Isopleth Maps

Isopleth maps are a type of thematic map used to display and model observations of phenomena with a continuous distribution by representing the values as a third-dimensional surface draped over the area. This common mapping technique is appropriate for diverse topics. Similar terms, mapping techniques, value interpretation, and specific isopleths, especially contours, are discussed.

Broadly called isoline maps, a distinction is sometimes made between isometric and isoplethic mapping, wherein isometric maps display real values such as rainfall amounts and isoplethic maps display calculated values such as population density. The terms isarithm and isogram are generally synonymous. Iso means “equal,” and pleth refers to “quantity.”

If something can be measured or generalized at any location, such as air temperature, then potentially, isopleth mapping can be used to show the spatial patterns. Observations cannot be made everywhere, so isopleth maps are constructed by interpolation from measurements at specified locations or associated calculations.

Isopleth maps use lines of equal value to portray patterns. These isopleths or isolines are plotted by assuming an even rate of change between the locations of known values. An isopleth interval identifies the difference in value between adjacent isopleths. The lines drawn are multiples of this isopleth interval. Smaller intervals mean more lines will be shown and more detail will be interpolated. This does not enhance accuracy. Isopleths enclose areas of either higher or lower values. They can neither cross nor split apart, because the line can have only one value. More closely spaced isopleths indicate more rapidly changing values.

Isopleth maps are read by identifying the value of the line through the point or the value of the adjacent higher and lower lines. For a point not on a line, a range is indicated. A value can be interpolated from the relative location between the lines, but the reader should remember the original generalization process used to produce the maps and not infer excessive detail.

The most difficult aspect of reading isopleths involves identifying the continuity between the high and low centers. A low center or a depression is an area completely surrounded by higher values. The primary skill involves maintaining the value trend. Any reverse in the trend is indicated by a repeated line value. If the value of the lines is increasing in the direction of a high center and a low center is encountered, the line value most recently passed will be repeated, indicating the change in the trend. However, if the trend is going down from a high center into a depression, the values do not repeat (Figure 1). Shaded isoline maps color the areas between the lines for clarification.

Figure 1 Continuity of values on isopleth maps

Source: Author.

A contour map shows elevation of the land surface and is a specialized isopleth map. These differ by the inclusion of hachures, small tic marks pointing downslope, added to the lines in the low centers. The same interpretation skills are applied to read the values. The first hachured contour line has the same value as the lower adjacent unhachured line.

Table 1 Names of selected isolines

Source: Author.

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