An accurate measurement of sea level is one of those things that is surprisingly hard to pin down! But it is important for two reasons:

• By having an accurate sea level measurement, it is possible to measure the height of everything on land accurately. For example, calculating the height of Mt. Everest is complicated by sea level measurement inaccuracies.
• By knowing sea level accurately, we can determine if the oceans are rising or falling over time. The concern is that global warming and other weather changes caused by man might be leading to an overall rise in sea level. If so, coastal cities are in big trouble.
The problem with measuring the sea level is that there are so many things that perturb it. If you could take planet earth and move it out into deep space so that the sun, moons and other planets did not affect it and there were no temperature variations world wide, then everything would settle down like a still pond. Rain and wind would stop, and so would the rivers. Then you could measure sea level accurately. If you did this, the level of the ocean's water projected across the entire planet would be called the geoid. On land, you can think of the geoid as the level that ocean water would have if you were to dig a canal from the ocean's shore to any point on land.

The problem with the earth is that it is not in deep space -- it is in the middle of a chaotic solar system. There are all sorts of things changing the water level at any given point, including:

• The tides, caused by the moon
• Large and small waves caused by wind and the tides
• High and low pressure areas in the atmosphere, which change the surface level of the ocean
• Temperature changes in the ocean, which change the density and volume of the water
• Rainfall and river water flowing into the ocean
• And so on…
If you were to stand on the ocean shore and try to measure sea level with a ruler, you would find it to be impossible -- the level changes by the second (waves), by the hour (tides) and by the week (planetary and solar orbit changes). To get around this, scientists try using tide gauges. A tide gauge is a large (a foot (30 cm) or more in diameter), long pipe with a small hole below the water line. This pipe is often called a stilling well. Even though waves are changing the water level outside the gauge constantly, they have little effect inside the gauge. The sea level can be read relatively accurately inside this pipe. If read on a regular basis over a time span of years and then averaged, you can get a measurement of sea level.

You can see that getting an accurate reading (for example, down to the millimeter level) is extremely difficult. Satellites are now used as well, but they suffer from many of the same problems. Scientists do the best they can, using extremely long time spans, to try to figure out what the sea level is and whether or not it is rising. The general concensus seems to be that the oceans rise about 2 millimeters a year (although the last link below is interesting...).

Here are several interesting links to learn more: