Learning Objectives
After completing this unit, you’ll be able to:
- Determine when and how to build a filled map.
- Add visual detail to a filled map.
A filled map, also known as a choropleth map, shows how values differ proportionally across locations. For example, instead of showing points where EV chargers exist in locations, as we showed with the point map in the previous unit, we can use a filled map to show the spatial patterns of the ratio of EV chargers across Washington State. Instead of providing a point in the middle of a geographic location, filled maps fill in the entire location, such as a postal code. Filled maps use color and shading to aid analysis.
When to Build a Filled Map
A filled map is a great choice when you want to show spatial patterns and relationships, or if you want to display a ratio of distribution across geographic locations. Filled maps are helpful while analyzing socioeconomic data across locations.
Basic Filled Maps Building Blocks
To create a filled map, your data source should include geographic data and quantitative or qualitative measure fields.
Location |
Data |
|---|---|
Columns shelf |
Longitude (continuous measure, longitude geographic role assigned) |
Rows shelf |
Latitude (continuous measure, latitude geographic role assigned) |
Detail |
One or more geographic units (dimensions with geographic roles assigned) |
Color |
Measure or Dimension field |
Mark Type |
Automatic or Map |
How to Build a Filled map
Follow along with this tutorial to build a filled map and answer the quiz questions at the end of the unit.
Connect Your Tableau Public Account
If you haven’t already, or if your Tableau session has expired, log in to your Tableau Public account in the Playground window to the right. If you don’t already have a Tableau Public account, sign up for one now, and be sure to activate your account before starting this interactive unit. You can find more detailed instructions here.
The playground resets if your Tableau Public login session expires or if you refresh the page before completing the unit. We recommend completing this unit in one sitting.
- In the window to the right, you should see the Tableau Build and Analyze Maps workbook in Tableau Public for Trailhead.
- In the Data pane, expand the Alternate Fuel Stations table and double-click ZIP.
There’s now a data point for every zip code in your data source, and it looks like the point map we made in the previous unit.
- From the Data pane, under Measures, drag EV DC Fast Chargers to Color on the Marks card.
The map view changes to a filled map mark type and the zip code boundaries and filled in and colored greenish-blue.
Add Visual Detail to a Filled Map
This map shows the count of stations that have an EV DC fast charger per zip code. But let’s say you want to know how many DC fast chargers there are in total per zip code, not just how many stations have one.
If it’s not already, change the measure from CNT to SUM:
- From the Marks card, expand the menu on the mark for EV DC Fast Chargers.
- Hover over Measure, and select SUM.
The map updates to show the sum of DC fast chargers per zip code, rather than the count of stations that contain them.
Like you did with the point map in the previous unit, you can change the color palette of the map marks, the background style, and add a data layer. You can also add labels so that the quantity shows in addition to the color on your map.
By default, Mark labels aren’t shown. It's a good idea to show labels in addition to color to make your map more accessible.
To show labels, select Label on the Marks card and check the Show Mark Labels box. In the Show Mark Labels dialog box, you can choose to show all labels, selected labels, highlighted labels, and Min/Max labels. What question are you trying to answer? If you’re just trying to locate the postal code that has the highest amount of EV DC fast chargers, perhaps the Min/Max labels suit your needs. That reduces the cognitive load of too many labels and shows you what you need.
Analyze a Filled Map
At the end of the previous unit, you learned which zip codes had at least one EV charger. What you didn’t know was if there were a lot of EV chargers for the area, or if only one charger served a large area. With the point map, you also learned that the Seattle metropolitan area had more EV charging stations than other parts of the state. This filled map shows how many fast chargers exist per zip code. Fast chargers are important when drivers need to recharge in a hurry—such as when they're on a road trip, and don't want to spend hours charging their car. What do we see in this map?
The postal codes with the highest sum of EV chargers aren’t all in Seattle. There’s a giant hub of EV DC fast chargers in the middle of the state. There are also hubs of fast chargers to the north on the way to Vancouver, BC; to the south on the way to Portland, OR; and to the east in Spokane, WA.
To find out what this means, let’s change the background style from Normal to Streets by selecting Map | Background Maps from the toolbar. Now you can see the pattern. These EV DC fast chargers are strategically placed in locations off major highways and allow people driving their EVs on road trips to get a convenient fast charge.
There is a higher ratio of fast chargers off Interstate 5, which is the major highway that runs north and south from Seattle, and off of Interstate 90, which is the major highway that runs east and west across the state. You can see the path of the major interstates by looking at the darker blue color. In areas not close to a major highway, you see a lower ratio of EV DC fast chargers.
This filled map does a great job of showing us ratios of data, and how the ratio per location relates to one another. But some of the highest ratios of EV DC fast chargers in the Seattle area overlap each other. You can create a density map that reveals patterns or concentrations for marks that may otherwise be hidden. You learn now to create a density map in the next unit.