GEOG 863
GIS Mashups for Geospatial Professionals

5.3.2 ClassBreaksRenderer

PrintPrint

The ClassBreaksRenderer class is used to symbolize features in a layer based on the values in a numeric field. A common example is using a color ramp to symbolize areal units by their population, with areas of low population shown in a light hue and areas of high population in a dark hue.

The first step in implementing this sort of visualization is specifying the field you want to base your rendering on (done through the field property). Optionally, you can normalize the values in this field by a) setting the normalizationType to “percent-of-total” and the normalizationTotal to the sum of the values, or b) setting the normalizationType to “field” and the normalizationField to some other field. For example, it’s common to normalize population by area to produce a map of population density.

Once you’ve specified what it is you’ll be mapping, you need to define the classes themselves. To define a class, you set its minValue, maxValue, symbol and label properties. The ClassBreaksRenderer actually provides two ways to specify classes. The first is by setting the classBreaksInfos property to an array of objects. The second is by using the addClassBreakInfo() method to add classes to the renderer one at a time.

This example shows the Jen & Barry’s counties symbolized based on population density. The code should be fairly straightforward to follow. What might be worth expanding on is how the class breaks and colors were decided. My suggestion is to use ArcGIS Desktop or ArcGIS Online to map the data using the desired classification method (e.g., Natural Breaks, Quintiles, etc.) and copy the min and max values for each class into your code. You can also select a color ramp in either platform, open the color picker for each class, and copy the hexadecimal color value into your code. Alternatively, you could use ColorBrewer to obtain the color values.

One thing I hope you noticed about the code from this example is that the section defining the class breaks is repetitive. This sort of repetitiveness should be avoided when possible. For example, what if you decided to change the app from 3D to 2D? After switching from a SceneView to a MapView, you would need to change from a PolygonSymbol3D to a SimpleFillSymbol five times (once for each class in the renderer).

Here is a version of the app that reduces this redundancy and makes the app easier to modify. Note that the code that adds a class to the renderer is moved to a function called addClass. This function requires five pieces of information to do its job: the minimum and maximum bounds for the new class, the color to display it with, its label (which appears if the layer is displayed in the Legend widget) and a reference to the Renderer. These values/objects are then used to set the appropriate properties in the addClassBreakInfo() method call.

With the function defined, it can be called upon five times to create each of the desired classes. This version of the code is an improvement over the previous one because a) changing the kind of symbol used requires just one change instead of five, b) the values that define the classes are clustered together, making it easier to edit them, and c) the length of the script is reduced.

Note that we’ll talk about the Legend widget and other GUI-related topics later in the course.