The pilot reported that after he refueled his airplane, he was cleared by the tower controller to runway 21. He added that as he taxied west toward taxiway delta, the asphalt was very rough, and he did not see the taxiway to his left, nor the unmarked drainage ditch immediately in front of him. The airplane entered the ditch travelling about 5 knots. A postaccident examination of the airplane revealed substantial damage to the empennage. The pilot reported that there were no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation. A review of the accident airport diagram revealed that the area around the ditch was not a depicted taxi way. A further review of satellite imagery revealed that there were two access points to taxiway delta southwest of the refueling point.