  On-the-fly mapping of cm-wave spectral lines has been implemented at the the
Tidbinbilla 70-m radio antenna. We describe the implementation and data
reduction procedure and present new H92$\alpha$ radio recombination line maps
towards Orion A and Sagittarius A. Comparison of the Orion~A map to previous
observations suggests that the lines arise largely from gas with electron
density of 100--200\,cm$^{-3}$. On-the-fly mapping is very efficient at
generating large maps of bright lines (such as radio recombination lines), but
will still yield strong efficiency gains for smaller maps of fainter lines,
such as the ammonia inversion lines at the 1.3\,cm wavelength.
