Altitude is a fundamental measurement describing the vertical distance between a specific point or object and a fixed reference surface. While essentially a measure of height, the context—such as aviation, geography, or atmospheric science—dictates the precise starting point for measurement. Measuring altitude is necessary for navigation, creating topographical maps, and understanding atmospheric conditions. The vertical distance is typically measured in feet or meters, providing a standardized way to quantify a location’s height above a designated baseline.
Defining the Reference Point
Determining accurate altitude relies on establishing a clear and consistent zero-point, or reference datum.
Mean Sea Level (MSL)
The most globally recognized baseline for large-scale geographic and aviation measurements is the Mean Sea Level (MSL). MSL represents the average level of the world’s oceans, providing a fixed, consistent reference used for defining the elevation of land features and the altitude of aircraft. MSL is the standard for air traffic control, charting, and long-distance navigation because it offers a stable, absolute measurement independent of local topography.
Above Ground Level (AGL)
Another critical reference is Above Ground Level (AGL), which measures the height of an object solely above the terrain directly beneath it. Unlike MSL, AGL is dynamic and changes continuously as the landscape below rises and falls. AGL is crucial for low-level flight operations and obstacle clearance, providing pilots and drone operators with a real-time sense of their immediate vertical separation from the ground.
The Relationship Between Altitude and Air Pressure
Altitude and air pressure share an inverse relationship: as altitude increases, pressure decreases significantly. This occurs because air pressure is the force exerted by the weight of the air column pressing down from above. At sea level, a person has the entire mass of the atmosphere resting on them, resulting in the highest pressure, approximately 1013.25 millibars.
As elevation increases, the column of air above a point shortens, reducing the total mass of air molecules pressing down. This reduction causes air pressure to fall exponentially with ascent. Gravity also pulls most gas molecules closer to the Earth’s surface, making the air denser at lower altitudes.
The decrease in pressure is directly linked to a drop in atmospheric density, meaning gas molecules are more spread out at higher elevations. This change is why altimeters in aircraft rely on barometric pressure measurements to calculate altitude. Although the percentage of oxygen remains about 21% at all practical altitudes, the partial pressure of oxygen—the force driving oxygen into the lungs—is greatly reduced by the thinning atmosphere.
Altitude Zones and Their Impact on Humans
The decrease in air pressure and density with ascent creates distinct altitude zones that affect human physiology. The primary consequence of ascending to these zones is hypoxia, a condition caused by the insufficient partial pressure of oxygen.
Altitude Zones
- Low altitude: Sea level up to 4,921 feet (1,500 meters), where the body functions normally.
- High altitude: 4,921 to 11,483 feet (1,500 to 3,500 meters), where some people may experience increased breathing rate or mild headaches.
- Very high altitude: 11,483 to 18,045 feet (3,500 to 5,500 meters), where symptoms of altitude sickness become more common.
- Extreme altitude: Above 18,045 feet.
In the extreme zone, the human body struggles to adapt to the severe lack of oxygen. The body attempts to compensate by increasing the depth and rate of breathing, but long-term survival is impossible without supplemental oxygen. Mountaineers refer to the area above 26,000 feet (8,000 meters) as the “death zone.” Even at the lower end of high altitude, blood oxygen saturation begins to decrease rapidly, underscoring the physiological challenge.