Simply, the units in which magnetic survey maps are often contoured. 1 nanotesla = 1 gamma. A unit for stating the magnitude of the magnetic field vector B represented by the number of lines of induction passing through a unit area perpendicular to the vector direction.

1 nanotesla = 10-9 tesla

=10-9 weber/m²

=10-1 lines/m²

=10-5 lines/cm²

=10-5 gauss = 1 gamma