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The isoline representation is the most used method to visualise quantitative phenomena
which occur comprehensively and which values vary continuously in space. They are therefore called
continua. Examples for such continua are temperature, air
pressure, precipitation heights or ground elevations.
Isolines are lines which connect points with identical values
inside a continuum. Isolines are virtual and abstract. Gradients are related to isolines and show
the direction of the biggest value differences at a specific point. Gradients are always perpendicular
to isolines.
The map below shows an example of an isoline map.
Imhof (1972, p. 127ff) distinguishes between natural and geometrical continua
For visualisations with isolines mostly equidistant
gradations are used. Depending on the topic or the data also other forms of gradations
are possible. For contour lines in areas like Switzerland, which include steep as well
as flat regions, a combination of two equidistant gradations are suitable.
Each of the following two pictures shows an extract of the
Swiss National map 1:25’000.
The left image shows a steep area in the Valais alps. To reach a good readability an equidistance of 20m is used. ![]() |
For the more flat area in the Swiss midland a smaller equidistance of 10m is more suitable. ![]() |
Besides this possibility also progressively increasing or random gradation can be used, whereby the second one is not recommended.
To increase the readability different levels of values can be summarised and coloured with a fill. The colours should be selected regarding the visualised topic. E.g. warm and cold regions could be coloured with reddish and blueish colours respectively (see isotherm map above).
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If there is an isoline map with different fill colours between the isolines there should be an explanation in the legend. Isolines without fill colours are labelled directly in the map with their corresponding values. Depending on its length and geometry a isoline can by no means have multiple labels. This prevents the map reader from laborious search for the label. If there is not enough space to label each line, only main isolines can be labelled (e.g. only 100 or 1000). These main isoline should also be emphasized with a slightly greater line width. |
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