(a): Map of survey area north of Greenland and Ellesmere Island. The colour code shows field strength ΔFsurvey in nT after removal of the main field and the external field (see text for details). Eureka and Alert in Canada are marked in blue and red symbols, respectively, while Station Nord in Greenland is marked in green; all three locations have airfields and permanently installed magnetometers. (b): Flight lines and cross-over differences after step three of the line levelling procedure (see text). Cross-over difference absolute values are represented by colours as well as symbol size as shown in the legends; positive values are indicated by a cross and a circle within the symbol, negative values without cross and circle. Cross-over differences occur also within traverse lines, e.g. from 360° turns of the aircraft for the gravity survey.