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QGIS for Tableau Users # 3: Getting Together and Breaking Up (Combining and Splitting Polygons)

We’re once again excited to have Sarah Battersby joining us for the third post in her series about QGIS for Tableau users. Sarah has been a member of Tableau Research since 2014. Her primary area of focus is cartography, with an emphasis on cognition. Her work is focused on helping everyone to visualize and use spatial information more effectively—without the need for an advanced degree in geospatial. Sarah holds a PhD in GIScience from the University of California at Santa Barbara. She is a member of the International Cartographic Association Commission on Map Projections, and is a Past President of the Cartography and Geographic Information Society (CaGIS). Sarah can be contacted at sbattersby@tableau.com or on Twitter @mapsOverlord.

 

Welcome back to the QGIS for Tableau Users series. In this third post, we’re going to walk through merging (and splitting) polygons in QGIS. While Tableau has an awesome feature for merging polygons together to create custom territories using the data built into the Tableau geocoding database, it doesn’t solve all cases where you need to group polygons from outside the Tableau geocoding database, or when you want to split polygons apart in special ways (e.g., Split California in half, but not along county boundaries).

 

So, today we’ll focus on manipulating polygon geography and cover merging and splitting polygons based on:

 

- Attributes

- Drawing lines for the split boundary


 

The Real Basics (Your Series Disclaimer Message!)

This post goes into how-to for specific tasks. If you need to take a step back and see where to even start (setting up QGIS, basics of adding and working with files, etc.) please refer back to the the first two posts in this series, Getting Started and Bending Text Files toYour Will!

 

There are also several great QGIS tutorials that will provide a broader (non-Tableau focused) introduction to the power of the software, such as QGIS Tutorials and Tips.

 

Now, we’ll get back to our regularly scheduled blog post message about awesome stuff you can do with QGIS!


 

Why Merge or Split Polygons?

Grouping smaller regions into bigger regions allows you to create custom groups of geography that act as single regions instead of as individual polygons. That’s a mouthful, so here’s an example. Perhaps you want to have two regions in California—Southern California and Northern California—but Tableau doesn’t have those specific locations in its geocoding database. So, you can make them by grouping the counties into just the right regions. 

 

Grouping (and splitting) regions has a few key benefits:

 

1) Regions Represent Your True Area of Interest

While many mapping needs are met by standard geographies (city, counties, states, provinces, countries, etc.), sometimes our questions require special geographic boundaries. When we’re lucky, they can built from combinations of counties or provinces...but sometimes you just need special geographic boundaries. Merging and splitting polygons can get you just what you need. And once you have that spatial file with just the boundary you need for your region, you can quickly use the file for visualization and spatial intersection functionality in Tableau to perform your point-in-polygon analyses (e.g., how many customers are in Territory 1?)

 

Tableau Territory map created using custom territories based on US Counties

 


2) Make Selections Easier

You can select everything in a single region instead of having to select each individual polygon that makes up the region.

 

 

3) Seamless Visualization

Grouping is also very helpful for general visualization. For instance, you can symbolize the region as a whole instead of as a bunch of individual parts as shown below.

 


 

When You Don’t Need QGIS

If the new geographies you wish to map can be built using geographies that are already in the Tableau geocoding database—for example, if your regions are made up of individual counties—then there is no need to use a GIS tool. For this, you can simply use Tableau’s custom territories. But, for more complex use cases, you’ll likely need to get your hands dirty with GIS.

 


How To

Okay, now that you know why you might need to do this and when QGIS is needed (as opposed to doing it directly in Tableau), let’s dig into how to do it.

 

I’m going to walk you through a few ways to manipulate your data, specifically:

 

- Merge polygons based on attributes

- Split polygons based on manually drawn lines

 

Merge Based on Attributes

Some data files have an attribute that defines how the polygons should be grouped together. For instance, the image below shows data at the Census Tract level. We can look at attributes for the selected polygon and see COUNTYFP, which is a unique identifier for which county each Census Tract falls inside. The map on the right shows the same Census Tracts colored based on their county. If we needed to combine these Census Tracts into Counties, we can do it easily here. 

 

Note: In real life, it doesn’t make sense to group Census Tracts into Counties since Counties are already in the Tableau geocoding database. I’m just using this example since it’s straightforward and easy to understand.

 

  

To combine the Census Tracts into Counties in QGIS, we’ll use a process called dissolving. This process will “dissolve” the borders between the census tracts and group them together into a larger geography based on the attribute(s) we choose. To do this, select Vector Geoprocessing Tools Dissolve. On the Dissolve dialog, specify your input layer (the file with polygons you wish to dissolve together), select one or more fields/attributes which will define how they should be grouped (in this example, we’ll just choose COUNTYFP), and then specify a name for the output file (and remember where you save it!!)

 

 

When the processing is done, close the Dissolve window and enjoy the results. In our example, the census tracts should all be merged into polygons based on the COUNTYFP attribute, as shown below.

 

 

You may still need to do some attribute cleanup (e.g., data from each of the original polygons haven’t been magically aggregated based on the merge), but that’s pretty easy. Most of the time I just delete all of the columns except for my unique identifiers for each polygon and then join the interesting data back in in Tableau—because Tableau is really good at aggregating and letting me adjust the aggregation however I want.

 

If you’d prefer to do edits to the attributes in QGIS, right-click on the layer name, open the attribute table and start editing. You can calculate new fields (2) or update existing values using the Field Calculator (4). When you’re editing (1), you can also use the Delete Fields (3) button to remove fields you no longer need.

 

 

Split Based on Lines You Draw

Sometimes you need to split instead of merge. For example, let’s say that you need to break California into two regions—Northern California and Southern California—and the border cannot be defined by county boundaries. Using the editing tools in QGIS you can easily split the polygon wherever you want. Here’s how:

 

First turn start the editor:

 

 

Next, use the Split Features tool on the Advanced Digitizing Toolbar.

 

 

If you don’t see the tool and toolbar shown above, you may need to turn it on (right click in the empty grey space on the toolbar at the top of the window and make sure the checkbox next to Advanced Digitizing Toolbar is checked).

 

 

Now simply click and draw the line you want to use to split your polygons. When done, just-right click (and make sure your line fully crosses every polygon you want to split with that line). Your lines can be simple or complex—whatever you need to draw the split just where you need it!

 

 

When you’re done, save the edits by clicking on the Toggle Editing button on the toolbar or right clicking on the layer name and selecting Toggle Editing):

 

 

You will want to do some attribute cleanup since the new polygon(s) will just have a duplication of the original polygon data values. Some basic tips on editing attribute tables are listed above. There is also a great set of documentation on working with the attribute table in QGIS.

 

Next Up!

And those are the basics of merging and splitting polygons that I often fall back on to make special geometry to work with in Tableau. Not too bad, eh? In the next post in the QGIS for Tableau Users series, we’ll talk about proximity analysis, including creating adjacency matrices, joining attributes based on location, and creating Voronoi or Thiessen polygons around points.

 

In the mean time, if great questions or ideas come to you, feel free to reach out on the Tableau Community Forums or to follow more of the random Tableau spatial thoughts that I share on Twitter (@mapsOverlord)...or to share the great maps that you’re making in Tableau. Thanks for reading!!

 

Sarah Battersby, September 20, 2021

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