Talk:Magnetic declination

Rewrite in progress

I'm using my Talk page to host a draft. I've managed so far to remove a lot of the redundancy, put some concepts where they belong, add some basic explanations, and upload a couple of new images. I still have to work on the compass usage and navigation sections. All suggestions are welcome! —Preceding undated comment added 05:15, 10 March 2010 (UTC).

Total re-write required

This page is badly written, very amateurish, verbose and confusing, it badly needs to be re-written. Frankly, the term 'Magnetic Declination' is a misnomer in itself. It should be Magnetic Variation. This removes a whole layer of confusion. Magnetic Deviation could then be explained perhaps as an adjunct in relation to compasses mounted on vehicles etc. The rest could be much more simply and clearly explained. As it stands, the page is, frankly, gibberish. —Preceding unsigned comment added by 88.111.22.90 (talk) 18:05, 19 January 2008 (UTC)

Terminology changes with time, and it is used differently in different communities. These days, "magnetic declination" is of used instead of "variation" so that there is no confusion, in a given context, with the time dependence of the magnetic field -- "variation" across a wide range of timescales that can apply to both the intensity of the magnetic field and its direction. For clarity of communication, one generally uses the terms your audience uses. Just saying the obvious. 174.16.77.15 (talk) 03:11, 28 December 2014 (UTC)

Agreed. I came to add a small image explaining how to compensate, and found about 5 different explanations, including a wrong statement. I'll do a bit of trimming now and will come back later arielCo (talk) 06:48, 8 March 2010 (UTC)

Navigation equations

If a easterly deviation is considered positive, and a westerly negative.. shouldn't the equation be standardized to reflect that? Currently it shows a "Compass Bearing +/- Deviation = Magnetic Bearing", but this is ambiguous.. do you subtract a negative westerly deviation or add it? csnoke 18:35, 17 April 2007 (EST)

Too many comments for one day! But this highlights exactly what I mean by "terminological thicket". The "east is least/west is best" stuff is likewise. Can the presentation be simplified? What do professional navigators actually do when on the job? I find it hard to believe any of them are muttering about virgins as they go about their computations -- as this article somewhat suggests(!). mdf 13:19, 13 September 2007 (UTC)

Definition of Magnetic Declination is in doubt

As it in the article: The magnetic declination at any point on the Earth is the angle between the local magnetic field -- the direction the north end of a compass points -- and true north.

According to FAA, it is the angle between true north and magnetic north. Direction compass points is not magnetic north, because bearing of compass is affected not only by the local magnetic variation but the carrier's own magnetic field as well. —The preceding unsigned comment was added by Natasha2006 (talk • contribs) 18:40, 25 April 2007 (UTC).

See my comment below: the magnetic field of the Earth doesn't care about your difficulties in making the measurement. Rather than confuse the matter, I strongly recommend a strict separation between the field itself, and to how the field is exploited by navigators. mdf 13:08, 13 September 2007 (UTC)

The image is not correct either

Explanation is the same as it for the definition.--Natasha2006 19:05, 25 April 2007 (UTC)

Fixed "East is Least/West is Best"

This was backward. A west variation means that magnetic is less than (counter-clockwise from) true.

The change made here is confusing. According to NOAA: "You can compute the true bearing from a magnetic bearing by adding the magnetic declination to the magnetic bearing. This works so long as you follow the convention of degrees west are negative (i.e. a magnetic declination of 10-degrees west is -10 and bearing of 45-degrees west is -45). Some example case illustrations are provided for an east magnetic declination and a west magnetic declination." NOAA FAQ 5D. I believe the way that the page read before is correct. "It is often combined with "West is Best, East is least"; that is to say, add W declinations when going True to Magnetic Compass headings, and subtract E ones." Elthorian (talk) 17:37, 20 December 2013 (UTC)

Grid magnetic angle

The article claims that magnetic declination is "also known as grid magnetic angle in military circles". However, grid north is a property of the map projection, not the planet: it will be close to, but not in general exactly "true north". Various checks on the net confirm this view, so I'll be removing it in a day, unless someone objects.

