Monday, April 21, 2014

What exactly is "sea level"?

Everyday we read facts that include an elevation. Whether is be the last known altitude of the Malaysian airlines flight, or the elevation of an avalanche on Everest. How is it calculated? What is the baseline for "sea level"?

Of course when you are speaking of such large measurements in respect to jet altitudes and Himalayan peaks, it is all based on barometric pressure. I used to known how to calculate it mechanically with a Brunton device, but that knowledge has long since been covered with dust in my brain. Most measurements have a known that all others are calibrated off of. For example the Kilogram is based on a physical artifact cast in platinum and iridium 120+ years ago, but even that is being replaced with a mathematical equation.

An accurate measurement of sea level is very hard to pin down. But it is an important measurement for two main 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, 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.

To truly measure sea level, you must remove all of the factors that cause the ocean level to fluctuate: rain, wind, undersea geological disturbances, rivers, planets, the moon and the sun (among other smaller factors). If you removed all of the factors and allowed to ocean to settle like a still pond, it would achieve a point of consistency called a geoid.

To mitigate these effects, scientists try using tide gauges. A tide gauge is a large (1 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.

Barometric altimeters are calibrated based on Mean Sea Level (MSL), but frequently, elevations are calculated on Local Mean Sea Level (LMSL) which is more accurate to a particular locale and not based on a universal benchmark

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