Ways to map quantitative data
Last modified September 15, 2008
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Quantitative data describes features in terms of a quantitative value measuring some magnitude of the feature. Unlike categorical data, whose features are described by a unique attribute value such as a name (see Drawing features to show categories), quantitative data generally describes counts or amounts, ratios, or ranked values. For example, data representing precipitation, population, and habitat suitability can all be mapped quantitatively.
Knowing what type of data you have and what you want to show will help you determine what quantitative value to map. In general, you can follow these guidelines:
When you map quantitative data, you can either assign each value its own symbol or you can group values into classes using a different symbol for each class.
If you are only mapping a few values (less than 10), you can assign a unique symbol to each value. This may present a more accurate picture of the data since you are not predetermining which features are grouped together. More likely, your data values will be too numerous to map individually and you'll want to group them in classes, or classify the data. A good example of classified data is a temperature map in a newspaper. Instead of displaying individual temperatures, these maps show temperature bands, where each band represents a given range in temperature.
Learn how to set a classification for your data
How you define the class ranges and breaks—the high and low values that bracket each class—will determine which features fall into each class and what the map will look like. By changing the classes, you can create very different-looking maps. Generally, the goal is to make sure features with similar values are in the same class.
Two key factors for classifying your data are the classification scheme you use and the number of classes you create. If you know your data well, you can manually define your own classes. Alternatively, you can let ArcMap classify your data using standard classification schemes. The standard classification schemes are