Contour Lines Explained

Contour lines are lines drawn on a map that connect points of equal elevation above a certain reference level, usually mean sea level. They are used to represent the three-dimensional shape of the terrain on a two-dimensional map. The spacing between contour lines indicates the steepness of the slope; closer lines mean a steeper slope, while widely spaced lines mean a gentle slope or flat area.

Contour Line Properties

Generally, contour lines of different elevations do not intersect or merge. Each line represents a unique elevation, so a single point on the map cannot have two different elevations simultaneously. However, there is a specific situation where contour lines of different elevations can appear to unite or merge on a map.

Contour Lines on a Vertical Cliff

A vertical cliff is a terrain feature where the elevation changes almost instantaneously over a very short horizontal distance. Imagine walking along the top edge of a vertical cliff. Points very close to each other horizontally might have significantly different elevations if one point is on the edge and the other is just below the edge face.

On a topographic map, this rapid change in elevation means that many different contour lines, each representing a specific elevation, would need to pass through the same very small horizontal area occupied by the cliff face. Since the change is so steep (vertical), the horizontal distance covered for a large change in elevation is negligible. Consequently, on the map, these contour lines representing different elevations essentially coincide or merge together to form a single, thick line representing the vertical cliff face.

Let's look at the given options in this context:

• Hilly terrain: In hilly terrain, slopes are present, but they are not vertical. Contour lines will be close together on steep slopes but will not merge.
• Flat terrain: Flat terrain has minimal elevation change. Contour lines will be far apart or absent. They do not merge.
• Lake: A lake surface is at a constant elevation. A contour line would follow the shoreline at that elevation. Contour lines above and below the lake level would be distinct. They do not merge at the lake surface.
• Vertical cliff: As explained, a vertical cliff is the unique case where contour lines of different elevations merge together on a map because the elevation changes vertically over a horizontal distance that is practically zero.

Therefore, contour lines of different elevation can unite to form one line in the case of a vertical cliff.