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Identify spatial patterns

In the previous lesson, you created a map of Houston evacuation routes. In this lesson, you'll determine areas of the city that are likely in need of evacuation assistance. To do so, you'll add a layer of Houston demographic data to your map and view the layer's attribute table to find attributes about vehicle ownership. Areas with a high percentage of people who don't own vehicles might need help evacuating, so you'll style the layer using more advanced symbology options to emphasize these areas. Lastly, you'll customize the layer's pop-up to show the relevant demographic information.

Add demographic data

First, you'll add a layer containing demographic data by census tract. United States census tracts divide counties into smaller geographic areas, which are useful for revealing spatial patterns.

  1. If necessary, open your Houston Evacuation Map. (Sign in to your account. At the top of the page, click Content. Click the options button next to your web map and choose Open in Map Viewer.)

    Open in Map Viewer option

  2. On the ribbon, click Add and choose Search for Layers.

    Unlike the routes layer, the demographic data you'll add isn't in the Living Atlas. Instead, it's owned by the Learn ArcGIS administrator account.

  3. At the top of the pane, choose ArcGIS Online.
  4. In the search box, type Houston Demographics owner:Learn_ArcGIS. Press Enter.
  5. Add the Houston Demographics by Census Tracts layer.
    Note:

    It may take a few moments for the layer to appear on the map.

    Houston Demographics by Census Tracts layer

    The layer is added to the map. It's styled by location and shows all census tracts with a single color. Right now, the layer doesn't tell you anything or provide any insight into areas in need of evacuation assistance.

  6. Click the Back button.

    The new layer is listed in the Contents pane above the Hurricane Evacuation Routes layer you added in the previous lesson.

    Layers are drawn on the map in the same order they appear in the Contents pane. In your map, the evacuation routes are partially covered by the census tracts because the Houston Demographics by Census Tracts layer is above the Hurricane Evacuation Routes layer in the Contents pane (you can still see the routes somewhat because the census tracts layer is transparent). To better see the routes, you'll reorder the layers.

  7. In the Contents pane, point to the Hurricane Evacuation Routes layer, click the More Options button, and choose Move up.

    Move up option

    Tip:

    You can also reorder layers in the Contents pane by dragging them.

    The layers are reordered and the evacuation routes are now displayed on top of the census tracts on the map.

    Map showing evacuation routes on top of census tracts

View the attribute table

Every layer has a table that contains all attribute data about the geographic features in the layer. You'll view the table for the Houston Demographics by Census Tracts layer to find data that will help you identify areas that are vulnerable during a hurricane.

  1. In the Contents pane, point to the Houston Demographics by Census Tracts layer and click the Show Table button.

    Show Table button

    The layer's attribute table opens. Each row in the table represents a feature (in this case, a census tract area). The columns, or fields, provide different types of information about the census tract features. For example, the FIPS Code field contains a code that represents the state, county, and census tract identifier for each census tract feature, and the Total Owner/Renter Households (ACS 2011-2015) field shows the total number of households in each tract.

    Tip:

    To make the table larger, drag the top of the table.

  2. If necessary, use the scroll bar at the bottom of the table to scroll all the way to the right.
  3. Locate the Owner/Renter Households without a Vehicle field.

    Table showing Owner/Renter Households without a Vehicle field

    This field shows how many households in each tract do not own a vehicle. You'll style the layer using the values in this field so the differences are visible on the map.

  4. Close the table.

Style the layer using an attribute

Next, you'll change the layer's style based on the attribute field to emphasize census tracts with a high number of households without a vehicle on the map.

  1. In the Contents pane, point to the Houston Demographics by Census Tracts layer and click the Change Style button.

    The Change Style pane opens. When you styled a layer previously, you styled evacuation routes with a single symbol based on location. This time, you'll choose an attribute and style the census tracts with multiple symbols.

  2. For Choose an attribute to show, choose Owner/Renter Households without a Vehicle (ACS 2011-2015).

    Choose an attribute to show

    Based on the attribute you chose, several styles become available. The list of available styles is determined by your type of data, a process known as smart mapping. In this case, the recommended smart mapping style is Counts and Amounts (Size), which is marked by a blue check mark and applied to the map. This style symbolizes each census tract with a circle sized based on households without a vehicle. You could use this style, but at this map extent the differently sized circles frequently overlap. Instead, you'll style the census tracts by color.

  3. For Counts and Amounts (Color), click Select.

    Counts and Amounts (Color) Select button

    The layer updates on the map with a default color ramp called High to Low. Census tracts with the lowest values have a light color, while those with the highest values have a dark color.

