We present observations of the galactic cepheids \eta Aql and \zeta Gem .
Our observations are able to resolve the diameter changes associated with pulsation .
This allows us to determine the distance to the Cepheids independent of photometric observations .
We determine a distance to \eta Aql of 320 \pm 32 pc , and a distance to \zeta Gem of 362 \pm 38 pc .
These observations allow us to calibrate surface brightness relations for use in extra-galactic distance determination .
They also provide a measurement of the mean diameter of these Cepheids , which is useful in constructing structural models of this class of star .