Explore lava flow risk
When a volcano erupts, how much time do residents have to evacuate? It depends. Lava flows downhill and travels faster over steep ground. In the early 1990s, residents of Kalapana, a town in the southeastern Puna region, had days to prepare for a lava flow that eventually covered the town. By contrast, a 1950 lava flow down the western flank of Mauna Loa reached the sea in about four hours. Although scientists monitor ground movement on the island continuously, there is no way to know how much advance notice residents living downhill of an eruption will receive.
What emergency responders and governments can plan for are evacuation scenarios. Knowing where lava flow hazard is greatest and how many people live in these areas can help responders understand where to set up evacuation shelters, send resources, and more. Before you start your analysis, you'll become familiar with areas of high lava flow risk and the distribution of population on Hawaii Island.
Filter the hazard zones
First, you'll open a web map with information about lava flow hazard zones and volcano shelters. Then, you'll filter the hazard zones to show only zones with the highest risk.
- Go to the Shelter Access Analysis web map in Map Viewer.
The legend shows that the island is divided into nine categories of lava flow risk, ranging from extremely high hazard zones (dark red) to extremely low hazard zones (light gray). The map also shows the locations of volcanoes and emergency shelters.
Next, you'll sign in and save a copy of the map.
- On the ribbon, click Sign In. Sign in to your ArcGIS organizational account.
Note:
If you don't have an organizational account, see options for software access.
- On the Contents (dark) toolbar, click Save and open and choose Save as.
- In the Save map window, set the following information:
- For Title, delete the text and type Hawaii Island Shelter Access.
- For Summary, delete the text and type (or copy and paste) Map showing areas of high lava flow risk and areas within 15 minutes of a shelter.
- Click Save.
You have saved a copy of the map. Automatically, you are taken to your copy. You're ready to make changes, explore the data, and perform analysis.
The map shows areas of both high and low lava flow risk. You're interested in high risk areas, so you'll filter the layer to only show them.
- In the Layers pane, click the Lava Flow Hazard Zones layer to select it.
When a layer is selected, a blue bar appears next to it in the Layers pane.
- On the Settings (light) toolbar, click Filter.
- In the Filter pane, click Add new. Confirm that the default condition reads Severity Code is High Hazard.
On the map, only high hazard areas are displayed. There are also very high and extremely high hazard areas, so you'll add them to the filter.
- Click Add new. Create the condition Severity Code is Very High Hazard.
- Click Add new. Create the condition Severity Code is Extremely High Hazard.
Your filter now has three conditions. By default, the filter only displays features that match all conditions, which means no data is being displayed on the map because each feature has a single hazard rating. You'll change the way the filter works so that it shows results that match one condition, rather than all conditions.
- For Show features where, click All of the following are true and choose Any of the following are true.
The three highest-hazard lava flow zones are now shown on the map. These are the areas you'll be analyzing.
- In the Filter pane, click Save.
Visualize where people live
Next, you'll use census data to understand where people live on the island. Understanding how many people live in areas of high risk is helpful for planning where emergency shelters are needed and how large they should be.
The map includes a layer of census blocks, which are the smallest geographic area for which the United States Census Bureau reports population information. By default, the layer is turned off, meaning it doesn't appear on the map. You'll turn it on and see what it shows.
- In the Layers pane, for Census Blocks (2020), click the Show Census Blocks (2020) button.
The layer is turned on and appears on the map. The census blocks are displayed below the hazard zones, which can make them difficult to see. You'll reorder the layers so the census blocks appear on top of the hazard zones.
- Drag the Reposition: Census Blocks (2020) button above the Lava Flow Hazard Zones layer.
You can see the layer more clearly now, but the layer's style doesn't communicate how many people live where. You'll change the style to one that is better for visualizing population.
- In the Layers pane, click the Census Blocks (2020) layer to select it.
- On the Settings toolbar, click Styles.
The Styles pane appears. It shows that the layer is currently styled using the Population (2020) attribute, which is what you want to display on the map.
By default, the layer is styled using Counts and Amounts (color). Census blocks with higher populations have darker colors on the map. At first, this seems like it's showing you what you want to know (where people live on Hawaii Island). However, this style doesn't account for the size of each census block, which can vary a lot. A large census block may have more people, but a population might be more densely concentrated in a smaller census block. To better visualize population density, you'll use the dot density style.
- For Pick a style, click Dot Density.
On the map, the layer is redrawn using the dot density style. In this style, each dot represents a constant number of things (in the case of your map, a certain number of people). By default, the dots are a red color, which makes them difficult to see against the red hazard zones. You'll change the color.
- For the Dot Density style, click Style options.
- Under Legend, click the symbol.
- In the Symbol style window, click the Dot color button.
The Select color window appears. You'll change the dots to black, so they'll show up on most backgrounds. The best way to set an exact color is to use its hex code, a six-character code following the # sign. The hex code for black is #000000.
- In the Select color window, for the hex code (#), replace the text with 000000.
