A system can generate and output boundary lines for controlling a drilling operation. The system can receive data, including an offset well surveys, measuring instrument information, and a well casing diameter, about offset wells in a subterranean formation. The system can determine reference well values. The system can generate boundary lines for the offset wells based on the received data and the calculated reference well values. The system can adjust the boundary lines, and can output the adjusted boundary lines for controlling a drilling operation.