Isolines, also called isarithms, are a type of two-dimensional cartographic symbolization that is commonly used for depicting a three-dimensional statistical surface on a flat piece of paper or a computer screen. This three-dimensional surface may be real (e.g., elevation or air pressure) or conceptual (e.g., crop yields). The location of each isoline depicts all of the places on the surface that have a particular value (e.g., a particular isoline on an elevation contour map shows all of the places that have a particular elevation; see Figure 6.cg.3, below). Over time, we have come to use specialized names for isolines that are used for representing certain types of phenomena (e.g., an isotherm is a line of equal temperature, while an isobar is a line of equal pressure). However, whatever the isolines are called, they are created using a similar procedure.
Isoline symbolization is generally appropriate for depicting phenomena that are continuous and smoothly changing (refer back to the section on Surfaces in the Lesson 4 concept gallery for a more detailed discussion of these characteristics of spatial distributions). Until the advent of remote sensing, cartographers were only rarely able to measure a phenomenon at all locations on a surface, so they used methods generally known as interpolation (see the section on Interpolation for more information on these methods) for estimating the values of variables at non-measured locations from measurements made at particular points (see Figure 6.cg.4, below). Even with the availability of remotely sensed data today, there are still many variables for which we cannot obtain measurements at each point on the earth's surface, so these interpolation methods are still very important. After a surface representation of the attribute of interest has been created (usually in the form of either a raster or a triangulated irregular network), the cartographer chooses which isolines s/he wants to represent on the map, and the computer can identify every location where the value of interest occurs and draw a line connecting those locations (i.e., the isoline).
When communicating data with isolines, there are several design factors to consider. Although isoline maps allow map readers to estimate values at particular locations with reasonably good precision, map readers often have a difficult time forming a mental image of the surface's shape from isolines alone (see figure below, left). Filling contours with some sort of sequential color scheme can help map readers to more easily form an overall impression of where highs and lows exist in the space (see figure below, right).
The choice of an appropriate contour interval can sometimes be tricky: if you choose a small contour interval, you may be left with a map that becomes a mass of closely packed lines that make it difficult to effectively read or display any other relevant information; if you choose a larger contour interval, map readers may miss important surface features, particularly in areas of slower attribute change.
By altering the base position or starting point for drawing the isolines, there may be features that get hidden or obscured. See Figure 6.cg. 7 below for an example.
Isoline maps often include symbolization in addition to the isolines themselves in order to communicate other features or data. It is important to be sure that your isolines stand out as figure against any other features you include in the map.
Sometimes it can be helpful to use additional visual variables within the isolines to add supplementary information. For example, using two types of line weights for elevation contours results in index and intermediate contours. The index contours, which are usually symbolized by a thicker line, are at set intervals (usually every fourth or fifth contour) and can help the map reader more quickly identify elevation information. For example, if you had a 20 meter contour interval, you might make index contours at every 100m. An advantage to this approach is that it is only necessary to label the index contours, making label placement easier.
A second type of visual characteristic that can be used for isoline maps is for depressions, or areas of decreasing value that are encircled by areas of increasing value. These are often represented with a line that is hatched towards the interior of the depression to give the map reader the impression that the surface is trending downward within the isolines. See Figure 6.cg.9, at right, for an example.
The placement of labels in isoline maps is very important. The result of a computer's default label placement can result in some lines falling directly on top of labels, rendering them illegible without masking; others are placed at angles that make reading them awkward or that make it difficult to tell to which line they refer to. Generally, label only the index contours, and only repeat labels in cases where the line is long, or interrupted. Labels should be placed "in-line" with the lines to which they refer, and techniques such as haloing can be used to mask or break the line so that the label is legible (refer back to the Text Effects concept gallery item in Lesson 3 for a refresher on line halos).