The recent development of highly sensitive magnetometers, such as the optical-pumping varieties, has made feasible the measurement of the first vertical derivative of the total field (delta Delta T/delta h) in aeromagnetic surveys. This is accomplished by using two sensitive magnetometer heads separated by a constant vertical distance, and recording the difference in outputs. The effect of diurnal is thus eliminated in the resultant differential output, and this is an especially desirable feature in northern Canada where the diurnal variation is usually much greater than is found in more southerly magnetic latitudes. Moreover, steeply dipping geological contacts in high-magnetic latitudes are outlined by the resultant zero-gradient contour. It is also possible to obtain the depth of burial of the contact from the graph of (delta Delta T/delta h) versus (xdelta Delta T/delta x) where x is the horizontal distance measured from the contact. Similar quantitative interpretations may be made for the point pole and dipole. The data reduction necessary to produce a vertical-gradient map is much simpler than with the total-field case because no datum levelling is necessary. Since the aircraft track will be available from the main compilation it is only necessary to plot the resultant vertical-gradient values on the track map and contour. Thus, two maps will be obtained for little more than the price of one but with a greatly increased gain in geophysical information concerning the geometry of the causative bodies. Actually, a first-derivative map is difficult (and therefore costly) to produce by any other means. The measurement of the vertical gradient would appear to be the main advantage to using hundredth-gamma magnetometers in aeromagnetic surveys, since those types presently in service are sensitive enough for the effective delineation of total-field anomalies.