Height Above Mean Sea Level: Understanding Elevation and Altitude
When we describe how high a mountain peak reaches or how far an airplane is from the ground, we are discussing height above mean sea level. This measurement, often referred to as elevation or altitude, represents a location's vertical distance relative to a specific vertical datum. In the scientific field of geodesy—the science of measuring Earth's geometric shape and orientation—this is formally known as orthometric height.
Because different countries use different reference points and historical measurement periods, the "zero level" can vary depending on where you are in the world. Furthermore, environmental factors such as climate change and geological shifts can cause both sea levels and land elevations to change over time.
ไม่มีภาพประกอบKey Facts
- Orthometric height is the formal term used in geodesy for elevation above a sea-level datum.
- The zero level varies globally based on local historical reference points.
- Altimetry is the scientific term for the measurement of altitude or elevation.
- Sea levels are not static; they can be influenced by climate change and land mass subsidence.
- Measurement units vary between the metric system (metres) and US customary/imperial units (feet).
Common Applications of Elevation Data
Measuring height above sea level is a fundamental requirement across various industries and scientific disciplines. It provides a standardized way to communicate vertical positioning.
Geographic and Structural Mapping
Elevation is essential for defining the physical characteristics of our world, including:
- Geographic landmarks: Determining the height of towns, mountains, and other natural features.
- Man-made structures: Measuring the height of buildings and other infrastructure.
- Mining: Managing underground mining infrastructure.
Aviation and Navigation
For pilots, altitude is a critical safety metric. Flying objects, such as airplanes and helicopters, operate based on altitudes defined by local regulations, often relative to a transition altitude.
Units, Abbreviations, and Terminology
Depending on the region and the specific industry, you will encounter various abbreviations. In the metric system, height is typically expressed in metres above mean sea level, while the United States and imperial systems use feet.
| Abbreviation | Full Meaning |
|---|---|
| AMSL | Above mean sea level |
| ASL | Above sea level |
| MASL | Metres above sea level |
| MAMSL | Metres above mean sea level |
| FASL | Feet above sea level |
| FAMSL | Feet above mean sea level |
| AOD / AODN | Above ordnance datum |
| MSL | Mean sea level |
Methods of Measuring Altitude
The science of altimetry employs several different technologies to determine how high a point is relative to sea level. The method chosen often depends on the required precision and the environment being measured.
Traditional and Satellite Techniques
- Surveying: Using techniques like levelling to establish precise heights.
- GNSS (Global Navigation Satellite System): Using receivers (such as GPS) to determine location via pseudoranges from multiple satellites. This requires a geoid—a model of Earth's mean sea level surface—to convert 3D coordinates into sea-level elevation.
- Stereoscopy: Analyzing aerial photography to determine depth and height.
Remote Sensing and Atmospheric Tools
- Pressure Altimeters: These devices measure atmospheric pressure, which decreases as altitude increases. Because weather affects pressure, these must be calibrated using a recent local pressure measurement at a known altitude.
- Lidar and Laser Altimetry: Using light (aerial lidar or satellite laser altimetry) to map elevations.
- Radar Altimetry: Utilizing radar from aerial or satellite platforms.
It is important to note that measuring historical sea levels is a complex task. For example, land mass subsidence (the sinking of land) can make it appear as though sea levels are rising. Conversely, geological processes that cause land to uplift can make it seem as though the sea level is lowering.
Frequently Asked Questions
What is the difference between elevation and altitude?
While often used interchangeably, elevation typically refers to the height of a geographic location or landform, whereas altitude is frequently used in the context of flying objects or atmospheric measurements.
Why do different countries have different sea level references?
The "zero level" varies because different nations use different reference points and historical measurement periods to establish their local vertical datums.
How does weather affect altitude measurement?
If using a pressure altimeter, weather changes can affect atmospheric pressure. To maintain accuracy, the device must be calibrated against a known local pressure reading.
Can sea levels actually change?
Yes, sea levels can vary over time due to climate change and other environmental forces. Additionally, geological movements like land subsidence or uplift can change the relative height of the land compared to the sea.
What is a geoid?
A geoid is a mathematical model of the Earth's surface used to convert 3D positions obtained from satellite systems (like GPS) into elevations relative to mean sea level.