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.
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.
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.
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.
From the TRI map layer, click the ANALYSIS tab.
Click ADD ANALYSIS and apply the Create areas of influence option. This is the input layer used for creating the areas of influence.
Select the following parameters for the analysis:
The following image displays these parameters selected for the TRI layer.
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.
Style the areas of influence with a dark color and a transparent opacity, to better visualize the results.
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.
From the LAYERS pane in Builder, click the Census Tract layer.
Click the STYLE tab.
Click the COLOR to open the color picker. The SOLID tab is selected by default.
Click the BY VALUE tab.
pop2010sqm, a number column from your map layer, which enables you to style the map layer with a Sequential or Diverging color scheme.
The color scheme picker appears, from which you can select a color scheme for your data.
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.
If you are interested in using the underlying functions in the SQL view of Builder, view the
ST_Buffer PostGIS documentation.
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