Drawing Contour Lines The process of interpolation is used to draw contour lines from a map of sounding or topographic data. Interpolation means that if, for example, you're drawing a 20 foot contour line and you don't have an actual point that's 20 feet above sea level but you do have one that's 15 and another that's 25 you can esn'mate where the 20 foot line would be between those two and draw your line through that estimated point. Watch the video Drawing contour lines for more information and a demonstration of this. Contour lines are usually plotted at equal intervals, although there are exceptions