Altitude is a fundamental measurement that defines the height of an object relative to a specific reference point. This measurement is foundational for a vast range of activities, from global navigation and air travel to civil engineering and cartography. To ensure consistency across different locations and disciplines, a single, universally accepted baseline is necessary for measuring height. Mean Sea Level (MSL) altitude serves as this standardized global reference point for vertical measurement.
The use of MSL provides a fixed zero point that allows for the accurate comparison of elevations and altitudes anywhere on Earth. This standardized reference is necessary for safety and practical application, ensuring that a measurement of 5,000 feet in one country is the same height as 5,000 feet in another. Understanding MSL altitude is therefore the starting point for interpreting nearly all vertical data used in modern science and industry.
Defining Mean Sea Level and the Vertical Datum
Mean Sea Level is a calculated reference surface that represents the average height of the ocean’s surface over a long measurement period. This period is typically set at 19 years to account for all cyclical variations, including the gravitational effects of the sun and moon, which cause tides, as well as seasonal changes and weather-related fluctuations. MSL is not a single, physical mark but rather an averaged value that smooths out the constant rise and fall of the sea.
This calculated average creates a theoretical surface that acts as the zero-meter or zero-foot reference point for altitude and elevation measurements worldwide. Since MSL is based on the Earth’s gravity field, it closely corresponds to what is called the geoid, which is an equipotential surface where gravity is everywhere perpendicular to the surface. The geoid is the fixed, standardized reference surface that extends the concept of Mean Sea Level over landmasses.
The geoid is incorporated into a framework known as the Vertical Datum, which is a collection of specific points on Earth with known heights. This datum allows MSL to be used as a consistent baseline for measurements far from the coast, ensuring that the elevation of a mountain in the center of a continent can still be accurately referenced to the global sea-level average. Surveyors use advanced geodetic tools, including GPS and leveling techniques, to precisely determine elevations relative to the established vertical datum, thereby linking land-based measurements to the calculated MSL.
The Primary Uses of MSL Altitude
MSL altitude serves as the fundamental basis for vertical navigation and mapping across the globe. In aviation, all aircraft altimeters are primarily calibrated to read height above MSL when they are set to the local barometric pressure. This standard ensures that every aircraft in a particular region is operating from the same vertical reference, which is paramount for safety.
Air traffic control relies entirely on MSL for assigning and maintaining vertical separation between aircraft, allowing for safe flight paths and collision avoidance. When a pilot reports an altitude, they are reporting their height above Mean Sea Level, which is the “true altitude” used for all operational communication and planning. The altitude of an airfield is always specified in feet MSL, providing a fixed reference for pilots before takeoff and after landing.
Outside of air travel, MSL provides the essential baseline for all large-scale surveying and mapping projects. Topographic maps, which detail the shape and elevation of the Earth’s surface, show contour lines that are all referenced to MSL. This consistency allows engineers and construction teams to accurately plan the grades and heights of roads, bridges, and buildings. The official elevation of geographical features, such as the height of Mount Everest, is universally stated as a measurement above Mean Sea Level.
Comparing MSL Altitude to Other Measurements
While MSL altitude is the primary standard for navigation, two other measurements are used depending on the specific operational environment: Above Ground Level (AGL) and Pressure Altitude. Above Ground Level, or AGL, measures the height of an object directly above the terrain or surface immediately beneath it. This measurement is dynamic, constantly changing as an aircraft flies over varying topography like valleys and mountains.
AGL is particularly relevant for low-level flight and obstacle clearance, where proximity to the ground is the main concern. While a pilot’s main altimeter reads MSL, specialized radar altimeters are used at lower heights, typically up to 2,500 feet, to measure the exact AGL for landing or terrain avoidance. The distinction is simple: MSL tells you how high you are from the global sea-level average, and AGL tells you how far you are from hitting the ground.
Pressure Altitude is a third type of measurement derived from the physical properties of the atmosphere, not sea level. It is the height above a theoretical datum where the atmospheric pressure is standardized at 29.92 inches of mercury (or 1013.25 hectopascals). Pressure altitude is used at high cruising altitudes, typically above 18,000 feet MSL, where all aircraft are instructed to set their altimeters to this single standard pressure.
By using this common reference, all aircraft operate on the same pressure-based scale, which is essential for uniform vertical separation in the upper airspace, regardless of local weather conditions. This standardized reading is referred to as a “Flight Level.” Below this altitude, MSL is generally used, requiring pilots to continually update their altimeter with the local pressure setting to ensure their indicated MSL altitude is as accurate as possible.