Level curve

An imaginary line connecting points on the ground that are at the same altitude relative to sea level measured at a reference point. These lines are called isolines or isopleths. In Spain, the mean sea level for the Iberian Peninsula has been established at Alicante. In archipelagos, each island has its own reference altitude. On the 1:50,000 National Topographic Map, these lines are shown with a 20-meter interval.