Elevation is a fundamental measurement describing the vertical position of a point on the Earth’s surface. It quantifies the distance a geographic location is situated above or below a standard reference surface. This measurement is essential for mapping, surveying, and studying the physical landscape. Elevation is typically expressed in meters or feet and is a core component in visualizing topography.
Defining the Baseline Reference Point
Elevation requires a globally established starting point for all vertical measurements. For centuries, the primary baseline has been Mean Sea Level (MSL), which represents the average height of the ocean’s surface over a long period (typically 19 years) to account for tidal and weather variations. MSL is assigned an elevation of zero, making it the common reference point for determining the height of landmasses.
However, the actual sea surface is not perfectly flat due to gravity variations, ocean currents, and temperature differences, making it a dynamic and imprecise reference. To achieve the stability and precision required for modern mapping, scientists use established fixed Vertical Datums. These datums are physical and mathematical models, like the North American Vertical Datum of 1988 (NAVD88), which provide a stable surface, often referred to as the geoid, that closely approximates the average sea level.
Measurements taken from these fixed datums are known as Orthometric Heights. This height is the precise vertical distance along a plumb line from a point on the Earth’s surface down to the geoid. Datums use a network of precisely surveyed benchmarks and mathematical models to ensure elevation measurements are consistent and comparable across vast geographic areas, providing a reliable zero-point for topographical data.
Distinguishing Elevation from Altitude
Elevation and altitude are often confused, but they describe vertical position relative to different reference points. Elevation specifically refers to the vertical distance of a point that is fixed on the Earth’s surface, such as a mountain peak or a town, measured from a standard vertical datum. It is a measurement used primarily in geography and cartography to describe terrain.
Altitude, by contrast, is the vertical distance of an object above a reference surface, which can be the fixed datum or the local ground level. This term is most commonly applied to objects that are mobile or airborne, like an aircraft, a satellite, or a weather balloon. For example, a mountain has an elevation, while a climber standing on its summit is at a specific altitude relative to the local ground or sea level.
In aviation, altitude is often measured by a pressure altimeter, which calculates height based on atmospheric pressure changes. The key distinction is that elevation describes the height of the ground itself, providing a permanent geographic value. Altitude describes the height of an object above the ground or sea level, providing a value that changes as the object moves.
Influence on Atmospheric Conditions
Elevation has a measurable influence on the physical conditions of the atmosphere. As elevation increases, the column of air pushing down decreases, leading to a reduction in atmospheric pressure. This decrease in pressure causes air density to drop, meaning that higher elevations contain fewer gas molecules, including oxygen.
The thinning of the atmosphere with height is a predictable physical consequence of gravity and elevation. This reduced air density is the reason why high-altitude environments present challenges for human respiration and engine performance.
Elevation also impacts temperature, a relationship described by the environmental lapse rate. This rate measures how air temperature changes with increasing elevation in the lower atmosphere, or troposphere. On average, the temperature decreases by approximately 6.5 degrees Celsius for every 1,000 meters of vertical gain. This cooling occurs because the Earth’s surface, rather than the air itself, is the primary source of heat for the troposphere.