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» Online Classroom   »   » Public Discussion of Inland and Coastal Navigation   » Problem 2-1

Author Topic: Problem 2-1
Peter Brown

 - posted December 21, 2004 09:38 PM      Profile for Peter Brown           Edit/Delete Post 
I'm using the electronic version of the course and the electronic charts. The chart supplied is 18465 30th Ed. Jan 7/95. There are four compass roses on the chart. The ones in the channel or on the US side of the channel all show the variation as 19 deg 45' East, 1995, annual decrease 6'. The one on the Canadian side gives variation as 20 deg East 1995, annual decrease 6'. The answer to question 2-1. (A) given in the course is that Variation is 19 deg 45 East, 1998. I don't see how to get to that answer using this chart. Am I working from the wrong chart? If so how do I get the correct chart?
David Burch

 - posted December 22, 2004 05:17 PM      Profile for David Burch           Edit/Delete Post 
Unfortunately the chart edition of the paper and echart are slightly different. It does not affect this problem, however, other than this initial confusion. The paper chart is edition 32 from 1998 and the echart is edition 30 from 1995.

Both have the same compass rose near Hein Bank, and both show a variation of 19° 45' E, with an annual decrease of 6'. Hence you can use either one to get the right answer, but we must keep in mind that something is not right in that these charts do not agree, ie 1998-1995 = 3 x 6' = 18', so the two editions are technically not in agreement by this amount. The issue must be how they round off the numbers, or that they got new data in 1998. The geomagnetic constants used to compute these values are published every 5 years, on the 5's, so they may have just changed the value. We have new data available in jan 1 2005, for example.

In 2005 we will be either 10 years or 7 years different, which is 10 x 6' = 60' = 1° or 18° 45' vs 7x60 = 42' = 19° 03'. Either one has to be considered right for this one. We will make a note in the tech support as a correction to the printed answers.

Thanks for pointing this out. There may be other issues that come up with chart ed 30 vs ed 32.

From: Starpath, Seattle, WA
Peter Brown

 - posted December 23, 2004 09:59 AM      Profile for Peter Brown           Edit/Delete Post 
Thanks for the answer. I suspect there are a few other differences between chart editions 30 and 32. I will compile a list of the differences I've found when I complete the course. You might consider including a paper chart with the electronic course. I thought one was included but must have missunderstood the sales information. I plan to purchase a paper chart so will be able to compare the two.

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