Center of the Roman Empire?

One thing I was not aware of was that Istanbul was the capital of the Roman empire.  Most of us attribute Rome as the birthplace of the Roman Empire, but I was not aware as the empire expanded, it moved it's capital to what at the time was called Constantinople (now Istanbul).  This was for two main reasons.  First, apparently the Roman's saw the value of the Bosphorus straight and how critical it was to trade and thus felt they could generate revenue from it.  Second, most of the expansion of the empire was towards Asia and Istanbul straddles into both Europe and Asia.

The monument below was the official center of the Roman Empire in Istanbul. Every Roman city was measured from this point.