Analysis Guides

Create Areas of Influence

This guide describes how to apply the Create areas of influence analysis to create buffers of distance (in meters, kilometers, miles), or of travel time, via a medium (walking, driving). The analysis creates an “area of influence”, or “isoline”, of a specified distance or time from any geographic point, polygon or line.

For a dataset with polygon geometries, the centroid of the polygon is used to create an area of influence with the provided distance.

An isoline is any line on a map that shares a common value. It uses an approximation of isoline functions to create straight-line areas of influence from the origin point, measured by distance (isodistance) or time (isochrone).

This guide focuses specifically on the distance functionality (isodistance) with the Create areas of influence analysis.

Areas of Influence

For this guide, the Create areas of influence analysis is applied to view the impact of environmental equity assessment in a city. Specifically, we will use the Toxic Release Inventory (TRI) data from the EPA, and census tract data for Des Moines, Iowa.

The result of the analysis will help us understand whether a preliminary overlap, or “area of influence”, can be observed between more populated census tracts and affected regions around the TRI facilities.

  1. Import the template .carto file packaged from “Download resources” of this guide and create the map. Builder opens with TRI point data as the first map layer, and Census Tract polygons as the second map layer.

    Click on “Download resources” from this guide to download the zip file to your local machine. Extract the zip file to view the .carto file(s) used for this guide.

  2. From the TRI map layer, click the ANALYSIS tab.

  3. Click ADD ANALYSIS and apply the Create areas of influence option. This is the input layer used for creating the areas of influence.

  4. Select the following parameters for the analysis:

    • Distance as the TYPE
    • mi, for miles of UNITS
    • 1 as the RADIUS
    • 2 as the TRACTS. This parameter creates an isoline within the created buffer, dividing the radius equally by the number of tracts.
    • Dissolved as the BOUNDARIES. This parameter enables you to define the polygon outlines as intact (keeps the original boundaries intact for overlapping buffers), or dissolved (dissolves the boundaries of overlapping buffers).

    The following image displays these parameters selected for the TRI layer.

    Create areas of influence parameters

  5. Click APPLY to create the areas of influence.

    The map displays the created areas of influence of 1 mile radius, demarcated with an isoline at 0.5 mile.

  6. Style the areas of influence with a dark color and a transparent opacity, to better visualize the results.

    Style the results of the areas of influence

Defining Context

Since we are interested in determining the equity assessment for Des Moines, it is important to visualize the population density that is affected within the area of influence of the TRIs. The Census Tract layer can be styled to visually depict the areas with more density.

  1. From the LAYERS pane in Builder, click the Census Tract layer.

  2. Click the STYLE tab.

  3. Click the COLOR to open the color picker. The SOLID tab is selected by default.

  4. Click the BY VALUE tab.

  5. Select pop2010sqm, a number column from your map layer, which enables you to style the map layer with a Sequential or Diverging color scheme.

CHEATSHEET: Color Schemes

Choosing the right colors for your data aids storytelling, engages the map reader, and visually guides the viewer to uncover interesting patterns that may otherwise be missed. When styling by value, different types of color schemes appear, based on the selected data column from your map layer. Builder provides you with CARTOColor and ColorBrewer schemes, and enables you to customize your own color schemes.

  • Sequential Scheme: Color schemes that use variations in lightness make these ideal for displaying orderable, or numeric data. The variations progress from low to high, using colors that range from light to dark (or vice versa).
  • Qualitative Scheme: Color schemes that demonstrate categorical differences in qualitative data, which use different hues, with consistent steps in lightness and saturation.
  • Diverging Scheme: Color schemes that highlight values above and below an interesting mid-point in quantitative data. The middle color is assigned to the critical value, with two sequential type palettes at either end, assigned to values above or below.

The color scheme picker appears, from which you can select a color scheme for your data.

Style Census Tract by numeric value

Notice that the overlap does seem to be visually strong around the areas of influence, which can be further studied and verified by applying other analyses with Builder. For example, apply the Detect outliers and clusters analysis to explore significant patterns in your data.

Download the final .carto file from the “Download resources” of this guide, and explore the styling and basemap changes that were applied.

External Resources

If you are interested in using the underlying functions in the SQL view of Builder, view the ST_Buffer PostGIS documentation.