On: “A case for upward continuation as a standard separation filter for potential‐field maps” by Bo Holm Jacobsen (GEOPHYSICS, 52, 1138–1148, August 1987)

This paper deals with gradients, not residuals. Computing a field up, then subtracting the “up” field from the original field to find residuals obviously involves shifting datums. Apparently the author got caught up in his equation (17) concerning the simple case of deriving the anomaly of a slab by subtracting the anomaly of one infinite prism from another of the same cross‐section but at a slightly smaller depth. That the anomaly of a slab behaves like the gradient (first vertical derivative) of the prism’s anomaly is apparent from the fact that the field of an infinitely thick slab attenuates by one power less than the field of a slab which approximates an infinitely thin body.