Two contours of different elevation can unite to form one line only in the case of:

Explanation:

Characteristics of contour lines:

• All the points on a contour line have the same elevation.
• Two contour lines do not intersect each other except overhanging cliff or vertical cliff.
• The contour line is a closed curve. It may close either on the map or outside the map.
• Contour lines with higher values inside represent a hill, similarly closed contour lines with lower values inside indicate a depression without an outlet, for example, a lake.
• Closely spaced contour lines indicate a steep slope, and contour lines indicate a gentle slope if they are far apart. If contour lines are equally spaced, a uniform slope is indicated. A series of straight parallel and equally spaced contours represent a plane surface.
• Contour lines of different elevations can unite to form one line only in the case of a vertical cliff.
• Contour lines with U shape and convexity towards lower ground indicate a ridge, and Contour lines with V shape convexity upward or towards higher ground represent a valley.
• Two contour lines having the same elevation can not unite and continue as one line, Similarly, a single contour can not split into two lines.