Every compass ... will suffer from local deviation effects, due to local magnetic interferences ... - almost any ferrous or magnetic object. These effects will vary at different points of the compass, for example only a degree or two at a due north heading, and 5 or 6 degrees at an easterly heading.
I enjoyed it even though I thought it lacked the impact the RTD finale and even last year's finale. The Brigadier's death was a nice gesture on their part. I am not sure of the Doctor being hidden is a good idea.