BUT
Every compass is different, hence you Have to ' turn " the aircraft or boat , to all four cardinal points of said compass & write that error on a
" proper " card. Not just a scrap of paper ( tea bag box is a strong cardboard ) , cut to fit in the appropriate ' holder beside your compass .
ALL very ' old school ' , just in-case your " gps stops working ,! & your brain retains the knowledge to use mental arithmetic
spacesailor