Configure smart mapping options for size
The choices you make when designing a map should be informed by both your audience and your data. In this module, you'll experiment with different symbology options both to learn more about your data and to determine the best way to display it.
Choose a style method to map vacant housing units
You'll begin by opening a web map containing vacant housing units data for the United States. You'll then assess which smart mapping style to use.
- Open the Vacant Housing Units web map.
- If you have an ArcGIS Online account, sign in with your credentials.
If you don't have an ArcGIS Online account, you can still complete this tutorial, but you will not be able to save the map at the end.
A map of the United States appears. Larger circles on the map represent areas with more vacant housing units.
The data in this map is from the U.S. Census Bureau's American Community Survey (ACS) 2018-2022 5-year estimates. For a version of this data that is kept up-to-date, see the ACS Housing Units Occupancy Variables layer from ArcGIS Living Atlas of the World.
- If the Layers pane is not visible, on the Contents (dark) toolbar, click the Layers button.
This map contains three layers, mapping housing at the state, county, and tract levels. These layers are configured to be visible at different scales. Layer names appear in gray when they are not visible.
- Zoom in and out on the map to observe each of the three layers. Zoom until the Counties layer is visible.
- In the Layers pane, click the Counties layer to select it.
A blue bar appears next to the layer to indicate it is selected.
- On the Settings (light) toolbar, click the Styles button to open the Styles pane.
The layer is symbolized using the Vacant Housing Units attribute and the Counts and Amounts (size) style.
Is this the best style for this data? The Vacant Housing Units attribute contains numeric values. More specifically, it contains counts—it counts the number of vacant housing units in each county. Counts and Amounts (size) is an appropriate way to symbolize count data.
You'll try another style to confirm that size is the best option.
- In the Styles pane, click the Counts and Amounts (color) card.
The map changes so each county is shaded with a blue color. This kind of map is called a choropleth map.
The desert region in the southwest is a dark blue and attracts the most attention on this map, giving the appearance that most vacant housing in the country is located there.
- In the Styles pane, click the Counts and Amounts (size) card to return to the former symbology.
Now you can see that while there are a lot of vacant housing units in Los Angeles and Phoenix, there are also many in New York, Chicago, and Houston. The choropleth map told a story that was skewed by land area. It emphasized large areas instead of large counts.
The Counts and Amounts (color) style is recommended when you are mapping percentages or rates. The Counts and Amounts (size) style is recommended when you are mapping counts. If you were mapping the percentage of housing units that were vacant, a choropleth map would be best. But you're mapping the number of vacant housing units, so you'll symbolize by size.
Adjust the histogram
The map currently uses the default suggested settings for size symbology. Next, you'll explore the data's distribution using the histogram and use this knowledge to fine-tune the symbology.
- In the Styles pane, on the Counts and Amounts (size) card, click Style options.
The Style options pane appears. It includes a configurable histogram, set to display the full range of your data. In this case, the minimum value is 15 and the maximum is 236,468. This means that one county has only 15 vacant housing units, while another has 236,468. All other counties fall in between this range.
You can also see that the average count of vacant housing units per county is 4,826, denoted by the x-bar label on the histogram. Counties vary a lot in size, so this average is not very informative without a denominator, such as total housing units.
The histogram shows gray bars along its side, which represent the spread of the data. For this layer, all of the gray bars are near the minimum value. None are visible near the maximum value.
This tells you that there is at least one outlier in the data: a few counties have far more vacant housing units than all of the others. When this happens, the outliers are represented on the map with large circles and all of the other features have very small circles in comparison.
- Drag the top handle of the histogram down and observe the changes it makes on the map. Leave the handle near to 100,000.
Many of the small circles on the map grow larger.
The handles—or breakpoints—correspond to the minimum and maximum sizes. In the image below, these are set to 4 and 50 pixels, but yours may vary depending on what scale you are zoomed to.
All counties that fall below the lower handle will draw with the minimum size. All that fall above the upper handle will draw with the maximum size. All other counties will draw with sizes in between. When you moved the upper handle down, it compressed the data range that corresponds to the size range, resulting in more variation on the map. The maximum value no longer dominates the size ramp, allowing mid-sized cities to stand out more clearly from rural counties.
