Mountains are typically measured from a standardized reference point that scientists call “sea level.” This measurement is termed “elevation,” and it represents the vertical distance from this agreed-upon zero-point up to the mountain’s summit. Determining a consistent global sea level requires sophisticated models of the Earth’s shape and gravity. The chosen reference surface, known as a vertical datum, is what allows for the comparison of mountain heights across continents.
Defining the Standard Reference Datum
The concept of “sea level” is not a simple, flat surface because the oceans are constantly affected by tides, currents, and atmospheric pressure. To establish a reliable zero-point for elevation, geodesists—scientists who study the Earth’s geometric shape and gravitational field—rely on a model called the Geoid. The Geoid represents the shape the ocean surface would take if it were extended without interruption through the continents, influenced only by the force of gravity.
This imaginary, undulating surface closely approximates Mean Sea Level (MSL), which is the average height of the ocean recorded over a long period, typically 19 years, at a specific location. Since the Earth’s mass is not distributed perfectly evenly, the force of gravity varies across the globe, causing the Geoid to have subtle “hills” and “valleys.” The Geoid serves as the accepted gravitational equipotential surface, which functions as the global zero reference for all accurate elevation measurements.
In practice, different regions utilize specific local vertical datums that are based on this Geoid concept. For instance, in North America, the North American Vertical Datum of 1988 (NAVD 88) is a commonly used reference system for elevations across the continent. These regional datums ensure consistency and accuracy for mapping and surveying projects.
Determining Mountain Elevation
Modern mountain elevation is primarily determined using advanced satellite technology, specifically Global Navigation Satellite Systems (GNSS), which includes GPS. Geodesists place highly precise GNSS receivers on a mountain’s summit to record signals from multiple orbiting satellites. This process initially calculates the height of the mountain above a simplified mathematical model of the Earth called an ellipsoid.
The ellipsoid is a smooth, theoretical surface that approximates the Earth’s shape, but it does not account for local variations in gravity. Because the final elevation measurement must be relative to the Geoid (the gravitational sea level reference), a conversion process is necessary. Scientists use a detailed geoid model to calculate the exact vertical difference between the ellipsoid and the Geoid at the mountain’s location.
This correction allows the initial satellite-derived height to be converted into an orthometric height, which is the final, official elevation above the standardized sea level datum. Before the advent of satellites, surveyors relied on methods like triangulation and geodetic leveling. Triangulation used trigonometry to calculate height by measuring angles from known points, while leveling involved measuring small height differences sequentially across the landscape.
Elevation Versus Topographic Prominence
Elevation is the height of a summit above the sea level datum, but it is only one metric used to classify mountains. A separate and equally important measure is topographic prominence, which defines a mountain’s independence. Prominence is the vertical distance between a mountain’s summit and the lowest contour line connecting it to any higher peak.
To calculate prominence, surveyors identify the “key col” or “key saddle,” which is the lowest point on the ridge that connects the mountain to the nearest higher mountain. The prominence is the difference between the peak’s elevation and the elevation of this key col. A mountain with high elevation but low prominence, such as a bump on a high plateau, is not considered a significant standalone peak.
Prominence helps to distinguish true mountains from subsidiary peaks of a larger mass. Mount Everest has the highest elevation on Earth, but its prominence is also equal to its elevation because its key col is considered to be at sea level. This demonstrates that a mountain’s topographic importance is determined by how much it rises above its surroundings, not just its absolute height above the sea.
The Total Height Question: Measuring from Base
The standard definition of elevation, measured from the Geoid, leads to Mount Everest being recognized as the world’s highest mountain. However, a different perspective emerges when considering a mountain’s total geological mass, which involves measuring from its true base. This alternative method addresses the total vertical relief from the lowest point of the mountain structure to its summit.
The most famous example illustrating this difference is Mauna Kea, a dormant volcano on the island of Hawaii. While its summit only reaches about 4,207 meters (13,803 feet) above sea level, the vast majority of its bulk lies beneath the surface of the Pacific Ocean. Its geological base extends deep into the ocean floor, making it a massive structure that rises from the abyssal plain.
When measured from this submerged base to its peak, Mauna Kea has a total height of approximately 10,210 meters (33,500 feet). This measurement makes Mauna Kea significantly taller than Mount Everest’s elevation of 8,848.86 meters (29,031.7 feet). The comparison highlights that Everest is the highest point above the conventional reference datum, but Mauna Kea is the world’s tallest mountain from a base-to-peak geological standpoint.