    Map showing the number of households without a vehicle

  4. In the Change Style pane, click Done.

    Now that the layer is styled to show households without a vehicle, you'll give it a more descriptive name.

  5. In the Contents pane, point to the Houston Demographics by Census Tracts layer. Click the More Options button and choose Rename.

    Rename option

  6. In the Rename window, type Households Without a Vehicle and click OK.

    The layer name is updated in the Contents pane.

Style the layer using normalized values

Currently, the layer is styled to show the total number of households without a vehicle in each census tract. However, it's possible that tracts with a higher population in general are the only ones being emphasized on the map. It'll be more meaningful for the city to concentrate assistance in tracts with a high percentage of households without a vehicle based on the total population. This percentage is called a normalized value.

  1. Point to the Households Without a Vehicle layer and click the Change Style button.

    When you opened the table, there weren't any attributes in this layer that contained normalized values. However, you can create normalized values by building an expression. This expression will divide the Owner/Renter Households without a Vehicle field by the Total Owner/Renter Households field. Then, the expression will multiply the result by 100 to get a percentage.

  2. For Choose an attribute to show, choose New Expression.

    New Expression option

    A new window opens. In this window, you can create an expression using ArcGIS Arcade, an expression language used in many ArcGIS programs. When you create an Arcade expression, it is saved in the map and can be reused to create pop-ups and labels. The name of the expression appears in the map legend, so it's important to give the expression a meaningful name.

  3. Next to Custom, click Edit.
  4. Delete the existing text and type Households Without a Vehicle (Percentage). Click Save.

    The window has several tabs to help build your expression. The Globals tab, which is active by default, lists all attributes in the layer. You can choose these attributes to create your expression.

  5. On the Globals tab, for Field: Owner/Renter Households without a Vehicle (ACS), click $feature["OwnerRenterHH_NoVehicle"].

    Households without a vehicle field

    The Expression tab updates to contain the field you chose. Next, you'll divide this field by the total number of households. A forward slash is the sign for division.

  6. In the Expression tab, click the end of the expression and type the division sign: /.

    Expression with division sign

  7. On the Globals tab, scroll through the list of fields and click $feature["OwnerRenter_HHs"].

    Total households field

    The field is added to the end of the expression.

    Expression with both fields

    Your expression divides the number of households without a vehicle by the total number of households. To get a percentage, you'll multiply the results of this expression by 100. An asterisk is the sign for multiplication.

  8. Add a set of parentheses around your expression. After the second parenthesis, type *100.

    Your expression reads as follows: ($feature["OwnerRenterHH_NoVehicle"]/$feature["OwnerRenter_HHs"])*100

    Next, you'll test your expression to ensure you didn't make an error.

  9. Click Test.

    Test button

    The Results table appears. The result of your test is a value with a large number of decimal places.

    First expression test result

    Tip:

    If your result differs significantly, check your expression for errors.

    To simplify the result, you'll use the Round function to round the percentage to two decimal places.

  10. Click the Functions tab. In the search box, type Round and click the information button for the Round function.

    Round function information button

    Information about the function and how to use it appears. The complete function is Round(value, numPlaces), where value is the number to round and numPlaces is the number of decimal places to round to. Your value will be your existing expression. For numPlaces, you'll use 2.

  11. Click in front of your expression and type round(.

    Beginning of expression with round function

    Next, you'll add the numPlaces and the closing parenthesis.

  12. Click after your expression and type ,2).

    End of expression to complete the round function

    Your completed expression reads as follows: round(($feature["OwnerRenterHH_NoVehicle"]/$feature["OwnerRenter_HHs"])*100,2)

  13. Click Test.

    The result now has only one decimal place. Although you set numPlaces to 2, the Round function removes trailing zeros, so only one decimal place is displayed in the test result.

    Second expression test result

    Your completed expression works correctly.

  14. Click OK.

    In the Change Style pane, for Choose an attribute to show, your expression is now chosen. The layer style updates on the map. The legend indicates that each census tract size is based on percentage instead of total numbers, but tracts are again styled by size. You'll change the style to use color instead.

  15. In the Change Style pane, for Counts and Amounts (Color), click Select.

    Map styled with normalized values

    Now, darker colors represent census tracts with a higher percentage of households without vehicles. The default color ramp can make it difficult to see the evacuation routes in areas with high percentages, so you'll change the color ramp to ensure all layers remain visible throughout the map.

