Conversation Topics: Sea Level
We're fascinated by height. For example, we're familiar with Mt. Everest because it's the tallest mountain in the world (it is a little over 29,000 feet). Many cities post signs that proudly display their elevation along side their population as if the elected officials were somehow responsible for it. Denver, Colorado, even carries the nickname "the Mile High City." And the Sears tower in Chicago and the Petronas Twin Towers in Kuala Lumpur have bragging rights to the distinction of being the tallest building in the world.

How can all three buildings be the tallest? By using different criteria for measuring them. The Council on Tall Buildings and Urban Habitat, in an attempt to resolve claims for the tallest building, has set four criteria (see
  • Height to structural top of the building (the Petronas Twin Towers)
  • Height to the floor of the highest occupied floor of the building (the Sears Tower)
  • Height to the top of the roof (the Sears Tower)
  • Height to the tip of spire, pinnacle, antenna, mast, or flag pole (formerly, the World Trade Center)
In each case, the height is measured from the sidewalk level of the main entrance of the building.

By comparison, determining the top of mountains is a lot easier. No one has ever suggested that any man-made structures on top should be counted. For mountains, the difficulty comes when trying to find the base since mountains are usually surrounded by valleys and other mountains; thus we do not have a clearly identified base to anchor the bottom end of the measurement. While climbers will calculate vertical ascents based on particular starting points, for all other purposes, the the starting height of mountains is ignored and mountains are ranked by their overall height as measured by their altitude.

Altitude is also used for cities, but in this case, the top is once more open for interpretation. Is it the lowest ground, the highest ground, or an average?

The concept of altitude suggests that we have a common, worldwide floor from which we are measuring the altitude. That floor is called "sea level" and represents the altitude of the oceans as zero. Water always finds it's level, goes the common saying. That's why when you spill a glass of water on a table, the water rushes out in all directions to form a thin, level pool. Consequently, since our world is three-fourths covered with water, using sea level as a standard, worldwide measuring point is ideal. However, there are some problems.

In the first place, as anyone who sails knows, there's the problem of the rising and falling tide, courtesy of the Moon's gravitational pull distorting the shape of the Earth. The tides are augmented by the winds (the dreaded 'storm surge' that accompanies hurricanes and tropical storms is a good example). Since the oceans are never still, we can only approximate the level of the ocean by averaging the measurements over a long period.

But then there's the problem of varying amounts of water in the ocean basins causing the ocean levels to change. During ice ages, large portions of oceans turn to ice and are thrust on top of land; this leaves less water to fill the ocean basins and thus lowers the overall water level. On the otherhand, global warming, it is believed by many, causes the level of the oceans to rise as ice at the polar caps melts and fills the ocean basins. Thus, climate changes can slowly affect what we call 'sea level.'

If that weren't enough, all of this is compounded by the shifting land masses (the process that causes earthquakes). Shifts in land can change the altitude of the land (and the objects on them) as well as change the size of the ocean basins, thereby indirectly changing the sea level.

Finally, unlike the table onto which we spilled our glass of water, the earth is not flat, nor is it a perfect sphere (its rotation makes it about 27 miles wider in diameter at the equator than through the poles). Thus, relative to the center of the earth, sea level can vary by miles.

Taking all of these factors into account, we find disparities such as that the Pacific Ocean is about eight inches higher than the Atlantic, and the water around New York is around three feet lower than that around Bermuda.

Despite these problems, sea level provides the best approximation of a floor. Averaging the fluctuating measurements gives us a relatively stable reference point that changes very little over a span of hundreds of years. Perhaps Denver isn't really a mile high, but it helps us better understand that it's far above the waves in Bermuda. Now, if they would just add altitude as a criteria for the tallest building, we could toss several more buildings in for consideration.

Source: The Proudman Oceanographic Laboratory (

Updated September 30, 2003 - go to our home or life advice page