Note:
You can use the same technique when the bulk of your data is close to the maximum value. In that case, you would move the lower histogram handle up.
Next, you'll make the legend easier to read by choosing rounded numbers.
- On the histogram, click the upper handle and type 100,000. Press Enter.
The histogram shows that there are many features close to the minimum value, so for the lower handle, you'll choose a number that is close to 15 to avoid losing variation in the map. Appropriate choices could be 40, 50, or even 100.
- Change the lower handle to 50.
The legend is improved by these changes.
Deciding where to place histogram handles is an iterative process. You should experiment with different breakpoint values before choosing those that are most effective for your map.
Next, you'll save your symbology and the map.
- At the bottom of the Style options pane, click Done. At the bottom of the Styles pane, click Done.
Compare classified and unclassed symbology
The symbols on your map vary continuously; each one is a slightly different size based on the number of vacant housing units. The size changes gradually from smallest to largest, allowing for the full variation within the data to be visible on the map.
An alternate way to size the symbols is to classify them. This means dividing the distribution of values into a small number of bins, so that there are only a few sizes of symbols that are easy to tell apart on the map, for example, small, medium, large, and very large. This technique can be helpful when your map's message or audience demands a more simplistic map.
- Reopen the Styles pane for the Counties layer. On the Counts and Amounts (size) card, click Style options.
- Scroll to the bottom of the Style options pane and turn on the Classify data option.
All symbols on the map now have one of four sizes. Most counties use the smallest symbol size.
You'll experiment with some of the symbology settings to see if they improve the map.
- For Size range, set the minimum size to 6 and the maximum size to 40.
- For Number of classes, choose 3.
These changes result in a map with only small, medium, and large symbols. The map has less variation and detail now that it is classified, which allows it to tell a simpler story.
You can make the map even easier to read by rounding the symbol class breakpoints.
- For Round classes, choose 10,000.
The legend updates to show rounded values.
Note:
You may have noticed that the Method option changed from Equal interval to Manual breaks. This will happen any time you modify the breakpoints. For more information on classification methods, including Natural breaks, Standard deviation, and Quantile, read the Classification methods documentation.
Most of the counties fall into the first class, depicted by the small symbol. You'll adjust the histogram to change the break points and move some of these counties into the medium size class.
- On the histogram, change the upper handle to 100,000. Change the lower handle to 30,000.
Some of the counties change from a medium symbol to a large one (for example, Detroit) while others change from a small symbol to a medium one (for example, Cleveland).
- Explore the classified map and assess if better serves your purpose and audience.
Classifying symbol sizes can help by making a map simpler and easier to read. This map is for an internal report for your colleagues in the housing advocacy group. Will classifying the data help them interpret the map? More likely this audience would appreciate the more nuanced unclassed map, so they can see more detail and variation across the country.
Classifying data can also help when there are meaningful breakpoints, for example, if you were mapping employers and there was a policy in your state such that employers with 50 or more employees had to adhere to a particular rule, while employers with 500 or more employees had to adhere to a different rule. In this case, it would make sense to use a small symbol for values from 1 to 49, a medium symbol for values from 50 to 499, and a large symbol for values of 500 and more.
Your group is not targeting counties with specific numbers of vacant housing units, so there are no meaningful breakpoints for this map.
It's important to experiment with different style options and methods. From this experimentation, however, you learned that an unclassed size ramp better communicates the data for this particular map than a classified one.
- At the bottom of the pane, click Cancel.
The map reverts to the unclassed symbology that you previously saved.
You now have a good sense of your data's distribution and how it can be displayed on the map. Through your experimentation, you made the following choices:
- To symbolize by size instead of color because your data represents counts
- To adjust the handles of the histogram so the symbols on the map show more variation in size
- To use rounded values so the legend would be easier to read
- To use an unclassed size ramp instead of classified size bins because your audience would not benefit from a simplified map
These are the best decisions for this particular dataset and your particular audience. In another map, other choices may be better.
