The elevation we call sea level is actually the average level of the sea. This means that at average sea level, the elevation is zero. In reality, the level of the sea rises and falls in what are called tides. Low tide is the point of lowest sea level in a tidal period. High tide is the point of highest sea level in a tidal period. The height above or below average sea level is called elevation. An elevation below sea level is considered a negative value, while an elevation above sea level is positive.

Winston lives near the ocean. He regularly brings an altimeter to the beach to measure the elevation of the ocean’s surface. You will use the data Winston gathered to complete tasks about the rising and falling of the ocean’s surface. When necessary, sketch a diagram of the situation on a separate sheet of paper to assist you.

On Monday at 1 p. m., when the ocean’s surface was at an elevation of 2.2 feet, Winston went to a surf shop that was 8.2 feet above the ocean’s surface. At 7 p. m. when the ocean’s surface was at an elevation of -2.4 feet, Winston ate dinner at a restaurant whose deck was 10 feet above the ocean’s surface.
Part A

What was the net change in the elevation of the ocean’s surface from 1 p. m. to 7 p. m.?