Panoramic Photo, Part 4. This is the view from an observation post (OP) in the First Army sector. Many OPs used this sort of photograph because it provided a standard reference for all officers using the OP. Given the casualties among officers and the rotation of units through different sectors, using this sort of photo reduced the level of confusion.
It also shows how limited the view from an ordinary OP was, and how difficult it was to spot a trench from ground level. Since OPs were located in the most advantageous positions practicable, the difficulties in spotting from forward positions can be imagined.