Elevation

Lore & Background

In aviation, the term elevation or aerodrome elevation is defined by the ICAO as the highest point of the landing area, often measured in feet and found in approach charts. It is not to be confused with altitude or height. In cartography, topographical maps depict elevation primarily through contour lines. Within Geographic Information Systems (GIS), digital elevation models (DEM) represent surface topography via raster datasets, while digital terrain models offer another representation method.

Reader's Guide

Elevation is a fundamental concept in geodesy, mapping, and aviation, providing a standardized way to describe the vertical position of points on Earth. Its definition relies on a reference geoid, which models sea level as an equipotential gravitational surface. The distinction between elevation, altitude, and depth is critical in fields like aviation, where aerodrome elevation refers specifically to the highest point of the landing area. In GIS, digital elevation models (DEM) and digital terrain models allow for spatial analysis and visualization of terrain. The USGS 3D Elevation Program (3DEP) provides high-quality LiDAR data for the conterminous United States, Hawaii, and U.S. territories, offering bare earth DEM layers at resolutions of 1/3, 1, and 2 arcseconds. Elevation data underpins topographic mapping, infrastructure planning, and environmental studies, though it must be distinguished from geocentric distance, as exemplified by Mount Everest and Chimborazo.

Did You Know?

• Elevation is not to be confused with distance from the center of the Earth; Mount Everest has the largest elevation, while Chimborazo has the largest geocentric distance.
• In aviation, aerodrome elevation is defined by the ICAO as the highest point of the landing area.
• The USGS 3D Elevation Program (3DEP) provides three bare earth DEM layers at resolutions of 1/3, 1, and 2 arcseconds.
• A topographical map is the main type of map used to depict elevation, often through contour lines.

Defining Elevation and Its Boundaries

Elevation describes how far a point on the Earth's surface sits above or below a fixed reference, most often a reference geoid — a mathematical representation of sea level conceived as an equipotential gravitational surface. This concept is reserved specifically for locations on the ground; when we speak of an aircraft in flight or a spacecraft in orbit, the appropriate terms shift to altitude or geopotential height, while points beneath the surface are described by depth. A common source of confusion is treating elevation as the same as distance from the planet's center. Because Earth bulges at the equator, the summit of Mount Everest holds the record for greatest elevation, yet Chimborazo actually sits farther from the geocenter. This distinction between a vertical measurement anchored to a gravitational datum and a simple radial distance from the core is fundamental to geodesy and to every vertical datum system — from the Dutch Normaal Amsterdams Peil to Germany's Normalhöhennull to the North American Vertical Datum of 1988 — that underpins modern surveying.

Elevation in the Aviation Context

In the world of flight, elevation takes on a precise operational meaning distinct from its geodetic definition. The International Civil Aviation Organization defines aerodrome elevation as the highest point within the landing area of an airport. Pilots encounter this figure routinely on approach charts, where it is typically expressed in feet. The ICAO's standardized definition ensures that every approach chart communicates the same unambiguous figure to every pilot who reads it, regardless of the country in which the aerodrome sits. It is important, however, not to conflate aerodrome elevation with altitude or height, which describe a different vertical relationship. While the geodetic definition of elevation anchors a surface point to a gravitational datum — a mathematical model of sea level as an equipotential gravitational surface — the aviation usage is a practical, site-specific number tied to a particular landing area. This distinction matters because the terms altitude and height carry their own technical meanings in flight operations, and mixing them with elevation could lead to misinterpretation of the data a crew relies on when reading an approach chart.

Mapping the Terrain: Topography and GIS

Capturing elevation on a two-dimensional surface has long relied on the topographical map, where contour lines trace the shape of the land by connecting points of equal height. In the digital age, Geographic Information Systems have transformed how we visualize, manipulate, capture, and store elevation data along with its associated attributes. A GIS provides tools for spatial analysis and cartography, enabling users to understand patterns and relationships in the landscape at a variety of scales. The workhorse of digital elevation representation is the digital elevation model, a raster grid dataset in which each cell holds an elevation value for a specific location. Digital terrain models offer an alternative representation of the same physical surface. Together, these datasets allow analysts to query, layer, and model terrain in ways that paper contour maps never permitted, making elevation a manipulable variable rather than a static illustration.

The 3DEP Program and Global Elevation Datasets

The United States Geological Survey is developing the 3D Elevation Program to meet the growing demand for high-quality topographic data. 3DEP is a collection of enhanced elevation data delivered as high-quality LiDAR imagery covering the conterminous United States, Hawaii, and U.S. territories. The program provides three bare-earth digital elevation model layers that are nationally seamless, available at resolutions of one-third, one, and two arcseconds. Beyond the United States, global elevation data has been made available through products such as GTOPO30, a digital elevation model for the entire world, and earlier databases like ETOPO2 (a two-minute grid) and ETOPO5 (a five-minute grid), both of which have since been deprecated. The existence of multiple vertical datums — the Dutch Normaal Amsterdams Peil, Germany's Normalhöhennull, the superseded Sea Level Datum of 1929, and the current North American Vertical Datum of 1988 — underscores that elevation is never an absolute number but always a measurement relative to a chosen reference surface.

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