How Do I Update My Charts for Variation/Declination?

[KetoNomad]

So I've never been a good navigator, never really needed to be as I've mostly stuck to known areas. Now, however, I am trying to learn proper methods.

My charts are between 12-20 years old, though; not a lot of new charts avail at the chandlery. Declination values on the compass roses are all off by several degrees; do I just update them with a pencil?

Also, how does one determine the magnetic deviation of a particular vessel? Must be a good method one can employ. All I can come up with is to compare my orienteering compass' readings (on land away from any metal) with my boat's and take the orienteering compass as accurate. Would that work?

[nigel1]

Best way to correct charts that old is to replace them with new charts.

Declination is more commonly known as Variation with regards to marine charts and compasses. In the UK, any significant change in Variation as shown on a chart may be included in chart corrections, most likely the same applies in other countries.

If you want to determine deviation for a particular compass on a boat, you need to swing the compass.
The following article will provide some clues as to how this is done.
How to swing a compass - Practical Boat Owner

[DeepFrz]

Don't throw out your old charts though. Use them for practice exercises.

[KetoNomad]

Originally Posted by nigel1

Best way to correct charts that old is to replace them with new charts.

Declination is more commonly known as Variation with regards to marine charts and compasses. In the UK, any significant change in Variation as shown on a chart may be included in chart corrections, most likely the same applies in other countries.

If you want to determine deviation for a particular compass on a boat, you need to swing the compass.
The following article will provide some clues as to how this is done.
How to swing a compass - Practical Boat Owner

Ah an excellent article on swinging a compass, thank you.

As for replacing charts, I haven't found newer ones at the store and even if I had I'm not paying $20 a chart for something as simple as a change in variation. My money could be spent more wisely elsewhere given the relatively unchanged area. We have all kinds of updates available to us from the local coast guard anyway, easier and much cheaper to just update In fact charts I just bought last week have old data on them. Blame Canada, I don't know, haha.

[MYTraveler]

Maybe I am missing something, but I don't see how changing declination affects your chart in anyway, except that the compass rose is set to the declination applicable about when the chart was produced. it should say its date and have a formula for adjusting in future years. As a practical matter, I can't imagine that 20 or 30 years would make any real world difference.

[KetoNomad]

Originally Posted by DeepFrz

Don't throw out your old charts though. Use them for practice exercises.

Winnipeg!? Jeez, I hope she's pretty

[KetoNomad]

Originally Posted by MYTraveler

Maybe I am missing something, but I don't see how changing declination affects your chart in anyway, except that the compass rose is set to the declination applicable about when the chart was produced. it should say its date and have a formula for adjusting in future years. As a practical matter, I can't imagine that 20 or 30 years would make any real world difference.

I'm thinking for the process of speeding up use of the parallel ruler I should change the roses.

Also, not sure if I mentioned it but the variation is about 4 degrees different now than it was on the date of print. Plus I get the impression one is better off using online resources to get variation figures for a given area now as the change is much larger and slightly less predictable from year to year now when compared to 20 years ago. Could be wrong.

[belizesailor]

The usual method is simply to calculate current year variation based on annual change printed on compass rose...not actually updating on chart. Pretty straight forward. Will this not work for you?

[Paul L]

Do you know how to translate between True and Magnetic bearings? You can plot in True bearings and then the chart is as-is.

[StuM]

Originally Posted by Paul L

Do you know how to translate between True and Magnetic bearings? You can plot in True bearings and then the chart is as-is.

This!

Do all your chartwork in True. Do conversions C->T->C as required on your pilotage plan using simple arithmetic before and after the chartwork.

[Kelkara]

Originally Posted by KetoNomad

My charts are between 12-20 years old, though; not a lot of new charts avail at the chandlery. Declination values on the compass roses are all off by several degrees; do I just update them with a pencil?

At $20 a sheet I certainly sympathise with wanting to get as much use out of old charts as possible ... but if you can't find new ones, then you're looking in the wrong places (assuming you're still talking about Western Canada). I've not failed to find the charts I've needed at local chandleries ... even the grocery store next to my dock sells local charts.

If you want to check the current variation for a chart you can calculate it here:
Magnetic declination calculator

Don't forget, it's a good idea to update your old charts if possible - the magnetic variation won't be the only thing out of date:
https://www.notmar.gc.ca/corrections-en.php

[toddster8]

The change in variation isn't a constant, and not entirely predictable. Hypothetically, one could carefully paste a new compass rose onto the chart, using the latest value. On the other hand, other things change too. Such as navigation aids and shoals. So I might say that updated charts are a worthwhile investment... except that the very latest chart for my area still shows a wharf that was destroyed before WWII, beacons that were abandoned in the 1950's, and shoals that were blasted out in the 1960's.

All that aside, I've been pretty satisfied with charts from frugalnavigator.com

[wjhutchings]

Good day all

All navigational charts have a correction to magnetic variation which is the angular difference between the magnetic pole and true pole where ever you happen to be on the earth, large scale charts I.e. coastal navigation charts the correction is found on the compass roses on the chart I.e 7 degrees east (1969) decreasing 5 minutes annually so if you want the magnetic variation for that spot now it 38 x 5 or 190' less than it was in 1969 so it will be roughly 4 degrees east.

On small scale charts I.e ocean charts the magnetic variation is found on the isogonic lines which are marked on the chart, to obtain magnetic variation for where you are on these charts take the closest lines either side of your position adjust both for differences then interpolate between those numbers to get the variation for where you are, sounds complicated and a bit hit an miss, but if you can steer to better than a degree using a magnetic compass you are a better helmsman than me and I did it for a job.

Deviation unless you are sailing in a yacht built of steel, nickel or cobalt then it won't worry you because your vessel won't have any.

If you have a steel boat then to obtain a table of deviations the best thing to do is take a series of compass bearings preferably one for semi cardinal point of heading of a known object stars are best over max ten minutes work out the true bearing, apply local magnetic variation to compass bearing and compare the results with calculated true bearing and the difference is the magnetic deviation of your boat,

From Compass to true is Compass, deviation, magnetic, variation, true
or Cadburys Dairy Milk Very tasty

[rhubstuff]

Annual change in variation is related to movement of the magnetic poles, and is very small, only minutes per year, so it would take many years for a change in a given area to amount to as much as a degree. Are you sure that you're seeing differences of degrees and not just minutes?

[Kelkara]

Originally Posted by wjhutchings

7 degrees east (1969) decreasing 5 minutes annually so if you want the magnetic variation for that spot now it 38 x 5 or 190' less than it was in 1969 so it will be roughly 4 degrees east.

Just out of curiosity I checked how accurate the change in variation really is over long(ish) times. I happen to have a 15 year old chart handy:

019° 30'E (9'W) 2002 implies that for 2017 it would be 17° 15'E
But the current (2017) variation is 16° 28'E

so quite good enough to steer by: a little less than one degree error - about the same error as forgetting that 1969 is actually 48 years ago.