A device or set of devices which measures the value of a field in at least two different points in space at the same time. The gradient is the difference in field values per unit of distance between the sensors. By measuring a field's gradient (that is, its first derivative or rate of change with distance), the total field itself may be computed with varying degrees of accuracy. For potential fields , the direction of the measurement relative to the earth is critical. Is the gradient being measured horizontally, vertically, and in the case of magnetics, what is the orientation relative to the earth's magnetic field? Even with these possible difficulties, measuring just the gradient has the advantage of removing non-geologic field signals, such as when measuring gravity, those introduced by the normal accelerations of the survey aircraft.