Meaning of water level in civil engineering?

Water level is flat on sea according to civil engineering because water level is same on two point of a flat road. Building top or two point of a wall is flat because water level is same. A top of a building is known as flat if water level is same from all of its side. Water level can be checked by means of two sides of a transparent nylon tube or hose also in civil engineering by putting water inside it as per the length of tube or hose and both the sides had to be raised at the two points where it has to be checked. The level depends on gravitational force on earth from the center of the earth. So surface of road is known as flat. Even table or bed is known as flat. But surface of earth is known as round from outside the gravitational zone of earth from the space, not from earth.From the surface of earth earth is flat because water level is same. From the surface of sea sea is flat because water level of sea level is same. The concept is from sea level as per physics. So it is known for altitude for a location on earth means height above sea level. For example altitude of New York City is 33 feet above sea level.