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 just couldn't get into the main character with Sarah Marshall. He was too hokey and sad. I liked Jonah's character, because he was smart and at times cagey enough to get out of some of the situations he was caught in even if he scarified himself.