  We present the modeling tool we developed to incorporate multi-technique
observations of Cepheids in a single pulsation model: the
Spectro-Photo-Interferometry of Pulsating Stars (SPIPS). The combination of
angular diameters from optical interferometry, radial velocities and photometry
with the coming Gaia DR2 parallaxes of nearby Galactic Cepheids will soon
enable us to calibrate the projection factor of the classical
Parallax-of-Pulsation method. This will extend its applicability to Cepheids
too distant for accurate Gaia parallax measurements, and allow us to precisely
calibrate the Leavitt law's zero point. As an example application, we present
the SPIPS model of the long-period Cepheid RS Pup that provides a measurement
of its projection factor, using the independent distance estimated from its
light echoes.
