  Determining radiation location observationally plays a very important role in
testing the pulsar radiation models. One-photon pair production in the strong
magnetic field, $\gamma-e^{+}e^{1}$, is one of the important physical processes
in pulsar radiation mechanisms. Photons near pulsar surface with sufficient
energy will be absorbed in the magnetosphere and the absorption optical depth
for these GeV $\gamma$-ray photons is usually large. In this paper, we include
the aberrational, rotational and general relativistic effects and calculate the
$\gamma$-B optical depth for $\gamma$-ray photons. Then we use the derived
optical depth to determine the radiation altitude lower bounds for photons with
given energies. As a case study, we calculate the lower bounds of radiation
altitudes of Crab pulsar for photons with energy from 5 GeV to 1 TeV.
