Types of Altitudes in Aviation
When it comes to flying airplanes, there are numerous types of altitudes you, as a pilot, must be aware of to ensure flight safety. If you're new to flying, think of piloting as being equivalent to what baking is to cooking. A chef can play around with different recipes for his or her Bolognese sauce, but a pastry chef (much like a chemist) must follow exact instructions to bake a souffle, or else it will fall.
Below you will find the meanings of different altitudes as well as examples of where you might come across each one.
Indicated altitude is what is indicated on the altimeter in your airplane. The altimeter is a simple and basic flight instrument that measures the atmospheric pressure at the airplane's flight altitude and compares it to a preset pressure value.
True altitude is the height of the airplane above Mean Sea Level (MSL). Area forecasts (FAs) report cloud height in MSL. Airport elevations, terrain, and obstacle clearance altitudes listed on the VFR sectional charts are often given in MSL or true altitude.
Pressure altitude is the altitude above the standard datum plane and is also the altitude that your altimeter reads when it's set to 29.92" Hg, which is the standard pressure setting. Pressure altitude is important when it comes to computing aircraft performance data including such things as takeoff and landing distances.
It's also the altitude that operators use while flying in Class D airspace, which requires everyone in flight to set their altimeters to 29.92" Hg in order to standardize the indicated altitudes. You can actually determine the pressure of the air by calculating the difference between the pressure altitude and the current altimeter setting.
Density altitude is important for determining the performance of an aircraft, or how the aircraft will "act" under certain conditions. Density altitude is pressure altitude corrected for nonstandard temperature. And, because the temperature is always nonstandard, it's very important for pilots to know the density altitude. At sea level, the standard temperature is 15 degrees Celsius. This temperature decreases, on average, about 2 degrees per 1,000 feet increase in elevation. At sea level, the pressure altitude will be the same as the density altitude when the temperature is standard (or 15 degrees Celsius). Density altitude is what your aircraft "feels like" when you are in the air. If the airport elevation is 5,000 feet MSL and the temperature is above standard, as it might be in Prescott, Arizona on a summer day, then your airplane might "act like" it's at 7,000 feet instead of 5,000 feet. What this means is that there could be a substantial decrease in performance characteristics and an increase in takeoff and landing distances.
Absolute altitude (AGL) is the exact height above ground level or the actual height above the earth's surface. METARs and TAFs report cloud cover in AGL so if you have a radar altimeter on your airplane, it will indicate an AGL altitude.