I'll also say that over the year or so since I last edited this, a fair amount of confusion has been added here. This no doubt reflects the "terminological thicket" that surrounds a great deal of the art of navigation. Perhaps some simplicity may be restored by isolating all the navigation stuff, either in this article, or moving it all to another. After all, "magnetic declination", as defined in the intro here, is a component of the local field, unsullied by whether or not the guy making the measurement is wearing an steel watch, and wants to get from where he is to where he wants to go (see: http://gsc.nrcan.gc.ca/geomag/field/magdec_e.php for more on this distinction). mdf 13:02, 13 September 2007 (UTC)

Grid Magnetic Angle (GMA) and Magnetic Declination are two completely different things. GMA is the difference between Grid North and Magnetic North whereas Magnetic Declination (AKA Magnetic Variation) is the difference between True North and Magnetic North. The difference between Grid North and True North is known as Convergence. —Preceding unsigned comment added by 84.139.36.214 (talk) 12:16, 24 May 2008 (UTC)

Animated image is too large

The animated map of changes over time is a very large file: it weighs in at 3.4MB. Can I suggest we put a link to the Commons instead? It just doesn't seem sensible causing users to load such a large file as part of the default page. El T (talk) 12:11, 28 December 2007 (UTC)

What about a movie? Another option would be another animated GIF with less frames, either by spanning less years or skipping some —Preceding unsigned comment added by Arielco (talk • contribs) 05:10, 10 March 2010 (UTC)

Declination Not the Same as Variation

Comment 1 still broadly holds. The term declination is more often used with regard to some specific vertical angles. In all the references that I have checked, both in American and English English, as far as maritime and air navigation are concerned, the terms Variation and Deviation are used exclusively. As a sign of the success of wikipedia, I was pointed to this page by a compass manufacturer whose confusing Operating Manual uses the term declination thereby conflating Variation and Deviation, and making a nonsense of their instructions. I suggest that the page is likely to be more useful if re-shaped, based on Variation & Deviation (and dip if you want completeness), even if it is necessary to cross refer to the term magnetic declination if some US Sources use that term. Simon Jackson Charts (talk) 00:37, 3 March 2009 (UTC)

Magnetic south ?

Besides the magnetic north, I read about the south pole also being magnetic; thus an obvious question is that when you travel to the southern hemisphere, does a regular compass point to the magnetic south ? I also haven't seen any article about the magnetic south at wikipedia

Another thing I read and which must be described in the article is that the magnetic north/south? "walks"; meaning that where your compass points to today, it won't point in a few years. This is also the reason why the charts are based on the "true north"; these charts don't need to be changed every few years and remain correct.

Finally, to explain why the variation changes with the location, it should be mentioned that the magnetic south is located at Bathurst, Canada. The variation changes as relative to this location, a higher or lower amount of inaccuracy occurs.

—Preceding unsigned comment added by 91.180.239.232 (talk) 14:09, 8 October 2009 (UTC) 

Air Navigation

The sentence "[f]or example, near San Francisco, TRUE north is about 14.5 degrees less than magnetic north" follows a claim that up-to-date values of magnetic variation are required for air navigation purposes. If this is true, then include the year during which the stated variation is valid and the annual rate of change. Without these additional pieces of information, the sentence might as well be removed, as it contradicts the preceding part. (Weirpwoer (talk) 20:10, 29 November 2009 (UTC))

Fixing this article

As several editors commented above, this article is a bit of a mess. It's sort of a hodge-podge of random information about the phenomenon of declination (the ostensible topic of the article), plus some random stuff (some of it wrong) about how to use a compass and a map, etc. It needs a major rewrite, but I'm not sure where to begin. -- RoySmith (talk) 17:59, 16 June 2010 (UTC)

Needs global perspective

The article is in need of some global perspective. As written, the article is strongly USA-centric. Every example in the article that is geographic is for the continental USA except for one bad example that is general to the Northern Hemisphere with no Southern Hemisphere equivalent provided. All of the organizations named are American. -- B.D.Mills  (T, C) 23:36, 16 June 2010 (UTC)

Non Adjustable Compass

The article's advice to 'add' 14° when the magvar is 14°E is clearly wrong. To get your True Heading in an environment where the magvar is 14°E, you should subtract 14° from your Magnetic Heading (or the compass pointer). Get some Boy Scouts, sailors, hikers or pilots involved in this article. No disrespect meant to any other navigators out there. 24.151.15.175 (talk) 16:50, 29 September 2010 (UTC)

