I seem to remember that Sun Tzu says to avoid mountains and valley entrances, and be close to a river with grass, but how did a Roman army decide where to camp off the beaten track, such as through a Germania forest or Britannia moorland? Did this vary based on size of the unit of soldiers. Follow up questions: were there any practices that soldiers followed when camping, such as avoiding making campfires under X circumstances, etc.? How would a friendly approach a camp to avoid being killed, and how would soldiers decide who was friendly? Apologies, I am a novice with terminology, dates, and periods.
This is covered in the book The Military Affairs of Ancient Rome & Roman Art of War in Caesar's Time by Lt. Col. S.G. Brady. Not sure how good this book is, so perhaps others can weigh in here.
Apparently the book is out of copyright; you can find a full copy online here: http://www.digitalattic.org/home/war/romanarmy/index.php Here's the relevant section (with OCR errors fixed):
LOCATION, SITE AND FORM OF THE CAMP
Military considerations, of course, governed the choosing of the camp site. In general what was desired was a gently sloping hillside at the top of which the rear of the camp could be placed. Near by must be abundant supplies of wood for fuel and fortifications, facilities for getting water (aquatio) and for foraging (pabulatio). The hillside would reduce somewhat the labor of fortification and would give a commanding position against the enemy, provided there were no dense forests or dominant hills in the the immediate vicinity. There should be sufficient ground in front of the camp for the legions to deploy, and another desirable feature was to have the rear or one side of the camp parallel a river. The shape of the camp was square or rectangular with rounded comers, or as near this shape as the configuration of the ground would permit.
A lot of this would sound familiar to students of Sun Tzu, with some Roman flairs owing to their extreme level of rationalisation, like the square/rectangular camp shape.