- Click Done. In the Style options pane, click Done.
On the map, the dots representing population appear in black.
Note:
The number of people each dot represents changes depending on your zoom extent. In the example image, each dot represents 45 people. Your map may have a different number of people for each dot.
The dots are scattered randomly through the census block polygons, so this visualization style is not showing you exactly where people live, either. However, because census blocks are a fine-grained geographic area, the dot density style is a relatively accurate representation of the population distribution on the island.
Based on this visualization style, most of the population lives around the perimeter of the island, with population centers concentrated around Holualoa to the west and Hilo to the east. Most of the Holualoa population is outside the high hazard zones, but a large portion of the Hilo population is inside them.
- In the Styles pane, click Done.
- On the Contents toolbar, click Save and open and choose Save.
You filtered the hazard zones to only show zones of high risk. You also visualized the population using the dot density style. Next, you'll perform analysis to find areas with quick access to volcano shelters.
Analyze shelter access
Now that you've explored the data and are familiar with the lava flow hazard zones and the population distribution on Hawaii Island, you'll use analysis tools in ArcGIS Online to determine whether people have quick access to emergency shelters. Analysis tools run processes that transform your data, which can provide new insights that might be impossible to find in the original data alone.
Generate travel areas
First, you'll generate travel areas around emergency shelters to analyze their accessibility. Travel area analysis uses either distance or expected travel time along a street network to show the area within reach of a location or set of locations. You'll use a drive-time value of 15 minutes to define a shelter as easily accessible. This value was chosen arbitrarily, and you might want to repeat the analysis with other values to see how the results change.
This analysis does not attempt to model emergency conditions. In a real emergency, all sorts of things may happen. Evacuation routes may be cut off by lava flow, roads may be jammed, emergency shelters themselves may lie in the lava flow path, and so on. Nonetheless, the ability to see which areas of the island have easy access to a shelter, and which do not, is a useful planning aid.
- On the Settings toolbar, click Analysis.
Note:
If you don't see the Analysis button, contact your ArcGIS administrator. You may not have the privileges required to perform analysis. Learn more about licensing requirements for spatial analysis.
- In the Analysis pane, click Tools.
- In the search bar, type Generate Travel Areas. In the list of results, click the Generate Travel Areas tool.
The Generate Travel Areas tool calculates the area that can be reached within a specific travel time or distance along a street network. In this case, you want to calculate a 15-minute drive to shelters. First, you'll set the shelters as the input layer.
- For Input layer, click Layer.
- In the Select layer window, choose Emergency Shelters.
Next, you'll set the driving time.
- For Travel mode, ensure Driving Time is selected.
- For Cutoffs, type 15 and click Add. Ensure Cutoff units is set to Minutes.
You want to analyze the time it takes to drive toward shelters, rather than away from them, so you'll change the travel direction.
- For Travel direction, choose Toward input locations.
You'll also have the analysis results dissolved into a single feature, rather than have overlapping travel areas for each shelter. You're interested in the total area that is has access to a shelter in general, rather than access to specific shelters, so dissolving the results makes sense.
- For Overlap policy, choose Dissolve.
Lastly, you'll provide a name for the result layer.
- For Output name, type Shelter in 15 Minutes and add your name or initials to make the name unique in your organization.
Note:
You cannot create two layers in an ArcGIS organization with the same name. Adding your initials to a layer name ensures that other people in your organization can also complete this tutorial. Once a layer has been created, you can rename it in the map to remove your initials, which will not affect the name of the underlying data layer.
Running an analysis tool consumes credits. Credits are the currency used in ArcGIS Online. They are consumed during specific transactions, such as performing analytics, storing features, and geocoding. Before running an analysis, it's a good idea to check how many credits will be consumed. When running analysis tools, the credit cost is typically calculated by multiplying the number of features by the credit cost of the tool.
- Click Estimate credits.
This analysis will consume 15 credits.
- Click Run.
The tool runs. It takes a few minutes to finish.
- In the Analysis pane, click the History tab.
The tool's status is shown. When the tool finishes, the output layer appears on the map. It shows the areas on the island that are within a 15-minute drive of a shelter.
Most of the populated places on the island, including the densest populated areas around Holualoa and Hilo, are covered by the travel area, meaning they have relatively quick access to an emergency shelter. There are some populated places, such as the southwest corner of the island and parts of the area south of Hilo, that aren't covered by the travel area. These places might be in need in a new shelter.
To ensure your analysis result layer had a unique name in your organization, you added your name or initials. In the map, you can rename the layer to remove your name or initials.
- In the Layers pane, for Shelter in 15 Minutes, click the Options button and choose Rename.
- Remove your name or initials from the layer name and click OK.
Note:
Renaming a layer in a map does not change the name of the dataset, which is saved in your account and exists independently of the map.
