In reply to brigsy:
IMHO
A 4 is the workhorse and I've only ever properly broken one (shut it in a car door) and worn out 3 or 4 (well, one of those was cracked, but some Araldite held it together for years.
For most purposes it's fine as you're taking a bearing off a map and following an indicated direction, with inevitable slight meanderings. A big base plate is easier with frozen, icy gloves.
Accurate bearings from an object is only really relevant when directing artillery. For resections, you'll be lucky to have two precisely identifiable points, or the conditions in which to plot accurate lines anyway (if you did, you wouldn't be lost to the extent that you need to do accurate resections).
For artillery though it is important to have an accurate bearing so that 'Left 200, Drop 200' actually gets the rounds to move in the right direction when that instruction from the observer's point of view is interpreted at the guns. The guns don't fire conveniently along the observer's line of sight and the observer doesn't know where the guns are. The guns however know the location of the target (if the observer can read a map) and the bearing from the observer to the target. They fire the first round at the grid ref the observer gave, and then make adjustments based on what the observer sees and tells them to do.
(This is the steam-driven version)
Post edited at 17:28