Capeccr says it nicely - The choice of magnetic Vs true bearings can somtimes be quite important, over shorter distances that you would imagine. In Battlerat and Pussycats example however, he (she ?) uses (inter)cardinal points (N, NW, W etc). Dont forget that the difference between N and NW is 45 degrees, and typically when a user gives direcions using this method, they are seldom being precise. 45 degree steps suggests an aproximate direction, although you could the finer points (N by NNW for example). Typically if the cache owner wanted to be that precice he would have used degrees, in which case magnetic Vs True bearings would be very relevant. My guess is that in this case, 20M NW-ish of the your location would be an obvious place to hide a cache.