That's spot on Paul - 'f' represents training trenches. Here is the full key for you:
The submarine base, coastal batteries, and aerodrome at Dover (April 1316), taken from a German aeroplane, (a) Submarine base and several submarines. (b) Reservoirs, (c) Military Prison. (d) Coastal batteries of Langton Fort. (e) Barracks, (f) Infantry training trenches, (g) Aerodrome and 7 aeroplanes.
From here:
http://www.allworldwars.com/The-German-Air-Force-In-The-Great-War-by-Neumann.html
I hope this helps:
The modern Coastguard station is right in the middle of Langdon Fort.