In the previous lesson, you completed your map of bombing missions during the Vietnam War with symbolized and labeled reference information. Your map provides a compelling geographic overview of the bombing missions, but fails to capture the data's temporal dimension―when did the missions take place? Additionally, while your map indicates general patterns in the data, it doesn't actually quantify the density of missions throughout the focus area. Trying to provide this information in your existing map will likely make your dense data illegible.
Instead, you'll create supplementary material to provide with your final map document. First, you'll make a time series chart. This bar chart will show the number of missions per month across the duration of the Vietnam War. Then, you'll make a hexagon bin (hexbin) inset map. This map will aggregate (or bin) the bombing missions into hexagons of equal area. You'll then symbolize the map by the quantity of missions within each hexagon, providing a quick resource for your users to better understand the data.
Create a time series chart
A time series chart is a bar chart or graph that shows quantity over time. For your map, you'll show bombing missions per month over the course of the war. If you remember, when you first opened your CSV file of bombing missions, the file not only contained coordinate information but also information on date. In particular, you'll make your time series chart using the Date Simple field, which only contains the month and year (not the day) or the mission.
When creating charts in ArcGIS Pro, you can use either a table or a layer as the chart's data source. However, when using a layer as a source, the layer's symbology will influence the chart's appearance. Because you set your bombing missions to 98 percent transparency, its default symbology is almost invisible when there aren't hundreds or thousands of missions stacked on top of one another. Instead, you'll generate the chart from the original CSV file.
- If necessary, open your Vietnam War Bombing Missions project in ArcGIS Pro.
- In the Contents pane, under Standalone Tables, right-click Missions.csv. Point to Create Chart and choose Bar Chart.
The Chart Properties pane opens, as well as the Missions.csv: Bar Chart 1 view. The Chart Properties pane contains parameters to configure your chart, while the other pane previews the chart you create.
- In the Chart Properties pane, for Category or Date, choose Date Simple.
It's also possible to create a time series chart from the full Date field, which includes the exact day of each mission. However, the Vietnam War spanned an entire decade, with bombing missions on most days. In a time series chart, that would lead to thousands of individual bars. With the Date Simple field, which simplifies the dates to only months, the number of bars is reduced to about one hundred.
After a waiting period, the default chart is displayed as a preview in the other pane.
The size of your chart, and the values used for each axis, depends on the size of your preview pane. Adjusting the preview pane's size will also adjust the chart's size.
The color of the chart's bars is a default color that doesn't match your map's visual aesthetic. You'll change the colors of the bars to match the same purple hue used for bombing missions, as the bars also represent missions.
- In the Chart Properties pane, for Symbol, click the default color. In the list of colors, click Color Properties.
- In the Color Editor, change HEX # to 4C0073 and click OK.
The color of the bars in the time series chart automatically changes to the same purple color as the bombing missions. Next, you'll change the chart title, as well as its axis titles, to be more informative.
- Near the top of the Chart Properties pane, click General.
- Change the Chart title to Bombing and ground attack missions, 1965-1975 (monthly totals).
- Change the X axis title to Date and the Y axis title to Missions.
The titles update automatically in the chart preview. You can optionally add a description, but this chart is probably understandable from its title and axis titles. However, you may want to add an explanatory note about the nature of your data. When you first downloaded the CSV file, you were told that some bombing missions had been removed from the file due to missing or incomplete data. You'll make clear to your map's users that some data might not be included in the chart.
- For Description, type Note: This chart does not include bombing missions with missing or incomplete data.
You'll give the chart a transparent background so you can later place it on your map without obscuring it.
- On the ribbon, click the Format tab. In the Current Selection group, choose Background.
- In the Symbol group, for Fill, choose No color.
Next, you'll set the aspect ratio of your chart preview. Your preview's aspect ratio determines the aspect ratio of the chart image when you export it. Because your chart will be displayed with your map, it should probably be long and thin in order to fit under the rest of your map's information.
- If necessary, adjust the size of the preview pane until the x axis labels the dates at intervals of one year and the y axis labels missions at intervals of 10,000.
This aspect ratio is only the recommended aspect ratio for the purposes of this lesson. Feel free to experiment with the shape of your chart.
The chart indicates that most of the bombing missions occurred between 1967 and 1970, with some months containing extremely high spikes in the number of missions. The 1970s saw a tapering number of bombing missions, corresponding with the United States' gradual disengagement from the war. The final two years of the war, 1974 and 1975, saw hardly any bombing missions at all.
Lastly, you'll export the chart as a Scalable Vector Graphics (SVG) file. This type of file will allow you to scale the chart to a size appropriate for your map when you add it to your print layout later. The scalability is important because you're not entirely sure what your layout will look like right now, so the ability to adjust the chart without compression is important.
- In the preview pane, click Export.
The Export window opens.
