What is the distance above sea level is it legend?

Answer 1

Distance above sea level is not legend. Often referred to as altitude ASL (ABOVE SEA LEVEL), it is the difference in altitude between the sea and the point you are referring to. You could say a building is 100ft high, but that's the difference in altitude between the rooftop and the ground. Aircrafts need a point of reference, that's the sea. If the pilot needed to avoid said building, he would have to know the altitude of the ground, and add the height of the building. Since the ground isn't perfectly flat, most altitudes are defined relative to the sea. If the ground was 50ft above the sea level, the pilot would be instructed not to fly below 150ft ASL.