Elevation is the vertical distance of a point on Earth’s surface above a reference level, almost always mean sea level. It sounds straightforward, but the measurement involves decades of geodetic science, multiple competing reference surfaces, and technologies ranging from nineteenth-century spirit levels to laser-firing satellites. Defining “sea level” itself turns out to be surprisingly complicated, and the tools used to pin down elevation numbers have changed dramatically in the last few decades.
Why “Above Sea Level” Is More Complicated Than It Sounds
When you read that a city sits at 1,600 meters elevation, you probably picture a simple ruler stretching from the ocean surface up to that city. In practice, the ocean surface is not flat. Tides, currents, atmospheric pressure, and the uneven distribution of mass inside Earth all pull sea level higher or lower from place to place. A tide gauge in one harbor may record a slightly different average sea level than a gauge a few hundred kilometers away. So geodesists define a mathematical surface called the geoid, which represents the shape the ocean would take if it could flow freely under the continents without wind or tides. Orthometric height, the kind you see on topographic maps and hiking signs, is the distance from the geoid up to a point on the ground.
There is another surface in the picture: the reference ellipsoid, which is a smooth, mathematically defined shape that approximates Earth’s overall form. GPS receivers naturally measure your height above this ellipsoid, not above sea level. Ellipsoidal height and orthometric height can differ by tens of meters at any given location because the geoid bulges above the ellipsoid in some regions and dips below it in others. Converting one to the other requires a geoid model, and the accuracy of that model matters for everything from engineering surveys to flood maps.1Journal of Geodetic Science. Local orthometric height based on a combination of GPS-derived ellipsoidal height and geoid model: A review paper A paper comparing these two height systems puts it plainly: orthometric heights are physical heights above the geoid, while the alternative “normal heights” are geometric heights tied to the ellipsoid.2Journal of Geodetic Science. On the geoid and orthometric height vs. quasigeoid and normal height
Countries formalize all of this through vertical datums, official reference frameworks that fix where “zero elevation” sits. In the United States, the official datum has been the North American Vertical Datum of 1988 (NAVD88) for decades. But NAVD88 is showing its age. It carries a bias of roughly half a meter relative to the best current global geoid models, and it tilts by about a meter from one coast to the other. The U.S. National Geodetic Survey is replacing it with the North American-Pacific Geopotential Datum of 2022 (NAPGD2022), which will tie elevations more tightly to a modern geoid.3Coastal Engineering Proceedings. REPLACING NAVD88: THE EFFECTS OF VERTICAL DATUM MODERNIZATION ON COASTAL ENGINEERING That half-meter bias might sound small, but for coastal flood modeling and infrastructure design it is the difference between “safe” and “underwater.”
Sea level itself is also a moving target. A study of tide gauge records and satellite altimetry along the coast of Peninsular Malaysia found that mean sea level at one station rose about 27 millimeters over roughly three decades. That kind of drift gradually shifts the zero point that a national vertical datum was pegged to, which is one reason datums need periodic updates.4The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. THE IMPACT OF SEA LEVEL RISE ON GEODETIC VERTICAL DATUM OF PENINSULAR MALAYSIA
Traditional Ways of Measuring Elevation
The oldest precise method is spirit leveling, sometimes called geometric leveling. A surveyor sets up an instrument that projects a perfectly horizontal line of sight, then reads graduated rods placed at two points. The height difference between those points can be measured to fractions of a millimeter over short distances. By chaining thousands of these short segments together, surveyors built the elevation networks that still underpin national maps. Spirit leveling is extremely accurate at small scales, but it is slow and labor-intensive: covering a whole country takes years of fieldwork, and errors accumulate over long distances.
