Some important terms……
Mean Sea Level – Since about 80 percent of the earth’s surface is water, it is natural to use sea level as an altitude reference point. The pull of gravity is not the same at sea level all over the world because the earth is not perfectly round and because of tides. To adjust for this, an average (or mean) value is set; this is the mean sea level. Mean sea level is the point where gravity acting on the atmosphere produces a pressure of 14.70 pounds per square inch. This pressure supports a column of mercury in a barometer to a height of 29.92 inches. This is the reference point from which you measure all other altitudes. The altitude you read from an altimeter refers to MSL.
Elevation And True Altitude – Elevation is the height of a land mass above MSL. Elevation is measured with precision instruments that are far more accurate than the standard aircraft altimeter. You can find elevation information on charts or, for a particular spot, painted on a hangar near an aircraft ramp or taxi area. True altitude is the actual number of feet above MSL. A ruler or yardstick is used to measure the altitude. In standard day conditions, pressure altitude and true altitude are the same.
Absolute Altitude – Absolute altitude is the distance between the aircraft and the terrain over which it is flying. It is referred to as the altitude Above Ground Level (AGL). Due to variations in terrain, AGL is typically unreliable information. However, it is useful when flying near the ground, such as in a takeoff or landing pattern. You find AGL by subtracting the elevation of the terrain beneath the aircraft from the altitude read on the altimeter (MSL). A radar altimeter indicates actual altitude above the terrain; you call this indication radar altitude.
Pressure Altitude – To measure altitude, instruments sense air pressure and compare it to known values of standard air pressure at specific, measured altitudes. The altitude you read from a properly calibrated altimeter referenced to 29.92 inches of mercury (Hg) is the pressure altitude.
If a pressure altimeter senses 6.75 pounds per square inch pressure with the altimeter set to sea level and barometric pressure 29.92 inches of mercury, the altimeter indicates 20,000 feet. This reading does not mean that the aircraft is exactly 20,000 feet above MSL. It means the aircraft is in an air mass exerting a pressure equivalent to 20,000 feet on a standard day. You can see that pressure altitude is not true altitude.
Indicated And Calibrated Altitude – Unfortunately, standard atmospheric conditions very seldom exist. Atmospheric conditions and barometric pressure can vary considerably. A pressure change of one-hundredth (0.01) of an inch of mercury represents a 9-foot change in altitude at sea level. Barometric pressure changes between 29.50 and 30.50 are not uncommon (a pressure change of about 923 feet). Indicated altitude is the uncorrected reading of a barometric altimeter. Calibrated altitude is the indicated altitude corrected for inherent and installation errors of the altimeter. On an altimeter without such errors, indicated altitude and calibrated altitude are identical.
When flying below 18,000 feet, the aircraft altimeter must be set to the altimeter setting (barometric pressure corrected to sea level) of a selected ground station within 100 miles of the aircraft. Altitude read from an altimeter set to local barometric pressure is indicated altitude. The accuracy of this method is limited because you must assume a standard lapse rate; that is, for a given number of feet of altitude, an exact change in pressure occurs. This exact change seldom happens, which limits the accuracy of the altimeter. Above 18,000 feet, all altimeters are set to 29.92 (pressure altitude). Although the altimeter is not accurate, as long as all aircraft have the same barometric pressure setting, aircraft vertical separation is controlled.