Sometimes when navigating with map and compass one will take a back bearing, i.e. a bearing taken directly opposite to the direction one is travelling?
When and why would one want to do this?
Sometimes landmarks that you are travelling from, are much easier to take bearings to than what you are travelling towards.
For example, in southern Utah near Bryce Canyon, there is a cinder cone that rises several hundred vertical feet above a relatively flat forest. Taking a bearing to the cinder cone is easy since it rises well above the forest. Navigating away is harder since there aren't any landmarks in front of you to see.
However, it would be possible to take a bearing from your position back to the cinder cone, and thereby be able to tell whether or not you are on course or have strayed to the right or left of it.
It would look like this,
As far as the calculations go, if you are on course the back bearing should be 180 degrees to your direction of travel. So if you had started out on a course of 270 degrees, then the back bearing should be 90 degrees.
Of course, you can also use the forward direction with your compass to navigate, but it can be nice to be able to take a back bearing in a situation like this and confirm that you are on the same heading as when you started.