Contour map illustrating elevation changes with labeled contour lines at 10 ft intervals, beginning at 20 ft.

- Identify all points with elevations that are multiples of 10 (20, 30, 40, 50, 60, 70) and connect them with smooth lines.
- For contour lines at 30 ft, 50 ft, etc., interpolate between points. For example, a 30 ft line will pass between a 20 ft point and a 40 ft point, approximately halfway if the terrain is assumed to be linear between them.
- Start with the 20 ft contour line; it will only pass through the single point labeled 20.
- The 30 ft contour line will connect interpolated points between 20 ft and 40 ft elevations.
- The 40 ft contour line will connect all points labeled 40 and interpolated points between 30 ft and 50 ft elevations.
- Continue this process for 50 ft, 60 ft, and 70 ft contour lines, ensuring each line connects points or interpolated positions of the same elevation.
- Ensure contour lines do not cross each other and form closed loops or extend to the map edge.
- Label each contour line with its corresponding elevation (20, 30, 40, 50, 60, 70).