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isoline, isometric line — English
A line drawn on a map to connect the points with equal values for a certain variable in a specific study area. The underlying assumption is that all the points on that line have the same value for that particular variable. Altitude (height above sea-level) may serve as a variable for explanation. No area on the Earth’s surface is as flat as a table top; even the “flattest areas” on the surface (such as the Springbokvlakte on the boundary between Gauteng and Limpopo Provinces) which seems to be as even and flat as anything could be, is uneven and some places (areas) are higher above sea-level than others, albeit by only a few centimetres. Land surveyors use navigational instruments (which nowadays are very sophisticated) to measure the altitude at a number of points in the relevant area. The height above sea-level (altitude) of the prominent high points (called spot heights when they are plotted on a map) and the lowest points are always measured (or surveyed) and the surveyors cover the area with as many measured points as necessary for their triangulations or whatever other objectives they might have (and their budget would allow). From their field survey they have produced a map with a collection of plotted points and their heights above sea-level, but no spatial pattern can be detected on such a map and no three dimensional visualization of the area is possible. However, geographers always study spatial patterns, and they need to visualize the Earth’s surface in three dimensions. Adding isometric lines to the aforementioned map produced by the surveyors, makes geographical study of the area possible. When altitude is the variable under investigation, the isolines are called contour lines and the map is called a contour map; ioshyets are lines that connect points with equal rainfall on a rainfall map, and isotherms are lines of equal temperature on any climatic map, and so forth. Contour maps form the basic working maps for just about any empirical geographical investigation because a geographer has a practised eye and immediately “reads” the area on a contour map in three dimensions. It is, however, rather tricky to convert a map with spot heights into a contour map, that is, to derive line data (linear data) from point data. It requires familiarity with the process of interpolation which is something one learns from hours and hours of practise. Yet, experienced geographers do it amazingly fast; in fact, it becomes a geographer’s second nature. (See “interpolation” and “spatial interpolation”.)