Trigonometric leveling offers a faster alternative for rugged terrain. Instead of keeping the line of sight horizontal, surveyors measure the angle of inclination from one point to another and combine it with the distance between them to calculate a height difference. The main drawback is atmospheric refraction: light bends as it passes through air layers of different temperature and density, and the bending gets dramatically worse over longer distances. A study of trigonometric leveling in China’s Qinling Mountains confirmed that atmospheric refraction error is one of the main factors limiting accuracy, and its influence on height difference grows sharply as horizontal distance increases.5IOP Conference Series: Earth and Environmental Science. Research on the Influence of Atmospheric Vertical Refraction on Trigonometric Leveling Survey in Qinling Mountains
A much simpler, lower-precision technique uses barometric pressure. Because air pressure drops predictably with altitude, a calibrated pressure sensor can estimate elevation. This is the principle behind the altimeters in aircraft cockpits and, increasingly, inside your smartphone. Researchers have been developing position-calculation methods that use the barometric sensors already embedded in most phones to estimate elevation indoors and outdoors.6Impact. Development of position calculation method using elevation data by barometric pressure sensor The accuracy is nowhere near survey-grade, typically within a few meters at best, because local weather conditions change the pressure reading. But for applications like emergency-call floor detection in tall buildings, a few meters is good enough.
Satellites and Lasers Changed Everything
The Global Navigation Satellite Systems, including the U.S. GPS constellation, transformed elevation measurement starting in the 1980s. A receiver on the ground picks up signals from multiple satellites and calculates its three-dimensional position, including its height above the reference ellipsoid. To convert that into the elevation values most people actually need, you apply a geoid model. The combination of GPS-derived ellipsoidal height and a geoid model can deliver orthometric heights that are good enough for many engineering and mapping applications, though the quality depends heavily on how well the geoid is modeled in your part of the world.1Journal of Geodetic Science. Local orthometric height based on a combination of GPS-derived ellipsoidal height and geoid model: A review paper
For mapping elevation across entire continents, satellite radar has been revolutionary. The Shuttle Radar Topography Mission (SRTM), flown aboard the Space Shuttle in February 2000, collected the first high-resolution near-global digital elevation data. The final datasets were released at one-arc-second resolution (roughly 30 meters) for the United States and three-arc-second resolution (roughly 90 meters) for other areas.7Geography Compass. Applications of Shuttle Radar Topography Mission Elevation Data SRTM data became one of the most downloaded geospatial datasets in history, used for everything from hydrology to military planning. But radar has limitations: it can bounce off tree canopies instead of bare ground, and it struggles in steep terrain.
LiDAR (Light Detection and Ranging) addresses some of those problems. Airborne LiDAR systems fire rapid laser pulses toward the ground from an aircraft and measure how long each pulse takes to bounce back. Because some pulses penetrate tree canopy and reach the ground while others reflect off leaves, the data can be separated to produce both a surface model (tops of trees and buildings) and a bare-earth terrain model. LiDAR is now a primary tool for generating high-resolution digital elevation models.8Quaternary International. Potential of airborne LiDAR data for terrain parameters extraction The vertical accuracy typically reaches 10 to 15 centimeters over open ground, which is good enough to map subtle drainage features and small-scale flood risks that coarser satellite data would miss entirely.
From orbit, NASA’s ICESat-2 satellite (launched in September 2018) fires a green laser toward Earth’s surface and measures elevation with much higher precision than older radar missions.9Academic Journal of Science and Technology. Surface Elevation Change Monitoring Based on ICESat-2 Satellite Altimetry Data Researchers have developed methods that combine ICESat-2 laser altimetry measurements with Landsat satellite imagery to correct the older SRTM elevation products, using information about canopy cover and surface slope to fix known biases.10Remote Sensing of Environment. Digital terrain model elevation corrections using space-based imagery and ICESat-2 laser altimetry The result is continuously improving global elevation data, each generation correcting the errors of the last.
Digital Elevation Models and Why Resolution Matters
All of these measurement techniques feed into digital elevation models (DEMs), gridded datasets where each cell stores an elevation value. DEMs are the backbone of modern geography: hydrologists use them to figure out where water flows, engineers use them to plan roads and pipelines, and climate scientists use them to model everything from glacial retreat to urban heat islands.
The choice of DEM has a real impact on the answers you get. A study comparing multiple DEM sources across four geographically distinct regions found that the choice of DEM significantly affected the accuracy of stream and catchment delineation, though its influence on simulated streamflow within an already-defined catchment was relatively minor.11PubMed. How does the choice of DEMs affect catchment hydrological modeling? In other words, a coarser DEM might put a watershed boundary in the wrong place, which changes how much land area drains into a given river, even if the flow calculations within that boundary are reasonable.
