How to create 0-deviation plan

video: https://youtu.be/4w28X2B6D2U

In some situations, it is required to make a 0-deviation plan. For example, in some states, the congressional plan is expected to be 0-deviation.

iRedistrict may produce the 0-deviation plans directly, as shown in the image below.

However, it is not easy to get a 0-deviation plan. Most of the time, you get very balanced plans but they are not 0-deviation.

There are two ways to produce 0-deviation plans in iRedistrict.

Use lock function to produce 0-deviation plans

When you create plans, there might be some 0-deviation districts produced. Lock the districts you would like to keep and run the calculation again. Repeat this process to get the 0-deviation plan.

The plan below is a 0-deviation 4-region Utah plan created by Utah block groups. It uses the repeating lock strategy combined with a small population threshold.

Use mix-layer function to produce 0-deviation plans

The mix-layer approach is to involve a small population layer to make it easy to get 0-deviation. We can combine the mix-layer approach with the lock function to produce 0-deviation plans.

Unless very necessary, we do not need to use the blocks to produce the mix-layer. Usually, we can achieve the 0-deviation goal with the block group layer. Using blocks will increase the calculation burden. If we can achieve the goal with a smaller dataset, why use the larger dataset?

For example, when we create a state wide congressional plan, we use county, VTD, tract, or block group layer as the redistriting units to create plans. If 0-deviation plan is expected, we pick a plan mixed it with the block layer to produce a block-boundary plan.

choose "Generate a mixed shapefile with the selected plan"

Select the blocks layer, set the path of the output file and click "OK" button to produce the mix layer.

Once the mix layer is created, it is saved as a shapefile. The plan boundary and the blocks are mixed.

Load this shapefile in iRedistrict to create plans. It is much easier to produce 0-deviation plans. Lock function can also be used if necessary.