- Confirm that the export location is your project's Vietnam War Bombing Missions folder. Name the chart Missions_Chart.svg (including the file extension) and click Save.
The image is saved.
- Close the preview pane.
If you want to open or edit the chart, you can access it in the Contents pane by clicking the List By Charts button and expanding the missions.csv table.
Create an inset map
Next, you'll create a hexagon bin inset map to quantify bombing density throughout the focus area. Inset maps are secondary maps that provide additional information about the main map. Frequently, they contain more geographic context for the main map. Your inset map will instead help users determine the magnitude of difference in bombing between the highest- and lowest-density areas. You'll create it by aggregating missions into hexagon bins (or hexbins) of equal size and area. Then, you'll scale the hexbins based on the number of missions that occurred in each bin. Effectively, you're normalizing your point data by area, allowing users to make like-to-like density comparisons across the peninsula. For more information on hexagonal binning, check out this post on the Esri Insider blog.
First, you'll create a new map and copy the relevant layers from your original map to it.
- On the ribbon, click the Insert tab. In the Project group, click the top half of the New Map button.
A new map is created and added to your project. You can switch back to your original map at any time by clicking the tabs at the top of the map viewer.
- In the Contents pane, open the Map Properties for the new map (double-click the map's name). In the General tab, rename the map Hexbins.
- In the Coordinate Systems tab, change the coordinate system to VN 2000. Click OK.
The coordinate system now matches your original map. Next, you'll copy the relevant layers for your inset map.
- At the top of the map viewer, click the Bombing Missions tab.
You return to your original map. You want to copy two layers to the new map. First is the layer for bombing missions, as you'll need it to aggregate the missions into hexagons. The other is the layer for your focus countries, in order to provide some geographic context for your inset map. You don't want to use the same purple highlight symbology for the focus countries in your inset map, as the inset map will be small and shouldn't command too much attention. Instead, you'll copy the original, unmerged layer for focus countries.
- Hold the Ctrl key. In the Contents pane, click both Bombing_Missions and Focus Countries (the unmerged one) to select them. Right-click the selection and choose Copy.
- Return to the Hexbins map. In the Contents pane, right-click the Hexbins map name and choose Paste.
If you have difficulty copying both layers at once, you can also copy them one at a time.
- Turn off the Topographic basemap and turn on the Focus Countries layer. Then, right-click the Focus Countries layer and choose Zoom To Layer.
Aggregate missions into hexagon bins
Your inset map now displays bombing missions in Vietnam, Laos, and Cambodia. Next, you'll use geoprocessing tools to aggregate the bombing missions into equally-sized hexagon bins.
- Open the Geoprocessing pane (in the Analysis tab, in the Geoprocessing group, click Tools). Search for and open the Generate Tessellation tool.
The Generate Tessellation tool has a complex name, but all it does is create a grid of equally-sized polygons (triangles, squares, or hexagons) that covers a given extent. You'll set the extent to match the extent of your bombing missions layer and give each hexagon a uniform size of 1,000 square miles. A larger hexagon size will make your data more generalized, while a smaller size will make it less generalized. 1,000 square miles is a moderate size given the size of your focus area.
- For Output Feature Class, confirm that the output location is the project geodatabase. Change the output name to Empty_Hexbins_1000mi.
- For Extent, choose Bombing_Missions.
When you choose the layer, the tool parameters automatically populate with the northern, southern, eastern, and western coordinates that define the extent of the data.
- Confirm that Shape Type is set to Hexagon.
- Change Size to 1000 Square Miles. Confirm that Spatial Reference is set to GCS_VN_2000.
- Click Run.
The tool runs and the layer is added to the map. Your layer's symbology may differ from the example image.
The hexagons span an area much larger than your focus area. When you first added the CSV file to your original map, you learned that there were some missions placed as far as Africa, possibly due to errors in the data. These outlier missions helped set the full extent of the bombing missions layer, which you used to define the extent of the hexagons. Later, you'll remove hexagons with no bombing missions from the map. To find out how many missions occurred in each hexagon, you'll run another geoprocessing tool to aggregate the points.
- In the Geoprocessing pane, click the Back button to return to the list of tools. Search for and open the Summarize Within tool.
The Summarize Within tool counts the number of point features that are within a specified layer of polygon features. It creates an output similar to the polygon feature layer, but with attribute data indicating the number of points within. You can then use this attribute data to symbolize the features by the count of points. For your purposes, the point features are the bombing missions and the polygon features are the hexagons.
- For Input Polygons, choose Empty_Hexbins_1000mi. For Input Summary Features, choose Bombing_Missions.
- For Output Feature Class, confirm that the output location is the project geodatabase. Change the output name to Vietnam_Hexbins_1000mi.
You can choose to remove polygons that contain no points. Because your hexagon layer extends far past the area where most missions are, you'll remove these extra polygons.
- Uncheck Keep all input polygons. Leave the remaining parameters (which allow you to summarize more fields in addition to the point count) unchanged.
