Height of magnetic anomalies

One of the important duties of the United States Coast and Geodetic Survey in recent years has been the preparation of aeronautical charts. On these charts are shown isogonic lines, magnetic azimuths between important cities, or both. Questions have been raised regarding the extent to which magnetic anomalies observed at the surface would ‐affect air navigation. In particular: (1) Should the irregularities shown by isogonic lines on a chart of the entire country be shown also on aeronautical charts? (2) Would anomalies observed at the surface of the Earth, but of too small extent to affect isogonic lines on the United States chart, be appreciable at the height of air navigation?