- Army area map no. 10 : Watten Bridge Head - Merckeghem Switch - St. Omer - Sercus Line, signal communications
- Series Number Id: UNKNOWN. Sheet number: part of sheet 36. Secret, rear defence lines, August 1918. "Secret" in red overprint. "[4th] Field Survey Bn. R.E. (9315) 8-7-18 [numbers stroked out in red overprint and followed with] (9491) 14-8-18." Grey base map., Location: Research Collections; Fonds: WW1 Trench Maps: France; Box number: PC22; envelope_number: 676.