Longitude can be between 0~360 (long3) and -180~180 (long1). To convert from long3 to long1, use the formula below:
long1=mod((long3+180),360)-180 # Python: long1 = (long3 + 180) % 360 - 180 # Or: if long3 <= 180: long1 = long3 else: long1 = long3 - 360
To illustrate the values:
long3: 180, 181, ..., 359, 360/0, 1, ..., 179, 180 long1: -180, -179, ... -1, 0, 1, ..., 179, 180