Specialized terrain also demands specialized algorithms. China’s Loess Plateau, for example, is carved into intricate positive ridges and negative gullies. Standard flow-routing algorithms designed for smoother landscapes can misrepresent water accumulation patterns there. A study on this terrain developed a mixed flow-routing approach, applying different algorithms to the positive and negative landform areas, and showed that accounting for the morphological structure of the terrain produced more realistic water flow simulations.12Hydrological Processes. Landform‐oriented flow‐routing algorithm for the dual‐structure loess terrain based on digital elevation models
For coastal flood modeling, DEM resolution is particularly critical. A study in coastal North Carolina used LiDAR elevation data to explore how horizontal resolution and assumptions about hydrological connectivity affected flood projections from sea-level rise. The variation in predicted flood extent was much larger at low sea-level projections (under about 0.4 meters of rise) than at high projections (above about 0.9 meters). At low rise levels, fine topographic details that only LiDAR can capture determine whether water can reach certain low-lying areas. At higher levels, everything is underwater regardless of detail.13International Journal of Geographical Information Science. Raster modelling of coastal flooding from sea-level rise This is a good example of how elevation data quality directly translates into the quality of real-world planning decisions.
How Elevation Shapes Climate and Ecosystems
One of the most familiar effects of elevation is that it gets colder as you go higher. The standard environmental lapse rate, the rate at which air temperature drops with altitude, is conventionally given as about 6.5 degrees Celsius per kilometer. A recent study that analyzed temperature and pressure data decoded from aircraft transponder messages over five months found an observed mean lapse rate of about 5.1 degrees per kilometer, somewhat lower than the textbook figure, and noted that the altitude of the zero-degree isotherm shifted seasonally, averaging roughly 2,874 meters during summer months.14PubMed Central. Variations of the zero degrees isotherm and environmental lapse rate recorded with ADSB and Mode S EHS messages The lapse rate varies with humidity, latitude, and season, so a single number is always an approximation.
This temperature gradient drives the distinct vegetation bands you see on mountains. Tropical African mountains typically display five to eight altitudinal belts, ranging from tropical vegetation at the base through montane forest, bamboo, cloud forest, and ericaceous scrub up to the nival (snow and ice) zone near the summit. The specific belts that develop depend on the combination of annual temperature and precipitation at each elevation. The vertical range of the highest forest belts, like ericaceous forest, and the lowest belts, like monsoon rainforest, can be predicted reasonably well from climate variables, while mid-elevation belts like bamboo and cloud forest are harder to model because they depend on more complex interactions between temperature and moisture.15PubMed Central. Patterns and Geographical Mechanism of Altitudinal Belts in Tropical African Mountains
Elevation also controls how precipitation is distributed. Mountains force air upward, cooling it and wringing out moisture on the windward side, which is why mountain ranges often have lush forests on one flank and dry grassland or desert on the other. High-resolution climate simulations suggest that future warming may weaken this orographic rain shadow effect over the western United States, with larger percentage increases in precipitation predicted on the dry lee side of mountain ranges than on their windward slopes.16Geophysical Research Letters. Weakened Orographic Influence on Cool‐Season Precipitation in Simulations of Future Warming Over the Western US If that happens, it would redistribute water resources in regions that already face serious drought and wildfire challenges.
What High Elevation Does to the Human Body
Above roughly 2,500 meters, the lower air pressure means each breath delivers less oxygen. Short-term visitors acclimatize over days to weeks through faster breathing, increased heart rate, and higher red blood cell production. But populations that have lived at high altitude for thousands of years have gone much further than acclimatization: they have evolved distinct genetic adaptations.