Style the symbol
The symbols on your map are orange circles. Next, you'll experiment with symbol properties to customize the symbol's shape, color, transparency, and outlines.
Change the symbol's shape
Circles are the most common shape used when symbolizing by size, but they are not the only possibility. Next, you'll change the symbol's shape.
- Reopen the Styles pane for the Counties layer. On the Counts and Amounts (size) card, click Style options.
- Under Symbol style, click the orange circle.
The Symbol style window appears with more options.
- Under Current symbol, click Basic point.
- Under Category, click Basic shapes to see more options.
More complex shapes are helpful for landmarks. In particular, they can help map readers distinguish between different kinds of features. For example, you might use a book shape to denote a library and a mortarboard hat to denote a school. However, for this map, there is only one symbol and it represents an aggregated count, rather than true locations. For this particular map, a more complex shape would introduce clutter without benefit.
- Under Basic shapes, click the square shape and observe the change on the map.
- Click the diamond shape and observe the change on the map.
While both symbols overlap in busy areas, the diamond shape overlaps less than the square. For the Counts and Amounts (size) style, symbols that are closer in shape to circles generally work best because they minimize the amount of overlap between neighboring symbols.
- Ensure the diamond shape is selected and click Done.
Choosing diamonds over circles is a matter of personal preference. You found that the diamond shape does not harm the map or introduce any misunderstanding, so you chose to use it.
Change the symbol's color and transparency
Next, you'll change the color of the diamond to a turquoise green to match the logo of your advocacy group. This color will be used throughout the housing report you're creating, so it will help the map fit in visually.
- In the Symbol style window, click the Fill color menu. In the Select color window, for #, type 2d9a8c. Press Enter.
- Click Done.
The map is now symbolized with green diamonds of different sizes.
There are still areas on the map where the symbols overlap each other. You'll make the symbol partially transparent so smaller diamonds are not hidden by larger ones.
- In the Symbol style window, adjust the Fill transparency slider to 30 percent.
- Adjust the Outline transparency slider to 25 percent.
A more pronounced outline helps to define the edges of symbols when their fill is transparent.
- Close the Symbol style window.
Style the county outlines
The Counties layer is represented with point symbols, but it is actually a polygon layer. Each diamond symbol is drawn in the middle of the polygon it represents, and the border of each county is drawn with a thin gray line. To complete this layer's symbology, you'll adjust the border symbol to make it a bit more subtle.
- In the Style options pane, click the Background symbol style menu.
You'll make the outlines the same green color as the diamonds.
- In the Background symbol style window, for Outline color, click the green square.
Note:
If the green square is not present, set the color to #2d9a8c.
- Increase the Outline transparency slider to 85 percent.
By default, the outline's width changes slightly as the map zooms in and out. You can turn this feature off to take full control over the look and feel of the symbols.
- Set Outline width to 1 px and uncheck the Adjust width automatically check box.
Now the county borders will be 1 pixel wide at all scales.
- At the bottom of the Style options pane, click Done. At the bottom of the Styles pane, click Done.
- Explore and interact with the map.
- If you have an ArcGIS Online account, on the Content toolbar, click the Save and Open button and click Save as.
- In the Save map window, click Save.
- Use what you have learned in this tutorial to adjust the symbology of the States and Tracts layers.
You can reuse many of the settings from the Counties layer. However, each layer has a different data range, so when you adjust the histogram handles, you'll need to experiment to find the best values.
You now know how to change a symbol's shape, color, outline, transparency, and background symbol. Small changes to these settings can make a big difference on your map, especially when symbols of different sizes overlap.
Your map is complete and ready to be included in your housing report. In this tutorial, you symbolized vacant housing units by size. You explored the data values in the histogram to pick the best breakpoints. You experimented with both unclassed and classified symbolization and chose the best option for your map. Finally, you changed the symbol's shape, color, outline, transparency, and background symbol.
Next time you need to map a count or amount, you'll know how to experiment with the style options to make the best choices for your map. Take your maps one step further by combining size with other mapping styles, such as Color and Size, Types, Predominance, and Relationship.
You can find more cartography tutorials on the Introduction to Cartography page.