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Old 23 June 2005, 00:37   #1
Country: UK - England
Town: Towcester
Boat name: TBA
Make: Delta
Length: 6m +
Engine: Suzuki 175
Join Date: Nov 2002
Posts: 328
Magnetic variation


I have 'swung' the compass to obtain a devation chart. I obtained a reasonably smooth curve.

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.

I am after the Magnetic variation off Harwich/Clacton read from an admiralty chart. Next to a compass rose it will say, something like - '7 deg 30 min W decreasing 10 minutes per year' and it will state the reference year.

I no longer have an admiralty chart for this area.

To assist and to pre-empt the first post reply
The reason and process is as follows.
The compass doesnt read accurately, particularly when located on the boat with metal and electrics near by. this is generally the case for all compasses.

The deviation chart will be a reference by which whem a course does need to be steered (from the compass) the correction or deviation can be got at a glance and allowed for. It can also be worked in reverse where a magnetic course is being steered and the true course is required.

It is possible on some compasses to make internal adjustments to reduce the deviation by moving small magnets on screw threads. This is beyond the amateurs knowledge and best left untouched.(by me anyway!)


Paul Beaurain is offline   Reply With Quote
Old 23 June 2005, 01:10   #2
Country: UK - England
Town: Southampton
Boat name: Darwin
Make: Ribeye
Length: 6m +
Engine: 150hp Yamaha
Join Date: Jul 2004
Posts: 382
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.
Solent Ranger is offline   Reply With Quote

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