AGL (Above Ground Level) and AMSL (Above Mean Sea Level) are two different ways to measure altitude, and they are used in various contexts such as aviation, meteorology, and geography. Here's a detailed comparison:

Above Ground Level (AGL)

• Definition: AGL measures the height of an object above the ground directly beneath it.
• Reference Point: The reference point for AGL is the immediate ground surface.
• Usage: AGL is commonly used in aviation for activities like flying helicopters or drones, and during takeoff and landing of aircraft to determine the height above the runway or terrain.
• Context: It is particularly useful for low-altitude operations where the elevation of the ground can vary significantly.
• Examples:
  • A helicopter flying at 500 feet AGL is 500 feet above the ground directly below it.
  • A building's height might be given as 200 feet AGL.

Above Mean Sea Level (AMSL)

• Definition: AMSL measures the height of an object above the average sea level.
• Reference Point: The reference point for AMSL is the average level of the sea, which is considered as zero altitude.
• Usage: AMSL is used in aviation for flight planning and navigation, in topographic maps, and in meteorology to define the altitude of weather phenomena.
• Context: AMSL provides a consistent and absolute reference for altitude that does not change with the terrain.
• Examples:
  • An airport at an elevation of 1,000 feet AMSL is 1,000 feet above the mean sea level.
  • A mountain peak might be listed as 5,000 feet AMSL.

Key Differences

• Reference Point: AGL uses the ground beneath the object as a reference, while AMSL uses mean sea level.
• Application: AGL is more variable and changes with the terrain, whereas AMSL provides a stable, absolute altitude measurement.
• Context of Use: AGL is critical for tasks requiring knowledge of height above the immediate terrain (like obstacle avoidance), whereas AMSL is crucial for tasks requiring consistent altitude measurements across different locations (like flight navigation).

Summary

• AGL: Height above ground level, used for immediate terrain awareness.
• AMSL: Height above mean sea level, used for consistent altitude measurement irrespective of terrain.