Petersburg, Virginia. Unidentified Man?
Although "unidentified" on official sites, this officer is Maj. Simon Forrester Barstow (1817-1882) of Boston. He was Meade's Assistant-Adjutant-General from after Gettysburg until the end of the war. Not my identification alone; others have made the connection in various settings, including Find A Grave. This page collects and expands the Barstow references.
A carte-de-visite (right) sold by Cowan's Auctions in 2015 correctly identifies the individual as Barstow. The CDV looks like an off-print of the above photograph.