  A simple 3D-reconstruction method for gamma-ray induced air showers is
presented, which takes full advantage of the assets of a system of Atmospheric
Cherenkov Telescopes combining stereoscopy and fine-grain imaging like the High
Energy Stereoscopic System (H.E.S.S.). The rich information collected by the
cameras allows to select electromagnetic showers on the basis of their
rotational symmetry with respect to the incident direction, as well as of their
relatively small lateral spread. In the framework of a 3D-model of the shower,
its main parameters -- incident direction, shower core position on the ground,
slant depth of shower maximum, average lateral spread of Cherenkov photon
origins (or ``photosphere 3D-width'') and primary energy -- are fitted to the
pixel contents of the different images. For gamma-ray showers, the photosphere
3D-width is found to scale with the slant depth of shower maximum, an effect
related to the variation of the Cherenkov threshold with the altitude; this
property allows to define a dimensionless quantity omega (the ``reduced
3D-width''), which turns out to be an efficient and robust variable to
discriminate gamma-rays from primary hadrons. In addition, the omega
distribution varies only slowly with the gamma-ray energy and is practically
independent of the zenith angle. The performance of the method as applied to
H.E.S.S. is presented. Depending on the requirements imposed to reconstructed
showers, the angular resolution at zenith varies from 0.04 to 0.1 degrees and
the spectral resolution in the same conditions from 15% to 20%.
