Contour lines are used when maps are designed to show the physical nature of the land. They do this by linking all points which are the same height above sea level. The width between the contour lines indicates the steepness of gradients or slopes in the area. The closer the lines are together, the steeper is the slope.
On physical maps giving the height of mountains, rivers, lakes and principal towns all areas between certain heights are generally shown in the same colour. This is known as layer colouring.
Other methods for indicating heights include relief maps moulded in plastic to the physical feature raised as on a model. Spot heights may be shown, but these merely give the heights above sea level of certain points of the map and it, does not follow that the ground rises evenly from one point to another.
Very old maps have mountains drawn on them. Later ones have lines called hachures radiating from a central, point, with longer lines to show gentler slopes. Another system is to show the form of the land by hill shading. But none of these methods is effective as the use of contour lines.
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