Tibetans, Andeans, and Ethiopians have each arrived at different physiological solutions to the same problem of chronic low oxygen. Tibetans tend to have a strong ventilatory response to low oxygen, efficient lung gas exchange, preserved heart function, and greater blood flow and capillary density in muscle tissue. Andeans, by contrast, show a blunted breathing response, elevated pulmonary artery pressure, mild right ventricular enlargement, and higher hemoglobin concentrations, essentially packing more oxygen carriers into their blood. Ethiopian highlanders, particularly the Amhara, maintain ventilatory patterns closer to sea-level populations but achieve higher oxygen saturation and enhanced tissue blood flow compared to other Ethiopian groups living at lower altitudes.17PubMed Central. Human adaptation to high-altitude: A contemporary comparison of the oxygen cascade in Andean, Tibetan and Ethiopian highlanders
Genome-wide studies have pinpointed some of the genes driving these differences. In Tibetans, strong selection signals have been found in the EPAS1 gene (which encodes a protein in the oxygen-sensing pathway) and in EGLN1, which encodes one of that protein’s regulators.18PubMed Central. Human high-altitude adaptation: forward genetics meets the HIF pathway Andean and Ethiopian populations show selection on different sets of genes, reinforcing the idea that evolution found multiple routes to the same destination. The practical upshot: acclimatization (what your body does over days) and genetic adaptation (what a population’s bodies have evolved over millennia) are genuinely different processes with different physiological outcomes, and studying high-altitude groups has provided some of the clearest examples of recent natural selection in humans.19PubMed Central. Measuring high-altitude adaptation
Elevation and Erosion
Elevation is not permanent. Tectonic forces push land up, and erosion wears it back down. In some landscapes these forces have reached a rough balance; in others, the land is still catching up to a recent burst of uplift. A study of the Northern Andes illustrates this well. In the southern Central Cordillera, erosion rates are relatively uniform, averaging about 0.3 millimeters per year, suggesting a steady state where uplift and erosion roughly cancel out. In the northern Central Cordillera, by contrast, rapidly eroding canyons cut into slowly eroding, low-relief surfaces that began rising sometime in the Late Miocene. The interpretation is that a change in the geometry of the subducting tectonic slab accelerated uplift in the north, and the landscape has not yet had time to erode down to equilibrium.20Elsevier / Earth and Planetary Science Letters. Erosion rate maps highlight spatio-temporal patterns of uplift and quantify sediment export of the Northern Andes The elevation you see on a map is a snapshot of that tug-of-war at a particular moment in geologic time.
How High Can Mountains Get
There is a theoretical ceiling on mountain height, and it depends on gravity. A planet’s surface gravity limits how tall a column of rock can stand before the material at the base is crushed under its own weight. Earth’s surface gravity is about 9.8 meters per second squared, which yields a theoretical maximum mountain height of roughly 9 kilometers, remarkably close to Everest’s actual peak. The Moon, with surface gravity of about 1.6 meters per second squared, could theoretically support mountains up to about 54 kilometers tall. Mars, at 3.7 meters per second squared, could hold mountains around 23 kilometers high, and its Olympus Mons, at roughly 21.9 kilometers, comes impressively close to that limit.21Elsevier / ScienceDirect. What is the maximum elevation mountains can reach on Earth, the Moon, and Mars?
Elevation on other worlds is measured relative to different baselines. Mars has no ocean, so elevations are referenced to a surface of constant gravitational potential, analogous to Earth’s geoid but called the areoid. The Moon uses a similar approach with a reference sphere. The laser altimeters aboard orbiters like NASA’s Mars Orbiter Laser Altimeter (MOLA) and the Lunar Orbiter Laser Altimeter (LOLA) work on essentially the same principle as ICESat-2 on Earth: fire a laser pulse, time the return, and compute the surface elevation. The tools are the same; only the gravity and the reference surface change.
The fact that Everest sits so close to the theoretical maximum for Earth is not a coincidence. On our planet, erosion by glaciers, rivers, and landslides attacks peaks relentlessly, while tectonic forces continue pushing rock upward. The result is a dynamic balance that keeps the highest mountains hovering near the physical limit. On Mars, with weaker gravity, no liquid water on the surface, and no active plate tectonics to recycle crust, Olympus Mons has been able to grow over billions of years largely unchallenged by erosion, which is why it towers so far above anything on Earth.