  Driftscan methods are highly efficient, stable techniques for conducting
extragalactic surveys in the 21cm line of neutral hydrogen. Holding the
telescope still while the beam scans the sky at the sidereal rate produces
exceptionally stable spectral baselines, increased stability for RFI signals,
and excellent diagnostic information about system performance. Data can be
processed naturally and efficiently by grouping long sequences of spectra into
an image format, thereby allowing thousands of individual spectra to be
calibrated, inspected and manipulated as a single data structure with standard
tools that already exist in astronomical software. The behavior of spectral
standing waves (multi-path effects) can be appraised and excised in this
environment, making observations possible while the Sun is up. The method is
illustrated with survey data from Arecibo and Nancay.
