People use altitude, which is height above sea level, to measure mountains, and they use the geoid to measure trenches. The geoid is an imaginary representation of the water level on Earth that is affected solely by gravity and the movement of Earth’s rotation. The actual surface of the Earth can be thought of as an average between the geoid and the shape of an ellipse.
Mount Everest, in the Himalayan range, looms at a height of 8,850 meters and is the highest point on Earth. Conversely, the Mariana Trench is known as the deepest point on Earth, extending to a depth of an astonishing 10,994 meters. So, how have people managed to calculate the height of Everest or the depth of the Mariana trench? It’s common knowledge that the surface of the Earth isn’t uniform, so more specifically, what is used as a reference point for height and depth measurements?
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Most of you have heard of the concept of ‘altitude’. In its geographical definition, it is a concept used to express the height, and sometimes depth, of a landmass, mountain, ocean etc. It is defined as height above sea level. The reason that the water level is taken as baseline is because, technically, there is a single body of water connected around the globe. Due to the property of water to obtain the same level all over, sea level is best suited as the base for carrying out such measurements.
However, we need to consider an opposing factor – global warming. One of the most profound impacts of climate change is rising sea levels due to the melting of the polar ice caps. Does this, therefore, mean that altitude measurements will change? Will the height of Mt. Everest decrease, or will the depth of the Mariana Trench increase?
Mean Sea Level (MSL)
Well, to clarify, although the sea level is constantly changing, these daily, seasonal or yearly changes do not drastically affect the Mean Sea Level (MSL). As you can guess from the name, the MSL is the mean, or average, of the sea level. Since water levels are constantly changing, MSL is a measurement calculated after obtaining data regarding water levels over a period of years and then averaging them out to obtain a mean value. Therefore, tiny changes in water levels do not drastically affect the MSL, but due to the various sea levels across the world, map makers no longer use MSL as a reference point today.
The sea level is rather unpredictable, even in the absence of global warming. There are many factors that cause the sea level to change, like rising and ebbing tides, weather changes, weather anomalies, etc. Considering that, it would be pretty hard – almost next to impossible – to get an accurate, fixed measurement of any structure. Therefore, the concept of geoid was introduced.
To better understand this, let’s first go over the facts that we know. The Earth is not a perfect sphere; in fact, it is an ellipse. Secondly, any form of measurement needs a reference point from which calculations can be made. Thirdly, the water level, or sea level, is used as the reference point. Keeping these things in mind, a geoid is an imaginary representation of the water level on Earth, a level that is affected solely by gravity and the movement of Earth’s rotation. To better picture the geoid and increase its scope of application, it is assumed that water runs through everything, even in places that actually have landmass.
If the Earth were perfectly spherical, or even elliptical, we have mathematical formulae to carry out such calculations. Unfortunately, the surface of the Earth is broken by mountains and trenches, among others, and water cannot stay in a single, straight line. Therefore, the outline of the geoid is undulating or wavy. Geoids are used as reference points for calculations of depth and height. The actual surface of the Earth can be thought of an average between the geoid and the shape of an ellipse.
Obviously, with changing conditions, the geoid cannot stay the same forever. Sometimes, when the difference is a major one, it is altered to better match the current prevailing conditions on Earth, thereby affecting altitude measurements minutely. However, these changes are not as frequent as what must be done if sea level, or even mean sea level, were used as reference points.