You are confusing getting a bearing and getting true north. If the declination is 14°E, you can in fact imagine the compass needle rotating 14° counterclockwise to where it will be pointing towards true north, and you may think to yourself that you have subtracted 14°, and you have—you have subtracted 14 from the compass’ reading of north to get true north—but you have also increased the total number of degress between your understanding of north and the bearing you were already facing. captain anonymous 21:53, 21 April 2015 (UTC)

I think the article is correct. And, I used to be a Boy Scout and I live in southern California where my magvar actually is 14E. If I set the bezel on my non-adjustable compass to 0° and orient it so that its needle lines up with the readred orienting arrow, then my "magnetic heading" is 0° but I'm actually facing 14° east of true north. Bruceadler (talk) 17:53, 23 September 2011 (UTC)

The article is wrong. Taking your own example, at 14°E declination, you are facing 14 degrees east of true north when you line up your compass on 0°. To be facing true north, you need to turn left. In orienteering, turning left is subtraction. So with 14°E declination, you need to set your compass to 346° (0=360, 360 - 14 = 346) to be facing true north. The non-adjustable compass graphic is wrong and should be corrected. Jelloman (talk) 14:12, 29 September 2012 (UTC)

What it’s saying is if your compass says you’re bearing 40°, and the declination is 14°E, then by adding 14 to your bearing you will get your actual bearing (of 54°) had the compass been pointing to true north already instead of magnetic north.
The question presented is not “What is the difference between 0° on my compass and true north given a declination of 14°?” (which is obviously -14 [or 346]), and it is also not “If I subtract 14 from 0, what is the sum of that number and 40?” (which is obviously 26 [or 40-14]); rather the question presented is “If I subtract 14 from 0, how many whole degress regardless of sign from that number is the number 40 which I’m already facing?” (the answer is 54 [or |-14| + |40|). It’s an illustration of how to know the bearing you were already going before (40°) and after (54°) taking declination into account, and not an illustration of how subtracting an easterly declination can get your compass pointing true.
It is confusing, and I think this illustration could indeed be improved by being replaced with an animation or a series of images, or preceded by at least one simpler image illustrating only finding true north and nothing to do with a bearing. captain anonymous 21:53, 21 April 2015 (UTC)

Navigation

The article reads

V>0, D>0 for West Variation and Deviation

V<0, D<0 for East Variation and Deviation

Shouldn't it be

V>0, D>0 for East Variation and Deviation

V<0, D<0 for West Variation and Deviation

jptelthorst 21:40, 28 July 2014 (UTC) — Preceding unsigned comment added by Jptelthorst (talk • contribs)

It seems that SineBot's suggestion has been implemented in the article. However, I think the relation is wrong now. To my knowledge it shouln't read T = M + V but instead it should read T = M - V. The following formulation, found in a paper (https://www.stampsinclass.parallaxinc.com/sites/default/files/downloads/29133-Compass-Module-Application-Note.pdf), supports my impression: "To account for the variation simply add, if Westerly, or subtract, if Easterly, the variation angle from the corrected heading computation." Alorgen (talk) 14:39, 20 August 2016 (UTC)

Obsolete example

"As a traveller cruises the east coast of the United States, for example, the declination varies from 20 degrees west (in Maine) to zero (in Florida), to 10 degrees east (in Texas),"

This example must be taken from a very old textbook or reference. As the current NOAA map shows, the magnetic declination anywhere on the coast of Texas is not 10 degrees east. Around 60 years ago, it was.122.106.205.74 (talk) 19:34, 14 January 2012 (UTC)

The above observation is correct. The text says, " ... Florida = zero and Texas = 10 degs east" The Map [2010] shows Florida = +5 and Texas -5 (on the average). — Preceding unsigned comment added by 71.171.104.135 (talk) 22:49, 15 February 2012 (UTC)

The example was extremely out of date, possibly by several decades. I've updated the values, taken straight from the map of declination given in the article. Isambard Kingdom (talk) 03:46, 15 June 2016 (UTC)

"add W declinations when going True to Magnetic to Compass, and subtract E ones"

