CoachSea level is the average level of the surface of the sea, between high and low tide. The altitude of a place is its height above sea level, in metres. Under the sea, we speak of depth: how far the sea floor lies below sea level.
Examples
Example 9.5 (Three places)
The mouth of the Amazon, on the coast of South America, is dark green: its altitude is between 0 and 200 metres. The middle of the Sahara is pale green or pale yellow: between 200 and 1 000 metres. The plateau north of the Himalaya, in Asia, is almost white: above 4 000 metres.