  Exotic magnetic structures, such as magnetic skyrmions and domain walls, are
becoming more important in nitrogen-vacancy center scanning magnetometry.
However, a systematic imaging approach to mapping stray fields with fluctuation
of several milliteslas generated by such structures is not yet available. Here
we present a scheme to image a millitesla magnetic field by tracking the
magnetic resonance frequency, which can record multiple contour lines for a
magnetic field. The radial basis function algorithm is employed to reconstruct
the magnetic field from the contour lines. Simulations with shot noise
quantitatively confirm the high quality of the reconstruction algorithm. The
method was validated by imaging the stray field of a frustrated magnet. Our
scheme had a maximum detectable magnetic field gradient of 0.86 mT per pixel,
which enables the efficient imaging of millitesla magnetic fields.
