  Stereoscopic arrays of Imaging Atmospheric Cherenkov Telescopes allow to
reconstruct gamma-ray-induced showers in 3 dimensions, which offers several
advantages: direct access to the shower parameters in space and straightforward
calorimetric measurement of the incident energy. In addition, correlations
between the different images of the same shower are taken into account. An
analysis method based on a simple 3D-model of electromagnetic showers was
recently implemented in the framework of the H.E.S.S. experiment. In the
present article, the method is completed by an additional quality criterion,
which reduces the background contamination by a factor of about 2 in the case
of extended sources, while keeping gamma-ray efficiency at a high level. On the
other hand, the dramatic flares of the blazar PKS 2155-304 in July 2006, which
provided H.E.S.S. data with an almost pure gamma-ray sample, offered the unique
opportunity of a precision test of the 3D-reconstruction method as well as of
the H.E.S.S. simulations used in its calibration. An agreement at a few percent
level is found between data and simulations for the distributions of all 3D
shower parameters.
