We present optical and infrared spectroscopy of the first two months of evolution of the Type II SN 1999em .
We combine these data with high-quality optical/infrared photometry beginning only three days after shock breakout , in order to study the performance of the “ Expanding Photosphere Method ” ( EPM ) in the determination of distances .
With this purpose we develop a technique to measure accurate photospheric velocities by cross-correlating observed and model spectra .
The application of this technique to SN 1999em shows that we can reach an average uncertainty of 11 % in velocity from an individual spectrum .
Our analysis shows that EPM is quite robust to the effects of dust .
In particular , the distances derived from the VI filters change by only 7 % when the adopted visual extinction in the host galaxy is varied by 0.45 mag .
The superb time sampling of the BVIZJHK light-curves of SN 1999em permits us to study the internal consistency of EPM and test the dilution factors computed from atmosphere models for Type II plateau supernovae .
We find that , in the first week since explosion , the EPM distances are up to 50 % lower than the average , possibly due the presence of circumstellar material .
Over the following 65 days , on the other hand , our tests lend strong credence to the atmosphere models , and confirm previous claims that EPM can produce consistent distances without having to craft specific models to each supernova .
This is particularly true for the VI filters which yield distances with an internal consistency of 4 % .
From the whole set of BVIZJHK photometry , we obtain an average distance of 7.5 \pm 0.5 Mpc , where the quoted uncertainty ( 7 % ) is a conservative estimate of the internal precision of the method obtained from the analysis of the first 70 days of the supernova evolution .