Two rubidium narrow line magnetometers were operated in a differential mode along a 12-km base line near Boulder, Colorado, and the total field gradient measured to high accuracy. A number of runs of difference field were made from Marcch through September of 1974. The results showed that daily averages were generally constant to within ¿0.1 &ggr; during quiet or disturbed periods with the exception of a few days in late August and early September. The agreement is excellent between the observed difference field and a difference field calculated from the measured east-west field Y and a transfer function appropriate to a local conductivity anomaly. Accounting for the effect of the anomaly typically permits a reduction of a factor of 3 in the standard deviation of the daily average; on very dsiturbed days a reduction of a factor of 5 in gradiometer noise was achieved. For periods of mroe than several days the corrections for anomaly edge effects are comparatively small.