  The INT Photometric Halpha Survey of the Northern Galactic Plane (IPHAS) is a
1800 square degrees CCD survey of the northern Milky Way spanning the latitude
range -5 < b < +5 (degrees) and reaching down to r' ~ 20 (10-sigma). It may
increase the number of known northern emission line sources by an order of
magnitude. Representative observations and an assessment of point-source data
from IPHAS, now underway, are presented. The data obtained are Wide Field
Camera images in Halpha narrow-band, and Sloan r' and i' broad-band, filters.
We simulate IPHAS (r' - Halpha, r' - i') point-source colours using a
spectrophotometric library of stellar spectra and available filter transmission
profiles: this gives expected colours for (i) solar-metallicity stars, without
Halpha emission, and (ii) emission line stars. Comparisons with Aquila field
observations show that simulated normal star colours reproduce the data well
for spectral types earlier than M. Spectroscopic follow-up of a Cepheus field
confirms that sources lying above the main stellar locus in the (r' - Halpha,
r' - i') plane are emission line objects, with very few failures. Examples of
Halpha deficit objects -- a white dwarf and a carbon star -- are shown to be
readily distinguished by their IPHAS colours. The role IPHAS can play in
studies of nebulae is discussed briefly, and illustrated by a
continuum-subtracted mosaic image of the SNR, Shajn 147. The final catalogue of
IPHAS point sources will contain photometry on ~80 million objects. (abridged)