  16. For Counts and Amounts (Color), click Options. In the more advanced options, click Symbols.

    Symbols option

    A window opens with options to change the fill color and outline of the symbols on the map. You'll choose a blue-themed color ramp to match the evacuation routes.

  17. For Fill, in the color ramp selector, click the blue-to-gray color ramp.

    Blue-to-gray color ramp

  18. Click OK.

    The new color ramp is applied to the map.

    Map with blue-to-gray color ramp

    In this map, census tracts with a higher than average percentage of households without a vehicle stand out in blue. Based on the legend, the average percentage is about 6 percent. A clear pattern of limited access to vehicles stands out in downtown Houston, close to the geographic center of the city. These census tracts would likely benefit the most from increased evacuation assistance, such as public transportation.

  19. In the Change Style pane, click OK. Then, click Done.

Customize the pop-up

Like the Hurricane Evacuation Routes layer, the Households Without a Vehicle layer has preconfigured pop-ups. However, the layer's pop-up was configured to show only total values of households without a vehicle, not percentages. Because you styled the layer with percentages, you'll customize the pop-up to show similar information.

  1. Click any dark blue census tract on the map.

    Default census tract pop-up

    The pop-up provides the name of the census tract, the number of households without a vehicle, and the total number of households. You'll change the pop-up to include the percentages you calculated with your expression.

  2. Close the pop-up.
  3. In the Contents pane, point to the Households Without a Vehicle layer. Click the More Options button and choose Configure Pop-up.

    Configure Pop-up option

    The Configure Pop-up pane opens. The pop-up already has a title, which uses the FIPS field attribute that identifies each census tract. It also has a custom attribute display, which shows the two attributes you saw when you opened the pop-up. The only thing you'll add to the pop-up is your expression.

  4. For Attribute Expressions, click Add.

    Add button

    The expression editor window you used previously opens. You don't need to create a new expression, however. You can use the one you already made.

  5. Click the Existing tab.

    Existing tab

    This tab lists the expressions you've created.

  6. Click Households Without a Vehicle (Percentage) (Color Style).

    You have the option to edit the expression, which you don't need to do. Because you're not making any changes, you'll give the expression the same name as before.

  7. Next to Custom, click Edit and change the name to Households Without a Vehicle (Percentage).
  8. Click Save. Then click OK.

    In the Configure Pop-up pane, the expression is added to the Attribute Expressions list. Next, you'll add the attribute expression to the pop-up contents.

  9. For Pop-up Contents, click Configure.

    Configure button

    The Custom Attribute Display window opens. The preconfigured pop-up includes some text and two fields: OwnerRenterHH_NoVehicle and OwnerRenter_HHs. Attribute fields are indicated by curly braces. In pop-ups, they work like variables: When you click a census tract, the pop-up displays the attribute value for that particular tract.

  10. Click the end of the existing text and press Enter twice. Click the Add field name button and choose Households Without a Vehicle (Percentage).

    Add field name button

    The expression is added with the name expression/expr0.

  11. After {expression/expr0}, add a space and type percent of households in this tract do not own a vehicle.
  12. Highlight the expression and text you added and click the Bold button.

    Your custom attribute display is complete.

    Completed custom attribute display

  13. Click OK. In the Configure Pop-up pane, click OK.
  14. Click any dark blue census tract on the map.

    Pop-up showing percentage of households without a vehicle

    The pop-up now provides the percentage of households without a vehicle, rounded to two decimal places.

  15. Click several more dark blue pop-ups in downtown Houston.

    What is the highest percent you can find? Most of the dark blue tracts have about 20 percent of households without a vehicle. However, there are census tracts where almost 30 percent of households do not have a vehicle, and one where over 40 percent do not. These census tracts would be especially vulnerable and in need of evacuation assistance during a hurricane.

    Additionally, there are some high-risk areas near the bayous you previously identified as flood risks. Even if these census tracts don't have the highest percentage of households without a vehicle, they would be good places to keep in mind when providing evacuation assistance.

  16. Close the open pop-up.
  17. Save the map.

In this lesson, you added a layer of Houston demographic data to the map and viewed its attribute table. Then, you created an expression and styled the layer to show the percentage of households without a vehicle in each census tract. Finally, you customized the layer's pop-up to highlight key demographic information. Now, you can better identify areas in Houston with limited vehicle ownership to determine where evacuation assistance is most needed during a hurricane. In the next lesson, you'll create an interactive web app to share your findings with the public.