Elevation is the vertical distance of a point on the Earth’s surface relative to a fixed, zero-reference plane, most commonly the Mean Sea Level (MSL). For precise measurements, elevation is referenced to a specific vertical geodetic datum, a standardized model that accounts for the Earth’s uneven gravitational field. Understanding a location’s elevation is foundational for cartography, civil engineering projects, and determining safe routes for activities like hiking and aviation.
Digital Measurement Using Satellite Data
The most accessible method for determining elevation utilizes Global Navigation Satellite Systems (GNSS), including GPS. GNSS devices calculate a three-dimensional coordinate, including an initial height value, by measuring the distance to multiple orbiting satellites using signal timing.
The raw vertical measurement produced by a GPS receiver is known as ellipsoidal height. This height is measured vertically above a smooth, mathematical model of the Earth called the reference ellipsoid, such as the WGS 84 model. The elevation value most people want, however, is the orthometric height, which is the height above the geoid.
The geoid is the surface that would approximate MSL if it were extended globally, defined by the Earth’s gravity. Because the geoid is uneven, the ellipsoidal height must be corrected using a geoid model to determine the orthometric height. Modern smartphones and dedicated GPS units contain this geoid model internally to perform the conversion automatically. While GNSS provides latitude and longitude with very high precision, the vertical accuracy is less reliable because of the geometric arrangement of the satellites.
Interpreting Elevation from Topographic Maps
A traditional and reliable way to find elevation involves interpreting a physical topographic map. These maps use contour lines to represent the three-dimensional shape of the terrain on a two-dimensional surface. A contour line connects all points on the map that share the exact same elevation.
The fixed vertical difference between any two adjacent contour lines is called the contour interval, which is constant across the entire map. To assist with reading, every fourth or fifth line is usually drawn thicker and labeled with its elevation number; these are known as index contours.
To determine the elevation of a point not directly on a line, users employ a technique called interpolation. This involves estimating the height by assuming the slope of the terrain is uniform between the two known adjacent contour lines. For instance, if a point lies halfway between a 100-meter line and a 120-meter line, its elevation is estimated to be 110 meters. The closer the contour lines are to one another, the steeper the slope of the land is in that area.
Calculating Elevation with Atmospheric Pressure
Elevation can also be calculated using the principles of atmospheric physics with a device called a barometric altimeter. The fundamental principle is that air pressure decreases predictably as the altitude increases due to the decreasing weight of the air column above the sensor. An altimeter is essentially a barometer that converts pressure readings into an elevation value based on a standard atmospheric model.
This conversion relies on the International Standard Atmosphere (ISA) model, which defines a theoretical relationship between pressure, temperature, and height. However, weather systems cause the local sea-level pressure to fluctuate daily. For the altimeter to show an accurate absolute elevation, it must be calibrated to a known reference point.
Users must input a known elevation or a current, local sea-level pressure reading, often referred to as the QNH setting, to establish a correct baseline. Without proper calibration, a change in weather, such as an incoming storm, will cause the pressure to drop, making the altimeter incorrectly report an increase in elevation. While barometric altimeters are highly sensitive to small vertical changes, they require frequent re-calibration to maintain an accurate absolute elevation reading.