Geophysical investigations maps, GP (series). 51. Total intensity aeromagnetic map [and accompanying aeromagnetic profiles] of Douglas County and part of Grant County, Minn., aeromagnetic survey 1947 by J. R. Balsley and M. E. Hill. Scale 1 m.=1 in. for map and 1 m.=0.5 in. for profiles, contour interval 50 gammas, n. p.