An attempt to simplify magnetic charts
Gustaf S. Ljungdahl · Terrestrial Magnetism and Atmospheric Electricity · 1936
Everyone who uses magnetic maps of the conventional type, where the magnetic elements are indicated by isomagnetic lines, may often meet difficulties in deciding how far the lines are sufficiently ascertained by observations, or merely obtained by interpolation, especially in incompletely surveyed or magnetically disturbed regions. The appreciation of the reliability of a map may, therefore, at times, be difficult without supplementary aid of tables containing the observational material, or of figures on the map, showing the corresponding values. Figures are not perspicuous in the same degree as curves, and often encumber the form of the map, especially if the scale used is small in relation to the distance between the observed stations.