bigjames,
On 13 departure the 31 backbeam was specified as the nav aid to track through the gap. The TH would be less accurate. IIRC, the TD was off to the left a bit.
On a GA the TH was used for tracking. I knew someone from another airline who forgot to turn right on the IGS GA - frightened the c**p out of the local controller as he went over the tower!
