Werneuchen Soviet Air Base on a map of the US Department of Defense from 1972 - The field was located at the border of the Berlin Control Zone (BCZ), which is marked by the blue dotted line here. Since western allied planes could move freely within the BCZ, the field was well reconnaissable for the west from the air.
Source: ONC E-2 (1972), Perry-Castañeda Library Map Collection, University of Texas at Austin