The Contour lines are intended to give on a topographic map an overview of the real relief. A contour line ( fig. 10.1. ) is the intersection of the real relief with a horizontal plane of altitude given in round dimention (generally a whole number).
The curves are equidistant in altitude; their horizontal spacing depends on the slope of the terrain to be represented and on the scale of the plan or map. The terrain drawn flat on the map is visualized in three dimension. This is reinforced on the maps by coloring to underline the ridge lines: they represent the shadow created by a fictitious light whicjh would come from the northwest of the map.
On the example below, we can read on the plan view the slopes of the natural terrain; wy identify the summits, the topographic passes, the basins (or sinkholes), the breaks in slope.
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