The Five Types of Altitude
Altitude is a relatively simple concept. In general, it is defined as an object’s height relative to sea level or ground level. However, what most people don’t realize is that there are many different types of altitude. There are actually five, those being indicated altitude, pressure altitude, density altitude, true altitude, and absolute altitude.
The first type, indicated altitude, is the simplest. It is merely the altitude you read directly off of your altimeter when it is set to local pressure at sea level. It’s important to adjust your altimeter for pressure changes, otherwise the reading will be inaccurate. The second type of altitude is pressure altitude. Pressure altitude is the altitude of the aircraft above the standard datum plane, an imagined location at which the altimeter equals 29.92 inches of mercury at 15 degrees Celsius. Any aircraft flying around 18,000 feet is required to set its altimeter to 29.92 inches Hg.
Density altitude is similar to pressure altitude, but it is adjusted for non-standard temperatures. During the warmer parts of the year, it may seem like your aircraft is not performing to its usual standards. This is a result of the air becoming less dense as temperature increases. In turn, high temperatures cause an increase in density altitude, making your aircraft feel like it’s at a much higher altitude than it really is. A decrease in density of air molecules means that there is less air mass flowing over your wings; therefore, less lift is generated. For this reason, it is important to be careful of flying at high altitudes on warm days.
True altitude is the distance of your airplane above sea level. It is most commonly expressed as ‘feet MSL’ meaning feat above mean sea level. A majority of the altitudes, terrain figures, airways, and obstacles listed on aviation charts are recorded in true altitude. The absolute altitude is the distance of your aircraft from the ground. It is constantly changing as your aircraft flies over obstacles and differing terrain. Absolute altitude is expressed in feet above ground level (AGL) and is measured by timing how long it takes for radio waves to go from plane to ground and back to the plane.