Boating variation?

Question

hi there all, You smart sailing folk.

I'm looking for an answer (obviously).

On my Imray (firth of clyde map (Imray C63) ( WGS84 DATUM) Within the compass rose it says, * "VARIATION 5 Degrees 10' W, (2005) DECREASING Annually About 10'. SO my *1st Question is what would the correct reading be for 2008?. thanks in advanced. my *2nd Question is on the same map FURTHER DOWN it says 5 degrees 25' W. so can you help.? dois there a website to work these things out.?

ANY HELP OR LINKS TO HELP ME OUT. well appreciated

regards again John.

Answer 1

We would have to know the year your chart was printed to calculate the variation annual correction for you. To do it yourself, take the difference in years from the edition you have and 2008 and multiply it by 10 minutes. Subtract that from 5 degrees, 10 minutes west to get the corrected variation this year.

Answer 2

You don't say what year the chart is from so how can you calculate for 2008? If its a couple of years old don't even worry about the variation. You will notice on all charts that there are differences in variation as you move about the chart. Thats why when you navigate you use the closest compass rose to your plotted course.

Answer 3

variation is the difference between true north and magnetic north. This can be found by using the compass rose on a navigation chart by NOAA. You could also ask a boatswains mate from the local coast guard in your area. They are pro's at that stuff.

Answer 4

Sounds like the chart is from 2005.  Therefore, the variation has decreased 10' per year for 3 years.  This year it will be 4 degrees, 40' west in proximity to that compass rose.

The variation is different at different compass roses - always use the compass rose on the chart that is closest to the location in which you are cruising.

Chapman's Piloting & Seamanship has a good discussion of navigation.