The German ship Meteor accomplished a three-year mission of measuring the depths of the South Atlantic Ocean in 1927. Their assumption was that the level of the ocean was uniform, and so depth measured only one thing-how deep the ocean floor was from "sea level." But sea level eventually turned out to be something quite complicated. In plain terms, sea level was not everywhere at the same level. The ocean contained protrusions, localized elevations, known as ocean bulges.
The researchers on the meteor were not entirely ignorant of the ocean bulges. At that time there were only two kinds of ocean bulge known. First, it was appreciated that the rotation of the Earth gave rise to centripetal forces in the equatorial regions, and that the liquid ocean was more affected than the solid earth. French scientists in the 18th century found that pendulum clocks near the equator ran slower than those in Paris, from which it was eventually deduced that the surface of the Arctic ocean at the North Pole was closer to the centre than the oceans in equatorial regions (it turned out to be 43 km closer). However, most of this difference was due to the centrifugal forces acting on the solid earth, causing the earth to take the shape of an ellipsoid. But some of it was also due to the rotating oceans.
The second kind of bulge known was that produced by the tides, result of the Moon exerting its gravitational influence on the ocean as a whole. In terms of bulges, this is tiny indeed. The ocean surface closest to the moon is said to rise 1 meter, and the same occurs on the diametrically opposite point of the globe, the point furthest from the moon. The thing that makes the tides significant is that the bulge is a global phenomenon, and that it goes through its cycle of rise and fall over a short space of half a day. This twice-daily heaving of the ocean involves a huge momentum. When this moment interacts with the continental bodies, the rise and fall in water levels can be significant, the difference of up to 12 m in places.
But subsequent to the Meteor’s expedition, it was found that local level. The ocean floor is just as uneven as the surface of the continents, or even more so. In other words, it contains mountains and depressions, crags and plateaus - in fact features of every kind. The islands must be viewed as nothing but enormous mountains, and in this sense Hawaii’s Mauna Kea is a whole kilometer taller than Mount Everest. The presence of huge landmasses under the ocean give rise to local gravitational effects. This is because the basalt of the ocean floor is over three times denser than water. Water tends to gather around the submerged landmasses, so that the ocean surface above it is higher than its surroundings. The difference can be as high as 20 meters.
And finally, there are the effects of the ocean currents. All the changes above are modeled on a stationary body of water. However, the ocean also has currents, great bodies of water that flow through it as if rivers. These are dictated by convection, the rotation of the Earth, and many other factors. The movement and water in general causes further variations in the sea level, though of a kind less easily determined.
The final shape that the ocean surface assumes is called the geoid. Now we can see that the Meteor was not floating on a level surface, but on the geoid. By the same token, the map of the ocean floor obtained by it was bound to be misleading. But the mistake is easily corrected once you know the true shape of the geoid. And for this you need a more secure vantage point then of a ship bobbing on the ocean surface.
Today the geoid is measured by a satellite in a polar orbit (i.e., going over the North and South Poles). The first such satellite, Geosat, was first launched in 1985. It orbited the Earth 14.3 times daily, while the Earth rotates under its plane of rotation. The distance of the satellite above the Earth centre is known, but since ocean floor measurements require exceptional levels of accuracy, relay stations were established at various points around the globe, so that Geosat could refer to them and calculate its own position better.
The next stage for Geosat is to calculate its heights above the ocean level. The technique used was nothing dissimilar from that used by the Meteor, except that instead of the basic echo sounding techniques Geosat uses microwaves of the frequency 13 GHz, which is particularly good for bouncing off the surface of water. From the time taken for the transmitted beam to bounce off the water and return to the satellite, it is an easy matter to calculate its height above the ocean level. The difference of the two measured quantities gives the distance of the ocean level from the center of the Earth. This is what determines the shape of the geoid.
Ocean bodies are, of course, transient phenomena. The contributions to a bulge from the rotation of the Earth, and from fixed landmasses, do not change. However, the contributions from the tides and the currents are variable. Local disturbances, such as oceanic earthquakes and violent weather formations, also have a role to play. The map of all the bulges at any single point of time may be expressed as ocean topography. This is exactly like the topography of the continental surface, which displays mountains and depressions. But where continental topography is largely stable, that of the ocean is more variable.