Mean Sea Level

The way we measure altitude low to the ground normally depends on how far that ground is from Mean Sea Level (MSL) since we are usually dependent on barometric altimeters to figure that out. But what is Mean Sea Level?

Photo: Tie Level, Juneau, Alaska (Gillfoto, Creative Commons)
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The odd thing is that the level of the sea is different all over the world because of local variation of the earth's magnetic field, but many occupations, including ours, need to have a frame of reference. So even though it is rarely correct in terms of, yes that is where the sea's level is, we have a standard. If you turn your altimetry to 29.92 inches of mercury on an ISA standard day, where it reads 0 is where mean sea level is. So we are defining mean sea level based on the definition of mean sea level. Nuts? It really doesn't matter so long as you understand it is a theory of convenience.

Everything here is from the references shown below, with a few comments in an alternate color.

[Pariona]

• The mean sea level is a scientific term that is used to describe the average height of the oceans around the world. This average is determined by using the measurements of less active water, otherwise known as still or calm water, to avoid any possible increases due to waves or wind. Several industries, particularly those geared toward aviation and ocean navigation, rely on the mean sea level measurement in order to carry out their tasks. On aircrafts, for example, pilots utilize the mean sea level by subtracting the altitude on land from it to determine flight elevation and appropriate cabin pressure. Additionally, cartographers also rely on mean sea level measurements to produce accurate maps of the world, which are then used by ship captains.

[Pariona]

• To obtain accurate measurements, scientists use a special tool known as a tide gauge. Tide gages come in many different forms, but one of the most common looks like a plastic tube. Special sensors are located inside of this tube, which is open at one end to let in water for the sensor to read. This tool is also used to transmit early tsunami warnings. Scientists compare the collected data to information from other sources and average the results over a specific time frame.

[Science 2.0]

• [. . .] geodesists would like to re-compute sea level on the basis of the gravitational force of the Earth and they want to have 1 cm accuracy - but that stretches the limits of conventional measurement procedures or GPS technologies via satellites.
• Optical atomic clocks offer a new approach, because the tick rate of a clock is influenced by gravity. This well-known tiny effect was measured with unprecedented precision in 2010 using two optical clocks – located at the same institute. Now, up to 2000 km may lie between them; using commercial optical fibers and a sophisticated amplifier technique, the frequency of one of the atomic clocks can be transported to the other where the frequencies may then be compared.
• Highly sensitive interferometry method allows the long-distance transfer and comparison to be performed with an accuracy of 19 digits. Optical atomic clocks offer the perspective of realizing a frequency with such accuracy that even the small frequency deviations, which are caused by a height difference of a few centimeters, eventually become evident. What lies behind this is Einstein's general theory of relativity, the so-called gravitational red shift: If a clock is further way from the Earth, time actually runs a little faster for it. For a height difference of one meter, the rate (i.e. the frequency) of a clock changes by 1 × 10.

Altimetry

Pariona, Amber, World Atlas, "What is The Mean Sea Level and What is its Significance?" May 3 2018

Science 2.0, October 2, 2013