Happy #MapInfoMonday!
Redistricting is a feature that has been in MapInfo Pro since the very beginning. Originally, it was built to help design sales territories but it can be used for various other districts too: distribution districts, school districts, voting districts, and more.
The general driver behind redistricting is your need to divide a certain area into smaller districts so that work related to this area can be divided into smaller pieces. Of course, there can be multiple parameters that need to be taken into account when dividing such areas. It can be the number of addresses, the number of people in total, the number of voters, the number of pupils, the total size of sales, and maybe even the size of the area itself.
Redistricting in MapInfo Pro can help you take this into account when building or rebuilding your districts.
Let's take a look at a simple example from my local area. In the map below, you can see my neighborhood which is out in the countryside in the southern part of Denmark. There is 335 address in this area that we, the local community, from time to time need to hand out some letters. Would be good to divide this into smaller districts so that we can divide that work amongst 10 people or so.
I can continue using the address points but I have decided to create Voronoi polygons from the addresses. This allows me to create subareas or districts as polygons and it can also be easier to select these polygons instead of selecting points. You can read more about
creating Voronoi polygons from points in a previous #MapInfoMonday article.
My area with Voronoi polygons around the addresses looks like this.
You need to make sure your district table has the columns you need. It should have a column for the district name and columns with the values needed. IN my example, I only have a column holding the number of addresses for each subdistrict. I could also have added a column with the size of each of these subdistricts (Voronoi polygons).
Starting a Redistricting Session
From the
Redistricter dropdown on the
Map tab, select
New Redistricter Window.
In the
New Redistrict Window dialog, you now have to select the table with subdistricts and the column holding the district names.
From the
Available Fields list, you can select which columns you want to see while designing your new districts. These are values that can help you design districts of a given size. Note that for your numerical columns you can choose the sum of the column and/or the percentage for the column.
Click
OK to create your redistricting window using the existing map window.
If you are starting from scratch with no districts assigned to your subdistricts, you will see a map like this. Also, notice the
Districts Browser window on the left of the map. This shows you the districts you currently have and the values for each of these.
To get started, you need to create the first district. Right-click in the browser window and select
Add District. Then give your district a name.
Now select your new district in the Browser window using the small square on the left of the list. This is your target district.
Select subdistricts from the map using one of the select tools in MapInfo Pro. When starting from scratch it can be useful to use
Radius Selection or
Marquee Selection as they allow you to select multiple objects. As you select subdistricts, you can see how the numbers for your target districts are changing. Use Assign District from the context menu of the browser window to update the district name for the selected subdistricts.
As you work your way through the area you add more districts and assign the different districts to the subdistricts, you can see how the map displays the districts using a thematic map and how the values change in the browser window too.
Moving Subdistricts between Districts
Once you have assigned a district name to each subdistrict, you may have to change these a bit to make your districts of the right size. The process for this is similar to the earlier process of assigning a district name to subdistricts that hadn't been assigned a district.
Start by selecting the target districts in the browser window. Then select the subdistricts to move from one district to another. Notice how the numbers change in the browser window for both districts.
This means that you can select subdistricts to move and see the change before actually applying this change.
Once you are satisfied with the new numbers, use Assign District to update the district name for the currently selected subdistricts.
When you have made all the necessary changes, you can close the
Districts Browser which will end your Redistricting session. Also, remember to save the changes to the table to make them stick.
Creating District Polygons
You can now create actual district polygons from your subdistricts by combining all the subdistricts with the same name into districts in a new table. On the
Spatial tab, select
Combine Using Column from the
Combine dropdown.
Select your table with subdistricts and the column holding the district name.
For the new table, select
Add to Current Map and select the option to create the table structure using an existing table. This table can either be your subdistrict table or an already existing district table. I have chosen the subdistrict table.
Validate the table structure and consider removing unnecessary columns. I have only kept the column with the district name and the column holding the number of addresses. In the next dialog, you need to select where to create this new table and what to name it.
In the dialog Data Aggregation, I decide how to assign the values to my districts. For the column with my district names and set it to get the name from the column with the district name in the subdistrict table. For the column with the number of addresses, I set it to summarize the values from the column with the number of addresses in the subdistrict table. Click
OK to start merging the subdistricts into districts.
My districts end up looking like this. I aimed at around 40 addresses per district. A few districts have more but then the districts are smaller in size. I could probably tweak this a bit more to get an even better result.
I hope you have found this article useful.
Do remember to sign up for the Precisely Knowledge Communities and Join the MapInfo Pro community to ensure you get notified about upcoming #MapInfoMonday articles and MapInfo Pro events.
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Peter Horsbøll Møller
Principal Presales Consultant | Distinguished Engineer
Precisely | Trust in Data
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