The height above sea level is defined by one of two terms: elevation or altitude. Both terms describe a vertical distance from a fixed reference point, which is typically the average level of the ocean surface. This measurement is fundamental to fields such as geography, where it defines the height of land features, and aviation, where it is used for navigation and safety. The specific choice depends on whether the object being measured is a stationary point on the Earth’s surface or an object moving through the air.
Understanding the Sea Level Baseline
The concept of “sea level” as a zero-point for height measurements is more complex than a simple water line. Because the ocean surface constantly changes due to tides, weather, and currents, a stable reference is required. The established baseline is called Mean Sea Level (MSL), which is the average level of the ocean surface measured over a long period, typically 19 years, to account for all tidal cycles.
MSL is not a perfectly uniform surface because the Earth’s gravity field is not uniform. The true zero-surface for precise vertical measurements is the Geoid, a theoretical surface that models the Earth’s shape based on gravity. The Geoid is the surface to which the ocean would conform if it were subjected only to the force of gravity and the rotation of the Earth, extending under the landmasses.
This gravity-defined surface approximates MSL but is mathematically modeled to account for the uneven distribution of mass within the Earth, which causes local gravitational variations. Surveyors and scientists use the Geoid as the true datum, or reference zero, for highly accurate elevation mapping, even though it cannot be observed directly.
The Difference Between Altitude and Elevation
The distinction between elevation and altitude depends on what is being measured and its relationship to the Earth’s surface. Elevation is the height of a fixed point on the Earth’s surface relative to the Mean Sea Level or Geoid. It is used to describe the height of mountains, towns, plateaus, and other permanent land features.
For example, when Mount Everest is stated to be 8,848 meters, this figure refers to its elevation above the Geoid. This measurement is primarily used in geography, surveying, and cartography to understand the shape of the terrain. Elevation is an inherent property of a specific location on the ground.
Altitude, by contrast, is the vertical distance of an object that is not resting on the Earth’s surface above a reference point. This term is predominantly used in aviation for objects like aircraft, satellites, and balloons. An aircraft’s altitude is typically measured above Mean Sea Level, known as True Altitude. A separate, more localized measurement in aviation is Absolute Altitude, which is the vertical distance of the aircraft above the actual ground directly beneath it (above ground level, or AGL).
Practical Methods for Determining Height
The height above sea level is quantified using various technologies, each suited to different applications and levels of precision. Barometric altimetry is the primary method used in aviation to determine altitude. This technique relies on the principle that atmospheric pressure decreases predictably as height increases.
An aircraft’s pressure altimeter measures the ambient air pressure and converts it into a height reading. This reading is then adjusted using a local pressure setting to reflect the aircraft’s altitude above Mean Sea Level. Because the measurement is based on air pressure, non-standard atmospheric conditions, such as temperature variations, can introduce inaccuracies.
For highly accurate land measurements, surveying techniques like spirit leveling are employed. Spirit leveling uses a specialized instrument to measure minute differences in height between successive points. This method is considered the most precise for establishing benchmarks and determining the orthometric height (elevation above the Geoid).
Modern height determination heavily utilizes Global Positioning System (GPS) and other Global Navigation Satellite Systems (GNSS). GPS receivers directly calculate a geometric height, known as ellipsoidal height, which is the distance above a simplified mathematical model of the Earth called the reference ellipsoid. To convert this geometric height into the practical elevation above Mean Sea Level (orthometric height), the receiver must apply a geoid model to account for the difference between the ellipsoid and the true Geoid surface at that specific location.