In airborne magnetometry consideration must be given to the necessity of multi-level observations. The problem of computing total intensity anomalies from data observed on lower levels is investigated in the case of contours exhibiting both two- and three-dimensional features. It is found that even fairly complex anomalies can be extended, with results differing but slightly from observations at the higher level. Maps for comparing computed and observed anomalies are presented. The mathematical basis is given together with numerical formulas and procedures for affecting the computations.