Height Above Mean Sea Level
Height above mean sea level is the measurement of a location's vertical distance—often referred to as height, elevation, or altitude—relative to a vertical datum based on a historic average sea level. In the field of geodesy (the science of measuring Earth's geometric shape and orientation), this is formally known as orthometric height.
It is important to note that the "zero level" is not universal; it varies between countries depending on the specific reference points and the historic periods used for measurement. Furthermore, sea levels and elevations are not static; they can fluctuate over time due to climate change and other geological forces.
Key Facts
- Definition: Vertical distance measured from a historic mean sea level datum.
- Variability: Zero-level references differ by country and change over time due to environmental factors.
- Standard Units: Measured in metres (metric) or feet (US customary/imperial).
- Core Application: Used for mapping landmarks, aviation, and underground infrastructure.
- Measurement Science: The process of measuring altitude or elevation is called altimetry.
Common Applications of Elevation Data
Measuring altitude above sea level provides a standardized reference essential for various industries and geographic records. Primary uses include:
- Geographic Landmarks: Determining the height of towns, mountains, and other natural or man-made landmarks.
- Infrastructure: Measuring the peak height of buildings and structures, as well as the depth and layout of underground mining infrastructure.
- Aviation: Tracking flying objects, such as helicopters and airplanes, when they are operating below a transition altitude defined by local aviation regulations.
Units and Standard Abbreviations
Elevation is typically expressed as "metres above mean sea level" or "feet above mean sea level." Because these terms are used frequently in technical documentation, several abbreviations are common:
| Abbreviation | Full Meaning |
|---|---|
| AMSL / ASL | Above Mean Sea Level / Above Sea Level |
| MASL / MAMSL | Metres Above Sea Level / Metres Above Mean Sea Level |
| FASL / FAMSL | Feet Above Sea Level / Feet Above Mean Sea Level |
| MSL | Mean Sea Level |
| AOD / AODN | Above Ordnance Datum / Above Ordnance Datum Newlyn |
In many contexts, the reference to sea level is implied. For example, Mount Everest may be listed as 8849 m MSL, or simply 8849 m.
Methods of Measuring Altitude
Altimetry employs several different technologies depending on the required accuracy and the environment being measured:
Surveying and Satellite Systems
Traditional surveying, specifically levelling, remains a foundational method. Modern systems utilize the Global Navigation Satellite System (GNSS), such as GPS. In this process, a receiver calculates a 3D position using pseudoranges to multiple satellites; this data is then converted to sea-level elevation using a geoid (a model of global mean sea level used to make corrections to GPS height).
Atmospheric and Remote Sensing
Pressure altimeters measure atmospheric pressure, which naturally decreases as altitude increases. Because weather patterns affect pressure, these devices must be calibrated using a recent local pressure reading from a known altitude. Other advanced methods include:
- Stereoscopy: Analyzing aerial photography to determine height.
- Lidar: Using aerial or satellite laser altimetry.
- Radar: Utilizing aerial or satellite radar altimetry.
Challenges in Accuracy
Establishing an accurate historical mean sea level is complex. Geological processes can create misleading data: land mass subsidence (the sinking of land) can make it appear as though sea levels are rising, while land uplift can make it appear as though the mean sea level is lowering.
Frequently Asked Questions
What is the difference between altitude and orthometric height?
While altitude is a general term for vertical distance, orthometric height is the formal geodetic term for height measured relative to a specific vertical datum based on mean sea level.
Why does the "zero level" vary by country?
Different countries use different reference points and historic measurement periods to establish their baseline, leading to slight variations in what is considered "zero" elevation.
How does a pressure altimeter work?
It measures the atmospheric pressure surrounding the device. Since pressure drops as you go higher, the device can estimate altitude, provided it is calibrated against current local weather conditions.
What is a geoid and why is it used in GPS?
A geoid is a model of the Earth's surface that represents the surface of equal gravitational potential (essentially global mean sea level). It is necessary to convert the raw 3D geometric position provided by GPS satellites into a meaningful elevation above sea level.
Can land movement affect sea level measurements?
Yes. Natural land mass subsidence can simulate a rise in sea level, while geological uplift can make the sea level appear to be dropping relative to the land.