Painting With Blocks

In maps built with 2020 shapes, you can now assign individual blocks to districts in addition to assigning whole precincts (or counties). In other words, you can split the blocks of a precinct between districts.¹

Motivation

States have a lot of precincts, so the average population in a precinct tends to be very small. For example, Maryland has a 1,859 precincts and a total 2010 population of over 5.75 million people. Hence, the average population per precinct in Maryland is less than half of one percent, which is smaller than the 0.75% deviation that is generally tolerated for congressional districts (see ‘Roughly’ Equal Population).

But some states have adopted the unwritten (and unnecessarily stringent) practice of requiring that their congressional districts deviate by no more than one person (or some other almost exactly equal threshold). You can’t draw districts with that level of precision, using just precincts.

Painting with blocks lets you achieve this:

Population deviation from target size

Use it judiciously though. You shouldn’t need to split more than one precinct per district. If you split more precincts than that, you’re either engineering (read: gerrymandering) the districts or accommodating communities of interest (COI) that don’t completely align with precincts.

Adding Blocks to a District

To add some but not all of the blocks of a precinct to a district:

• Select the district in the District Selector
• Keep the District Selector drawer open, so you can see the population deviations update as you make changes
• Pan & zoom into the region of the district border that you want to adjust, and
• Find the precinct that you want to split

Then:

• Choose the Block option in the Paint control — the other options are County and Precinct (default)
• Click on the paintbrush to enter Paint mode, and
• Click on the precinct that you want to split

Paint with blocks

The map will zoom to the precinct which will “shatter” into its constituent blocks. The precinct will also show with an aqua border, so you can easily differentiate these blocks from other (small) precincts nearby.

A precinct “shattered” into blocks

If precinct labels are turned on, the same settings are used for these blocks. So you don’t have to hover over each block, you can turn on “Total Pop” Precinct Labels to see total population over each block.

Blocks with Total Pop Precinct Labels turned on

Then you can assign blocks to the selected district, with the normal painting gestures. Tip: Paint “out” from the border of the selected district, so it stays contiguous — you don’t want to assign a disconnected island of blocks to a district!

Some blocks re-assigned to another district

As you assign blocks to the selected district, the total populations for the affected districts will update in District Selector.

Unlike normal edits that are saved to the server periodically, your block changes are not saved to the server until you explicitly commit the changes by clicking on the checkmark control or you implicitly commit them by moving away from that modified precinct, be it by clicking to shatter a different precinct, switching the paint option back to precincts or counties, or leaving Map view altogether (going to Analytics or Advanced view or back to the list of maps). The message in the bottom right corner of the maps tells you the current saved/modified state.

Once you’re done assigning the blocks within a precinct, we merge all the blocks in the same district into a virtual precinct for performance and consistency. These virtual precincts act in every way just like real precincts.²

Note: When you open a map with split precincts, we have to download additional fine-grained data from the server that slows map load speed. If a map has lots of split precincts, this impact can be significant.

Find Split Precincts

Whether a map splits precincts because you split them intentionally or because they had to be split when a block-assignment file was imported to create the map, you can find split precincts using the “[Find]Split Precincts” tool in the dropdown menu next to the magnifying glass on the toolbar (see Finishing Maps). You can un-split a precinct, by simply assigning all its pieces to the same district.

Footnotes

1. The astute reader will recognize that for a long time we have automatically split precincts as necessary when importing block-assignment files — assigning some blocks to one district, and the other blocks to another — for 2010 shapes. That enabled us to show the official maps exactly. We continue to do that automatically on import, but now we’re letting you split precincts yourself in Map view for 2020 maps.
2. This is what we do under the covers when importing block-assignment files that split precincts across districts.