A method of obtaining aircraft altitude to within 50 feet at altitudes of 10,000 to 20,000 feet is described. This accuracy is necessary in a few special cases. The method consists of determining the height of a pressure level at some convenient point by radar or other means, and then calculating from a knowledge of the wind vector the change of height of this pressure level between the check point and the point at which an accurate altitude is required. The limitations of alternative methods of height finding, such as the use of an aneroid altimeter corrected for temperature at the upper level, are discussed.

For most purposes the aircraft type aneroid altimeter gives a sufficiently accurate indication of height, provided corrections are made for the temperature at flight level. In the application of radar to topographical surveying, however, it is necessary to know the altitude of the aircraft used in the measurement to within 50 feet when flying at heights of 10,000 to 20,000 feet. This can be achieved only by special techniques. The method here outlined has been used extensively during investigations into the use of radar for surveying. It is simple and appears to give satisfactory results.