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Jimmyboy
RO# 2477
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 Posted - Jul 28 2010 : 20:22:06
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Compass rose's on charts have a date and how many dergees to add or subtract for each year after that. My question is how many years after that date before it goes back to zero?
I know that you should buy new charts every few years to be current, but aside from that how long?
Jimmyboy
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Radioactive
RO# 3238
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Posted - Jul 28 2010 : 20:31:36
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Without getting all technical on a subject that I know enough about to know I don't know enough about it to make a definitive statement  , Earth's magnetic pole moves around a bit on a normal basis, as do the lines of magnetic force ( ie: the waves ripple a bit over time ). Add to this the fact that the local direction of magnetic North is also influenced by local conditions.
The best answer is that the "time till expiration" will be variable for each printed compass rose, and the more frequently you update your charts ( every few years vs every decade ) the more accurate your compass rose will be.
btw: It may never "return to zero", but over a few tens of thousands of years continue around as the pole "flips". The magnetic pole is not fixed.
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For the rabid technicians: I am prepared to be corrected on this, but I think I've got it substantially correct.
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Bill
"No matter where you go, there you are." -- Buckaroo Bonzai
"There is nothing--absolutely nothing - half so much worth doing as simply messing about in boats." - Kenneth Grahame |
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Homeport: MS Gulf Coast
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littlebookworm
RO# 27413
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Posted - Jul 28 2010 : 20:54:21
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Bill: Yes, you do. And considering the new charts are not always published every few years, i wouldn't worry too much unless you plan to cross the Atlantic or Pacific. Then you should be using some electronic equipment anyway, with your charts as backup.
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Homeport: Blue Point, NY
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SCORPIO
RO# 4810
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Posted - Jul 28 2010 : 20:57:03
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I'd be more worried about shifting shoals and channels and the relative positions of obstructions than I would about the miniscule increase in magnetic variation. Charts are almost obsolete the minute they are published so keep current charts available for the reasons mentioned.
As an example, a few years ago, we went with some friends to watch fireworks on the fouth of July. We were cruising on plane several miles off the beach when my friend uttered an expeltive and cut power. I asked what was wrong and he replied that the water got very skinny. We got out the charts and he gave me a position from the GPS. I plotted where we were exactly, problem was the chart was several years old and the plotted position showed adequate depth, but it was obvious that the depth had changed. There was a shoal shown on the chart, a small one, like a dot almost, quite some distance inshore of our position. Apparently that shoal had shifted over time and we had run smack over that "pimple" on the bottom. We got off by running back down our origional course then headding further off shore.
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| Chris USPS AP |
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Homeport: Lewes, Delaware
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November Charlie
RO# 824
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Posted - Jul 28 2010 : 22:14:13
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Annual increase or decrease is usually expressed in minutes - so if you have a 3' annual increase, you'll need to correct by one degree on a 20 year old chart.
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-Mike
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Homeport: Northeast
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Sandy
RO# 1159
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Posted - Jul 29 2010 : 00:06:47
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But FWIW , most GPS/chartplotters with acquired signal should calculate or receive and display approximately correct variation automatically.
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| Sandy |
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Homeport: The Vineyard
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Jimmyboy
RO# 2477
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Posted - Jul 29 2010 : 20:42:37
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Great info everyone. Thanks you!
Jimmyboy
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Compass rose on charts
