Discrete and Continuous Data

Discrete and Continuous Data are two ways of classifying data used in cartography and GIS to portray spatial elements and applications. All the data featured in maps and models are either discrete or continuous. Discrete data may only be recorded or reported as certain values while continuous data may be any value within a certain range. Another way of saying this is that discrete data is counted while continuous data is measured.

Some geographic features fall between the extremes of purely discrete and purely continuous. Examples of this include soil types, edges of forests, and boundaries of wetlands. One can determine if a feature is continuous or discrete by how easily it's boundaries can be defined.

Discrete Data
Data in a mapped area are discrete when they are only found at fixed locations or when the data represent only specific values. Buildings and roads are features that have distinct boundaries or limits are considered discrete. These are sometimes referred to as discontinuous data. In mapping, discrete data can be shown as a point, line, or a polygon. Points could be cities, lines could be road networks, and polygons could be provinces in a country.

Discrete data is helpful in showing the exact location, perimeter, and length of objects. Discrete data can be portrayed by either vector or raster data. Discrete data are also referred to as objects.

Continuous Data
Continuous data geographic do not have well-defined boundaries and sometimes have no boundaries. This type of data is seen throughout the mapped area and smoothly transitions from one value to another. An example of a map containing continuous data would be one displaying temperature measurements across a region. Other examples of continuous data are slopes, elevations, relative humidity, and atmospheric pressure.

One of the most common types of continuous data is a topographic map showing elevation on a color scale. Continuous data are often shown in a color scale in order to show change over an area. In the example of a topographical map, sea level could be shown in white and progress through the gray scale as it gains elevation until the highest elevations on the map appear black.

Continuous data can be called nondiscrete data or it can be referred to as a field. See also Field (geography).

Application
In a GIS, discrete data layers may be created for analysis. A layer of continuous data may be converted to discrete data where all values fall into either the acceptable value or the not acceptable value. For example, a set of continuous slope data may be grouped into slopes below 25 degrees and slopes above 25 degrees to help an urban planner decide where to put a new road. This grouping converted the continuous data into discrete data suitable for analysis."