Redistricting for Dummies

by Dave Barter on

Firstly, I apologise in advance for the use of "dummies", I did it to attract your attention to a piece of work we've recently completed linked to redistricting. But trust me that "for dummmies" does have a certain relevance that I shall reveal as we go on.

Redistricting (verb) - the process of drawing and revising electoral boundaries

The vast majority of people reading this are eligible to vote. As a voter you usually live in a voting district along with other voters. These districts need to be carefully constructed in order to properly represent the electorate. The voting proportions need to be maintained across all districts to ensure that each vote counts equally.

Now this seems straightforward. Swindon South can elect one MP to London and so can Swindon North. It is important that they have a similar proportion of voters in order to make their representation fair. But what happens when we build a large estate on Swindon South? It could be that 2000 extra residents of voting age will be present at the next election. We now need to rebalance the borders between North and South and make sure that the future representation continues to be fair.

That's a simple introduction to redistricting. It's a complex process as you can imagine of balancing development plans, mortality and birth rates and new voters coming of age to ensure that our electoral wards are fair both present and future.

In the UK redistricting happens at a number of levels. We elect local councillors at a ward level and these define the political makeup of our Borough/District councils. Members of Parliament are elected using constituency boundaries. This is clearly shown in the diagram below with the two Swindon constituencies and the wards below them.


Work (noun) Activity involving mental or physical effort done in order to achieve a purpose or result

As you can guess redistricting is hard work. You need a lot of data to help you understand the impact of each change to each boundary. This includes addresses, voter counts, age metrics, local development plans, mortality rates, birth rates and all sorts of other gubbins simply to count voters. These need to be built into a data layer and your boundaries overlaid on top.

Then you change your boundaries to achieve the required balance and every time you change the boundary the model needs to be re-run to check that you have the required balance of voters. Forget doing this on paper as it is going to require a whole lot of counting and sums or (even worse) guesswork. Luckily there are a number of redistricting tools on the market that will help you do this on a computer. Enter the data, move the boundaries and the computer will say "yes" or "no".

However, therein lies our problem. These tools are specialist pieces of software that require expertise in G.I.S to setup, load and manage. They're also a tad pricey which goes counter to the way ward level redistricting tends to work. The exercise is usually done in consultation with the local councillors the changes will affect and a team of electoral experts within the council itself. Neither of these parties are trained in G.I.S and there can be quite a few of them making the licensing cost eye watering.

So when consulting on redistricting there's often a reversion to a paper process, printing out maps, going backwards and forwards between the consultees and the redistricting package, time, cost and wastage.

Reigate and Banstead Borough Council were about the feel this pain. They were required to complete a redistricting exercise which included not only the alteration of ward boundaries but a reduction in the number of wards. They were staring at a potential mess of paper, debates, cost and time wastage.

Luckily they bumped into us.

We love a challenge and if it includes the words "maps", "data", "consultation" and "quick!" they're right up our street. Reigate tentatively asked us if we could create a site for redistricting capable of being used by those not so familiar with G.I.S that would not cost the earth...and be live in five weeks time. We looked at each other nervously, carried out a late night feasibility study, prototyped some new map controls and then said "sure". The challenge was set, create "Redistricting for Dummies" and make it live as an early Christmas present for Reigate.

Teamwork (noun) the combined action of a group, especially when effective and efficient

The five weeks following decision time were fairly intense. This was not a project where we could simply receive a specification and happily code away. We as a business had to quickly learn the mechanics of redistricting and understand the issues that our "dummies" would face. We were fortunate in having Sally and Robert from Reigate who brought three things that helped us deliver on time; enthusiasm, experience and flexibility. Everything you need to be part of a team tasked with something hard in a short period of time.

Our virtual team made decisions in real-time, arrived at practical compromises where needed and supplied each other with the data or information required instantly. It was a pleasure to work in such a manner and made our lives so much the easier as we dealt with a number of technical challenges:-

  • challenge 1 - maintaining ward geometries as boundaries are changed

Look at the picture below this is the intersection of three wards. I may want to alter the location of this intersection to add more houses (and voters) to the wards on the right. I need the system to change the boundaries of the adjacent wards at the same time. If this is not done, gaps or overlaps will appear as wards are edited.


We achieved this with a clever new map control that understands geometry relationships and alters all related geometries together as shown below:-


  • challenge 2 - update the user in real time with the impact of their ward change

This was another head-scratcher. The edit above has changed the shape and coverage of three wards. We now need to recalculate the voter counts for these wards and update the user immediately. This required some nifty SQL and we're fortunate in our use of PostGIS which made the task simpler. We're pretty proud of our query that updates the voter count against changed geometries in milliseconds passing the results back to the user in near real time.

  • challenge 3 - tell the user that their job is done.

Robert and Sally had already thought this through. A simple bar chart and colour scheme updated with each change to show the user where they are at in the process


Simple isn't it. Red means "more work", amber means "nearly there" and green is "good to go". Just keep returning to the map until everything is green and you're done.

Success (noun) the accomplishment of an aim or purpose

To cut a short story shorter we collectively made the deadline and as Christmas began Reigate started their consultation on redistricting. We sat tentatively waiting for a deluge of support calls that never came. Our tool appeared to be working, we'd put redistricting into the hands of "dummies", and sorry councillors I mean that in an affectionate sense. The consultees got on with their job and Reigate were able to arrive at a redistricting proposal on time and with a lot less hassle than before.

And what's more we can't take all of the credit. Without their willingness to work as a team, clear vision upon the outcome and determination to get a result our technology would have floundered. This has been deservedly underwritten by two unexpected results of the project. Some awards:-

Robert and Sally deserve these and more as the two of them sought to bring about real change in the public sector and use technology to properly ease the burden upon the public purse. Proper public service heroes. We're obviously dead proud to have been involved and in deep discussion with Reigate as to where we jointly take this next.

Between us we've truly delivered "Redistricting for Dummies", if you want to know more have a look at what Geoplace had to say


The Boundary Commission reviewed the Reigate proposals and asked for additional changes. This meant that the exercise needs to be repeated from a new model of ward boundaries and has further underlined the value of such a tool as we're ready to repeat it almost instantly and our audience knows exactly what to do.


Redistricting for Dummies

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