The effect of instrument errors in the basic pressure pattern formula (I) has been discussed. Calculations have been based on a 150 n.m. distance interval between altitude determinations, and the permissible instrumental tolerances are wider if the length of this run is increased. A particular latitude of N. 50° has been assumed, but this involves no loss of generality since ageostrophic errors and those due to the latitude term in the formula are believed to be compensatory. In general, both ageostrophic errors and pressure altimeter errors increase together with altitude, and if instrumentation is adequate for 10,000 feet (the level for which the probable error of 3 knots holds good) it will be so at higher levels. It appears that satisfactory results may be obtained by using a Mk. VI radio altimeter together with a Mk. XIV Kolsmann pressure altimeter, and taking the mean of four observations of D at the beginning and end of the run.