to the next precinct. You can advance without assigning a precinct to a district, by clicking on the next chevron.
You can stop going through the list at any time, by clicking on the ‘X’ to the right of “Unassigned Precincts.”
Checking Contiguity
Valid maps are also contiguous, meaning that every district is, in turn, contiguous. A district is contiguous if every precinct assigned to it is reachable from every other precinct assigned to it without crossing a district boundary.²
The Requirements section in Analytics view indicates whether a map is complete. You can also see this in the table in the Statistics view: The “Contiguous / Not Contiguous” indicator in the summary row at the bottom tells you whether or not the map is contiguous overall.
Note: A district will show as not contiguous, if you have one or more “stray” precincts assigned to it that are embedded within other districts.
If all districts aren’t contiguous yet, you can use the Find tool to find the pieces. Click on the dropdown menu next to the magnifying glass and choose “Non-contiguous Districts” option.
Here you iterate through the distinct pieces of districts that aren’t contiguous. For example, if there are two disjoint pieces of a district, the tool will zoom to each in succession. In general, the pieces consist of many precincts, but sometimes — as noted above — you can have individual “stray” precincts.
You can zoom in & out and re-assign precincts the same as above.
Checking Embedded-ness
Valid maps are free of holes, meaning that no district is fully embedded within another (i.e., no donut / donut holes).
The Requirements section in Analytics view indicates whether a map is free of holes. You can also see this in the table in the Statistics view: The “Embedded / Not Embedded” indicator in the summary row at the bottom tells you whether or not the map has any embedded districts.
Stray Precincts
A special case sometimes combination of these three scenarios — unassigned precincts or districts that aren’t contiguous or are embedded — is stray precincts: individual precincts that are unassigned or assigned to one district but are fully embedded within another district — using the “Reassign Stray Precincts” tool.
Checking Equal Population
Finally, valid maps have districts with “roughly equal” populations, meaning that the population deviation percentage is less than the accepted threshold — districts don’t have to have exactly equal populations or populations that deviate by just one person!
The Requirements section in Analytics view indicates whether a map has equal population. As above, you can also see this in the table in the Statistics view: The “+ / –” percentage in the summary row shows the map’s population deviation, which is defined as (max - min) / average expressed as a percentage. A note below the table tells you whether that deviation is tolerated by the courts for that type of district.³
Other Considerations: Split Precincts
While it is legal to split precincts, it still frowned on, i.e., all else equal, a map that doesn’t split precincts is considered better than one that does.
If one or precincts are split — e.g., many of the Official Maps that were created by importing block-assignment files split precincts — you can use the Find tool to find them.⁴ Click on the dropdown menu next to the magnifying glass and choose “Split Precincts” option.
As with the other Find tools, you iterate through the precincts that are split. The tool zooms to each precinct in its entirety, and then successively highlights the pieces with a yellow border. You can unsplit them, by re-assigning all the pieces to the same district.
State-specific Redistricting Requirements
In addition to making sure your map meets those basic requirements for well-formed maps that generally apply in all states, you should also understand any state-specific requirements.
These are three outstanding resources that you can use to learn more about the requirements in your state:
- The Brennan Center for Justice’s, 50 State Guide to Redistricting
- The Princeton Gerrymandering Project’s, State-by-State Redistricting Reform: The Local Routes
- Ballotpedia’s, Redistricting in <state> series — the URL for a state is of the form https://ballotpedia.org/Redistricting_in_North_Carolina.
Good luck!
Footnotes
- Actually, we don’t exclude maps that aren’t free of holes — i.e., that have one or more embedded districts — but we recommend that you avoid ‘donut hole’ districts anyway!
- Water, specifically islands, complicates this, but the principle is the same. From Ballotpedia Contiguity: “Generally, a district is also considered contiguous if the district is split by a body of water, but there is a method of transport over the water, such as a bridge.” We evaluate whether maps are contiguous or not, using shapefiles. Sometimes the shapes in the shapefiles aren’t fully connected the way people in the real world consider shapes to be adjacent. Let us know if you have a map that is reported as not contiguous that you think is considered contiguous in the real world, so we can fix it.
- The threshold typically allowed by courts is 0.75% for congressional districts and 10% for state legislative districts.
- We automatically split precincts as necessary, when importing block-assignment files. We don’t, however, support splitting precinct in the app. There is, however, a workaround described in Tips & Tricks.
