D-value


D-value

i. The difference between the actual height of a certain pressure surface over sea level and the height of the same pressure surface of the International Standard Atmosphere. It is the difference between the reading of a radio altimeter and a pressure altimeter while flying over the sea. Also known as a D-factor.
ii. The amount (positive or negative) by which the altitude (Z) of a point on an isobaric surface differs from the altitude (ZP) of the same isobaric surface in the ICAO Standard Atmosphere (i.e., D-value = Z—Zp) (ICAO).