Definitions

The terms Altitude, Elevation and Depth all reference different methods of measuring vertical distance:

• Altitude is the vertical distance to a point/object above a specified reference level.
• Elevation is the vertical distance between a point/object on the Earth's surface and mean sea level (MSL).
• Depth is the vertical distance to a point/object below a specified reference level.
  
  

Altitude

Altitude is the vertical measurement of the distance to an object from a surface reference point. Altitude is commonly used in aviation and atmospheric sciences to express the height of aircraft, weather phenomenon, or even satellites orbiting the Earth.

• Altitude measurements use a variety of reference points:
  
  • Indicated Altitude - The altitude value displayed by an altimeter.
  • True Altitude - The height above mean sea level (MSL).
  • Absolute Altitude - The height above ground level (AGL).
  • Pressure Altitude - The indicated altitude of an altimeter set to 29.92 "Hg (1013 hPa).
  • Density Altitude - The "pressure altitude corrected for nonstandard temperature variations."
    
    
• Negative Altitude:
  
  • Altitude values can be negative if an object is below the surface of the ground. For example, if a bird were to fly into a canyon that is below mean sea level, the altitude of that bird’s flight would be negative.
    
    
• High Altitude:
  
  • Geographic areas above an elevation of 8,000 feet (2,400 meters).
    
    
• Common altitude measurement units:
  
  • Feet (commonly rounded to the hundred value - "Climbing to 3-0-5" would indicate 30,500 feet)
  • Meters
    
    

Elevation

Elevation is the vertical measurement of the distance between a geographic feature or location and mean sea level. Elevation is commonly represented on topographic maps by contour lines, which connect points of equal elevation, enabling users to visualize the shape, slope, and relative height of the terrain.

• Elevation measurements use a fixed reference point, mean sea level.
  
  • Mean Sea Level (MSL) is the average height of the ocean’s surface (measured midpoint between high and low tides).
    
    
• Negative Elevation:
  
  • Negative elevation values indicate geographic features or locations on dry land that are below mean sea level.
    
    
• Common elevation measurement units:
  
  • Feet
  • Meters
    
    

Depth

Depth is the vertical measurement of the distance between a surface reference point and a geographic feature, location or object below. Depth values may be represented by contour lines (similar to elevation contours) on topographic maps, or actual measurement values for nautical maps.

• Common depth measurement reference points:
  • Mean Sea Level (MSL) - The average height of the ocean’s surface (measured midpoint between high and low tides).
  • Water Level (WL) - The undisturbed free surface level of a body of water.
  • Ground Level (GL) - The surface level of dry land.
  
  
• Common depth measurement units:
  
  • Feet
  • Meters
  • Fathoms (One fathom is equal to six feet or 1.83 meters)