TRACON Controller reported Tower departed two C172 aircraft and they had to turn one of the aircraft below the Minimum Vectoring Altitude to ensure required minimum separation.

Narrative

Aircraft X was a ZZZ departure issued runway heading off of RWY XX and 4;000 by the Tower. I climbed the aircraft to 5;000. Prior to Aircraft X reaching the MVA (we cannot turn below the MVA due to unmapped obstructions) the Tower departed Aircraft Y and coordinated a 190 heading; which is a turn below the MVA. I approved the heading because without it I could not ensure adequate lateral or vertical separation with the proceeding aircraft. The tower left me no choice but to violate the MVA or lose separation with another aircraft. ZZZ Tower is frequently departing aircraft; even jets behind single engine props; anticipating us to approve a heading that violates the MVA. This is happening daily and the Tower has been made aware of the issue yet it keeps happening. They really need to understand that we cannot turn aircraft below the MVA before a more serious incident occurs.Suggestion:1. Educate the Tower Controllers about our requirements and that the turns on departure they frequently request are illegal for IFR aircraft.2. Develop a SID off of ZZZ that allowed the Tower to issue headings to aircraft. This was an issue at my previous facility (ZZZ1) and a SID completely fixed the problem.3. Display the required obstructions in relevant MVAs that will allow us the utilize the 900 ft rule and turn aircraft once they are clear of the obstruction climbing to an altitude above the MVA.