Sea level, actually mean sea level, is calculated as the average height of the sea surface taking into account all stages of ocean tides.
It is measured at various locations by a network of tide gauges over a nineteen-year period.
Scientists have long recognized the inaccuracy of these tide measurements, and satellites are offering more precision, showing from the vantage of space that even calm ocean surfaces have large, constantly changing hills and valleys.
But for now these tidal measures are the standard for measuring all things up and down, including mountains.
Mount Everest is said to be the tallest, just over 29,000 feet above sea level.
But another way of measuring mountain peaks is by their distance from the center of the earth.
On this basis, Ecuador’s Chimborazo, at 20,561 feet above sea level, would be taller than Everest by a whopping two miles.
This is because the earth bulges at the equator, near Chimborazo, and flattens toward the poles.
Using the center of the earth as a benchmark, “sea level” at the equator is 14 miles higher than at the poles.