- Click Run.
Due to the large number of bombing missions, it may take the tool several minutes to run. Once the tool finishes, the new layer is added to the map.
You no longer need the empty hexbins or the bombing missions on this map.
- In the Contents pane, remove the Bombing_Missions and Empty_Hexbins_1000mi layers.
Symbolize the hexbins
Lastly, you'll symbolize the hexbins so that hexagons with a higher number of bombing missions appear larger on the map, while hexagons with almost no bombing missions will not appear. To create this symbology style, you'll use graduated symbols, which allow you to change symbol size or color based on a value of your choice (in this case, number of missions).
- In the Contents pane, right-click Vietnam_Hexbins_1000mi and choose Symbology.
- In the Symbology pane, for Primary symbology, choose Graduated Symbols.
The Graduated Symbols style increases symbols size for larger values. The default symbol is a circle (the color is random and may differ each time). It makes more sense for the symbol to be a hexagon. Rather than adjust each of the five symbol classes individually, you'll change the symbol's template.
- Click the symbol next to Template.
The pane changes to show various symbol styles you can use. As before, you'll create a custom symbol that matches the aesthetic of your map.
- If necessary, click Properties. In the Layers tab, for Form, choose the regular hexagon (first row, fifth column).
- For Color, open the Color Editor. Change HEX # to 4C0073 and the Transparency to 20%. Click OK.
- Change the Outline color to No color. Click Apply.
The color is correct, but the hexagons have a background that shows the original hexagon polygon.
- In the Symbology pane, click the back arrow to return to the parameters for graduated symbols.
- Click the symbol next to Background.
- For Appearance, change Color to No color. Click Apply.
The backgrounds are removed. Your symbol is complete, so you'll change the field that determines how large each hexagon is from the default Shape_Length field to the Count of Points field you calculated.
- Click the back arrow to return to the parameters for graduated symbols.
- For Field, choose Count of Points. Confirm that Method is set to Natural Breaks (Jenks) and Classes is set to 5.
The map updates automatically. Now, the size of each hexagon on the map corresponds to the number of bombing missions that occurred within the hexagon. The Natural Breaks (Jenks) classification method is a mathematical formula that attempts to sort all possible values into five groups with minimal variance within each group. Basically, for your purposes, it's a way to automatically determine how many bombing missions are needed for a hexagon to be a larger or smaller.
Currently, the size of your hexbins seems fine. However, your intention is to display this map as a much smaller inset map. Symbol sizes are absolute even when the map extent changes (for instance, if you zoom in or out), so a symbol size that is appropriate at this scale is likely inappropriate for a map that is smaller. To determine symbol size, you'll zoom to an approximate scale for an inset map.
- Below the map viewer, change the scale to 1:18,000,000.
The map zooms out.
At this scale, the symbols are far too large for the map. They overlap one another and are difficult to read. You'll change the size of the symbols appropriately.
- In the Symbology pane, change Minimum size to 1 and Maximum size to 9.
The symbols on the map update to sizes much more appropriate for the map scale. The last thing you'll change are the break values for your classification scheme. As mentioned previously, the Natural Breaks (Jenks) classification scheme finds natural clusters of data to create classes without too much variance. However, the numbers they use to define each class can appear arbitrary (such as the values chosen for your map: 1,444, 4,781, and so on). You'll round the break values to help users better intuit the data.
- In the table of values, double-click the first value in the Upper value column. Change the value to 1000 and press Enter.
- Change the second value to 5000, the third value to 10000, and the fourth value to 20000. Leave the final value unchanged (it represents the maximum value).
You'll also change the labels for each class to more accurately describe the values.
- Change the first label to <1,000, the second label to 1,000-5,000, the third to 5,000-10,000, the fourth to 10,000-20,000, and the fifth to >20,000.
The labels are updated in the Contents pane.
Your inset map is almost complete. The one problem that remains is that there are a large number of the smallest hexagon class on the map, which clutters the map with a lot of unnecessary symbols. Many of these hexagons have only a very small number of missions, so they're not exceptionally relevant. You'll run a definition query to filter out hexagons with fewer than 100 missions to reduce visual noise in the map.
- In the Contents pane, open the Layer Properties window for the Vietnam_Hexbins_1000mi layer.
- In the Definition Query tab, add a clause that reads Count of Points is Greater Than 100. Click OK.
The inset map has far fewer small hexagons now, allowing the user to visually focus on more relevant symbols. Most of the symbols that you removed were outside of your focus area anyway.
- Save the project.
In this lesson, you created two supplementary elements to add to your finished map. First, you created a time series chart that showed the number of bombing missions per month. Then, you created an inset map of hexbins that showed bombing density across normalized areas. In the next lesson, you'll arrange all your map elements into a final print layout and add text and annotations to explain the story of bombing missions during the Vietnam War.