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Contour lines

Text author: Luděk Krtička Last update: 2022-10-26

Contour lines (contours) are imaginary lines connecting places of the same elevation. They are the most common way to express the slope and shape of relief on the maps. With them we are able to capture common relief features such as ridges, valleys, saddles, knolls, depressions, terraces and changes in slope gradient. In places with local abrupt changes in relief slope, contour lines are usually replaced by other map symbols representing micro-relief features such as earth banks and cliffs. In flat areas, contour lines are often far apart and the change in relief may not even be apparent at the location of the contour line.

Contours on terrain model
Contours visualized on a twice-exaggerated relief model using an oblique orthographic view. Illustration by: Luděk Krtička (CC BY-SA 4.0)

The vertically measured distance between two contours is defined as the contour interval. A 5 m interval is most commonly used on forest maps for orienteering, however, in flat terrain where many details need to be captured, a half 2.5 m interval may be used. The contour interval chosen must be consistent over the entire map area, so you cannot combine two different intervals on the same map. The following table lists the allowable contour interval values according to each map specification.

Possible contour interval according to map specifications
Possible contour interval according to map specifications. Interval values in smaller font are only used in special cases such as very steep or very flat terrain. Illustration by: Ainārs Lagzdiņš (CC BY-SA 4.0)

There are three types of contours: contour, index contour and form line. To make it easier to estimate the elevation and height of parts of the terrain, every fifth contour line is thickened and drawn using index contour. If there are important terrain details or changes in relief slope that cannot be captured within the standard contour interval, they can be captured using form lines. However, form lines should only be used on a limited basis where the terrain detail is present. They should not be inserted between normal contours unless there is a different relief shape, as this gives the visual impression of a steeper slope than is actually present.

Contours Long Lake map
Contours example from the Long Lake map near Whiterhorse, Yukon, Canada. Map sample by: Yukon Orienteering Association (CC BY-SA 4.0)

The legibility of contours is essential to the competitor as the contours describe the relief used to navigate. It is therefore also necessary to maintain minimum spacing between contours in steep slopes to prevent their merging into brown areas on the print. The smoothness of their drawing with Bézier curves is essential here. Drawing with freehand and straight lines reduces their legibility and the quality of the map. The direct use of contour lines automatically generated from airborne laser scanning data is not acceptable. Such contours are often jagged and with many fictitious shapes (especially in the flat area). They must always be checked and redrawn in the terrain.

Nowadays, in many countries, contour lines can be based on accurate aerial laser scanning data. Working with such a basemap has its own specifics. The mapping is based on a basemap with a more detailed height resolution (often 1 m) in order to be able to perform the localization on the terrain shapes and to capture even small objects such as knolls and depressions.

  • Places on contour lines that have merged together can often be used to identify and map cliffs.
  • Lidar contours are a mathematically accurate representation of the elevation model. However, within the selected interval, the details of the relief may not be expressed with sufficient fidelity. Absolute height accuracy is less important here and it is acceptable to vary the height of the contour line slightly to better represent the shapes. This variation should not exceed 25 % of the contour interval and consideration should be given to adjacent shapes.
  • It is often necessary to simplify and smooth the contour line for better legibility.
  • A positional shift of the contour line can also be applied to better express the relief characteristics (e.g. capture of terrain edges).
  • By drawing smaller terrain features bigger and distinct, better legibility of the map can be achieved.