(i). Contour line. It is a line of intersection of a level surface with the ground surface. It is also defined as a line passing through points of equal reduced levels. Contour maps are then prepared to show the nature or topography of the ground surface. (ii). Contour interval. It is defined as the vertical distance between any two consecutive contours. If two successive contour lines have elevation 2000m and 1500m, then the contour interval is 500m (i.e. 2000m-1500m). Contour interval is constant for a particular map.