Originally Posted by Paul Beaurain
What I am missing to complete the job is the current magntic variation for the area where the test was done. driving on different magnetic headings (every 15 degrees) a true course was obtained from the gps.
Why? Where the compass was swung should make no difference! Deviation is set up to make allowances for any magnetic effects on the boat which will "deviate" the compass. Variation allows you to make the adjustments from reading true north to getting your magnetic heading - remembering of course (excuse the pun) - variation WEST, compass best, variation East compass least!
Oh hang on, I think I see where you are coming from now - you checked the magnetic heading against the "true" reading on the GPS and now want to apply the variation at the time! Hmmmmm, not the best way to go about it IMHO. Although the chart will state the magnetic variation, the reduction/increase will normaly say "about X' west or east of course on Admiralty charts it just indicates the minutes and what direction, on IMRAY it will say if it's increasing or decreasing. As the magnetic north pole moves every year, you would need to update your information every few years as the decrease/increase in annual variation is not accurate. Why perhaps did you not check your reading against the magnetic heading of the GPS? This would have been more accurate as the magnetic heading from GPS is sattelite derived and therefore not affected by any ferrous/electronic activity on your own boat.