GEOG 863
GIS Mashups for Geospatial Professionals

5.3.4 Visual Variables

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In addition to the Renderer functionality we’ve seen already, Esri also makes it possible to produce thematic depictions of data through what they call visual variables. For example, you might create a map of country populations using a light-to-dark color ramp. If you’re thinking you already saw how to do this with the ClassBreaksRenderer, that’s true. The difference with visual variables is that you’re not creating a limited number of classes to handle all of the values in the field you’re mapping. Instead, you’re creating a continuous ramp of color based on the field’s minimum and maximum values. A feature whose value is midway between the min and max values will be drawn in a color midway between the color you associated with the min value and the color you associated with the max value.

Color is one commonly used visual variable. Size is another. Opacity and rotation are the two others, though those options are less commonly used.

Esri refers to the use of visual variables as data-driven mapping, since the features are not being partitioned into classes whose bounds can be rather subjective. The symbolization is instead driven by the data.

A data-driven visualization can be created using any of the three Renderers we’ve discussed. Each Renderer class has a visualVariables property that can be set to an array of JS objects. At minimum, each object in the array should have a type (set to “color”, “size”, “opacity” or “rotation”) and a field (set to the name of the desired field). Optionally, you can also set a legendOptions property to specify if and how the visualization should appear in the legend.

The critical step in setting up a visual variable is defining the ramp. For both the color and size types, this can be done by creating an array of at least two stops. The code might look like this:

// Stops for a color visual variable
stops: [
 { value: 0,
   color: "#ece7f2", // light blue
   label: "< 0"
 },
 {
   value: 100,
   color: "#2b8cbe", // dark blue
   label: "> 100"
 }
]
// Stops for a size visual variable
stops: [
 { value: 0,
   size: 8
   label: "< 0"
 },
 {
   value: 100,
   size: 24,
   label: "> 100"
 }
]

This example shows the use of a color visual variable to depict average household income in PA census tracts. Note that a diverging red-to-green color ramp (found at ColorBrewer) was employed, with red being associated with the minimum data value, yellow/beige associated with the mean, and green with the maximum. These data values were obtained by mapping the field in ArcGIS Online.

As mentioned, you are not limited to just one visual variable per renderer. Here is another example that builds on the previous one by adding a second visual variable to the renderer and adding the layer to a SceneView rather than a MapView. This second visual variable extrudes (adds height to) the census tracts proportional to their population. (Note that this is not the best example since by design there isn’t a great deal of variation in census tract populations.)

Speaking of polygon extrusion, one of its common uses is in extruding building footprint polygons by the building height to produce a more realistic depiction of a cityscape. Here is an example, using buildings in Philadelphia.