Altitude refers to the vertical distance of a point or object above a fixed horizontal plane of reference. This measurement is fundamental across various fields, including aviation safety, topographical mapping, and large-scale construction projects. The necessity for reliable altitude data has led to the development of distinct calculation methods, each relying on different physical principles and reference surfaces. Understanding these methods reveals why various instruments can provide different readings for the same location.
Determining Altitude Using Atmospheric Pressure
The most traditional method for determining altitude utilizes the principles of atmospheric physics, specifically the relationship between height and air pressure. As elevation increases, the column of air above decreases in mass, causing the barometric pressure to drop in a known and generally consistent manner. This principle allows a pressure altimeter, which is essentially a sensitive barometer, to be calibrated to display altitude rather than just pressure.
Altimeters rely on the International Standard Atmosphere (ISA) model, a standardized mathematical representation of how pressure and temperature change with height. Because local air pressure fluctuates constantly due to weather, the instrument requires frequent calibration to a known pressure setting (QNH), often obtained from local weather stations. When QNH is entered, the altimeter indicates the height above Mean Sea Level (MSL).
When an aircraft is on the ground, setting the altimeter to the airfield’s QNH should cause it to display the published elevation. For flight at higher altitudes, all aircraft switch to a universal standard pressure setting (1013.25 hectopascals or 29.92 inches of mercury). This standard setting ensures safe vertical separation, and altitudes measured this way are referred to as Flight Levels. However, reliance on the ISA model means readings are subject to error if the actual air temperature significantly deviates from the model’s assumptions.
Determining Altitude Using Satellite Technology
Modern altitude calculation frequently relies on Global Navigation Satellite Systems (GNSS), such as GPS. The receiver determines its three-dimensional position by measuring the time it takes for signals to arrive from multiple satellites. This process, known as multilateration, requires signals from at least four satellites to accurately calculate latitude, longitude, and altitude, while simultaneously correcting for the receiver’s internal clock error.
The altitude calculated directly by a GPS receiver is based on a geometric reference surface called the World Geodetic System 1984 (WGS84) ellipsoid, not sea level or air pressure. The WGS84 ellipsoid is a smooth, mathematical model that approximates the Earth’s average shape, and the height reported is the distance above this theoretical, flattened sphere.
Its vertical accuracy is typically less precise than its horizontal accuracy. Since all usable satellites are generally positioned above the antenna, the geometric configuration results in a less precise vertical solution compared to the horizontal fix. To convert the raw ellipsoidal height into a more practical Mean Sea Level altitude, the receiver must apply a correction based on a complex gravitational model called the geoid.
Defining Altitude Reference Points and Geometric Measurement
Any discussion of altitude requires defining the zero point, or vertical datum, from which the height is measured. The primary difference between satellite and barometric altitude lies in their reference surfaces: the geoid and the ellipsoid. The geoid is an imaginary surface of equal gravitational potential that closely follows the shape Mean Sea Level (MSL) would take if extended under the continents.
The ellipsoid, such as the WGS84 model used by GPS, is a simple mathematical model of the Earth’s shape and does not account for local gravitational variations. Because the Earth’s mass is not distributed perfectly evenly, the geoid surface undulates and can be tens of meters higher or lower than the smooth ellipsoid. The elevation used on maps and charts is known as orthometric height, which is the distance measured vertically from the surface of the geoid.
Altitude is commonly expressed either as Above Mean Sea Level (AMSL), referring to the height above the geoid, or Above Ground Level (AGL), which is the height directly above the terrain below. Highly precise measurements, necessary to establish these initial reference points, are achieved through geometric surveying methods. Surveyors use sophisticated instruments like a Total Station, which combines a theodolite for measuring angles with an electronic distance meter (EDM).
This instrument uses a technique called trigonometric leveling, which determines the difference in elevation between two points by measuring the slope distance and the vertical angle between them. By applying trigonometry, surveyors can calculate elevation changes with high accuracy to establish fixed physical benchmarks on the ground. These ground-based geometric measurements provide the initial reference points used to calibrate and verify the more dynamic pressure and satellite-based systems.