How is pressure altitude determined in aviation?

Pressure altitude is determined by setting the altimeter to a standard pressure setting of 29.92 inches of mercury (inHg) and then reading the indicated altitude. This method is used because pressure altitude is defined as the height above the standard datum plane, which is a theoretical level where the atmospheric pressure is 29.92 inHg.

When the altimeter is adjusted to this standard pressure, it allows pilots to understand their altitude in terms of what the pressures would be in a standard atmosphere. This is particularly important for aircraft performance calculations, air traffic control, and ensuring safety as pilots navigate through varying atmospheric conditions.

Setting the altimeter to a different pressure, such as 30.00 inHg, would yield a different indicated altitude than the true pressure altitude, which is why that method does not yield the accurate reading needed. Additionally, using GPS devices to calculate altitude does not provide the same standardization as using the altimeter method, as GPS altitude can vary based on satellite signals and does not directly reflect the atmospheric pressure changes in relation to sea level. Thus, utilizing the altimeter set to 29.92 inHg is the standard and correct method for determining pressure altitude in aviation.