Combine the hazard zones
Now that you've performed your analysis, you'll make sure your map displays the important information about shelter access as clearly as possible. Your map has many layers, many of which overlap, reducing the map's legibility. On top of that, the hazard zones layer has three different symbols (one for high, very high, and extremely high hazards), further complicating the map.
You're not particularly interested in the differences in severity of the hazard zones, since you filtered out the medium and low hazard zones already. You'll run another analysis tool to dissolve the hazard zones into a single feature and reduce the map's complexity.
- In the History pane, click the Tools tab.
You return to the Generate Travel Areas tool.
- Click the Back button.
- In the search bar, type Dissolve. Click the Dissolve Boundaries tool.
This analysis tool dissolves multiple features in a layer into a single feature. It works similarly to the Dissolve option in the Generate Travel Areas tool, which combined the travel area for each shelter into a single travel area feature.
- For Input features, click Layer. In the Select layer window, click Lava Flow Hazard Zones.
Analysis tools account for filters or selections on the input features, meaning the tool will only run on the high hazard zones and ignore the medium and low hazard zones you filtered out. This tool has parameters to change the way the features are dissolved, but for this analysis, you don't need to change any of them.
Tip:
To learn more about any parameter in an analysis tool, click the information button next to the parameter.
- For Output name, type Combined High Hazard Zone and add your name or initials.
- Click Estimate credits.
This tool will consume only 0.011 credits.
- Click Run.
The tool runs. After a few seconds, a new layer is added to the map, displaying the high hazard zones as a single feature. You no longer need the original hazard zones layer, so you'll remove it from the map.
- In the Layers pane, for Lava Flow Hazard Zones, click the Options button and choose Remove.
- For Combined High Hazard Zone, click the Options button and choose Rename. Remove your name or initials and click OK.
You'll also reorder the layers on the map. By default, the analysis result layers were added to the top of the Layers pane. Both of these layers have polygon features, which are best displayed under point features (like the shelters and volcanoes).
- Drag the Shelter in 15 Minutes layer under the Census Blocks (2020) layer. Drag the Combined High Hazard Zone layer under the Shelter in 15 Minutes layer.
On the map, these layers now display under the other ones.
Style the map
Your map is more legible now, but can still be improved. The default blue color of the combined hazard zone doesn't intuitively convey danger, so you'll change it to an orange color with a red outline.
- In the Layers pane, confirm Combined High Hazard Zone is selected. On the Settings toolbar, click Styles.
- In the Styles pane, for Location (single symbol), click Style options.
- Click the Symbol style button.
Like when you changed the dot density style, you'll change the color of the hazard zone by inputting a hex code. Unlike the dots, which had a single color, you'll have to change both the fill and outline colors of the hazard zone polygon.
- In the Symbol style window, click the Fill color button.
- In the Select color window, for the hex code (#), type ff7900. Click Done.
- In the Symbol style window, click the Outline color button. Change the color to the hex code ff0000 and click Done.
- In the Styles pane, click Done.
The hazard zone now has an orange and red polygon, which is more appropriate for conveying hazards.
To ensure the travel areas around shelters stand out against the hazard zone, you'll style them with a complementary blue color.
- In the Layers pane, click Shelter in 15 Minutes to select it.
The Styles pane automatically changes to display style options for the selected layer.
- In the Styles pane, for Location (single symbol), click Style options.
- Click the Symbol style button. In the Symbol style window, click the Fill color button.
- Change the hex code to 047ccc and click Done.
The travel area polygon has many thin parts that follow roads. In some places, these parts are too thin to include a fill color, making them only have an outline. The result is that these thin parts can look irregular or be difficult to see. You'll resolve this problem by removing the outline so all parts of the polygon are displayed with the fill color.
- In the Symbol style window, for Outline color, click the No color button.
The outline is removed.
- In the Styles pane, click Done.
By default, the travel area polygon has transparency applied to it. Transparency is useful for seeing features under a layer, but it can also make the layer more difficult to see in some places. This layer is one of the most important ones on your map, so you'll remove its transparency.
- On the Settings toolbar, click Properties.
- In the Appearance section, for Transparency, drag the slider to 0 percent.
Similarly, you'll apply transparency to the dot density layer so you can see the travel area under it.
- In the Layers pane, select the Census Blocks (2020) layer. In the Properties pane, for Transparency, drag the slider to 30 percent.
Your map is now complete. It's possible to see the areas within 15 minutes of a shelter, the hazard zone, and the population dots at the same time. With these layers, you can determine where populations do and don't have quick access to an emergency shelter.
- Save the map.
In this tutorial, you used analysis tools to determine who has access to emergency shelters on Hawaii Island. Based on your analysis, it might be beneficial to identify more shelter locations near populations who are unable to reach shelters within 15 minutes. To improve your analysis, you could also try calculating other drive times or add layers showing population risk factors like vehicle ownership to help responders identify areas where residents might need assistance with evacuation.
While the conditions you used to analyze the island today may not be the same during a disaster, analyses like these can help local governments and other responders prepare.
You can find more tutorials in the tutorial gallery.