Isn't this backwards? If you're in the western U.S., there's roughly a 10 degree variation. If true (map) is 360 degrees, then the compass heading would be 350 degrees. "West is best" means you add 10 degrees to the compass heading to get a true heading of 360 degrees. — Preceding unsigned comment added by PDWriter (talk • contribs) 18:32, 27 April 2012 (UTC)

Maps / Charts

The article states that "Aviation sectionals (maps / charts) and databases used for air navigation are based on True north rather than magnetic north, and the constant and significant slight changes in the actual location of magnetic north and local irregularities in the planet's magnetic field require that charts and databases be updated at least 2 times per year to reflect the current magnetic variation correction from True north." Well, this is so poorly worded that it conveys woefully incorrect information to the reader. It is true that aviation sectionals are drawn in terms of true north. This is exactly the reason why they DON'T ever have to be updated because of magnetic north changes. What does have to be updated is the databases that are built in terms of magnetic north. While the latter certainly exist, the aviation sectionals don't belong to that category. The use of word "charts" in both the first and the last part of the sentence create an impression that aviation sectionals also need to be updated, which is not even remotely true. — Preceding unsigned comment added by Calligrapher (talk • contribs) 22:06, 15 June 2012 (UTC)

@Calligrapher: Much of what you have written is also partially incorrect, it is important that we are verifying our sources before we criticize the existing information. Aviation sectionals are indeed updated every 6 months, though not always for magnetic declination changes. In 2021 this update cycle will change to a 56 day cycle. The sectional charts show isogonic lines which change over time, so occasionally the sectional will indeed need an update due to magnetic variation. Additionally, the FAA and other government bodies update the magnetic declination calculation model every 5 years to ensure accuracy, currently the article says 2 years. I'd also like to change the section saying →

"When plotting a course, most small aircraft pilots plot a trip using true north on a sectional (map), then convert the true north bearings to magnetic north for in-plane navigation using the magnetic compass. These bearings are then converted on a pre-flight plan by adding or subtracting the local variation displayed on a sectional."

It is correct that the sectional is used to plot a course in true north bearings, then the bearings are converted for use in the air to magnetic north bearings. The last sentence is confusing because it re-states the conversion from true north to magnetic bearings a second time which is redundant.

I can also add some FAA and AOPA (Aircraft Owner and Pilot Association) citations to these claims Psobchak (talk) 15:41, 1 September 2020 (UTC)

Sign convention - Lead diagram

The illustration in the lead paragraph would be improved if the curved arrow indicating delta were to be changed so that it has an arrow at one end only. Presumably it should be at the clockwise end, indicating that delta is measured from True N to magnetic N and is positive when this angular displacement is clockwise. 77.96.58.212 (talk) — Preceding undated comment added 18:42, 26 February 2015 (UTC)

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History? =

It would be interesting to include some history. When, and by whom was it first understood that magnetic north and true north generally don't line up? And when was it first realized that the magnetic poles drift, so that the magnetic variation at a given point on Earth changes with time?

Asr34 (talk) 20:01, 24 May 2020 (UTC)

“degrees magnetic”

The article International maritime signal flags links here from the text “course in degrees magnetic”, originally inserted by Denelson83. The term “degrees magnetic” does not appear in this article, and I've never heard of a unit with that name. If that isn't simply an error then presumably it's a pseudo unit (much like vertical metre) to express “course relative to magnetic north measured in degrees”. I didn't find that term directly, but I found https://mediawiki.ivao.aero/index.php?title=Introduction_to_navigation, which uses the term “magnetic direction” for the same. ◅ Sebastian 09:19, 30 October 2020 (UTC)

Magnetic Declination Chart for the International Geomagnetic Reference Field

• There were no pole-to-pole maps of magnetic declination published after 1950 anywhere on Wikimedia Commons or Wikipedia, so I found one from USGS and uploaded it to Commons. However, this is a very large file and doesn't seem to be displaying correctly in either PDF or PNG (converted with GIMP). I don't know how to make the file size smaller for an image thumb without the map lines disappearing, so this file may need to be edited to make it visible at lower resolutions. Nicole Sharp (talk) 13:40, 3 April 2023 (UTC)
• I reduced the resolution to 2048p and converted to GIF so the image displays. I can read the declinations but not all of the country names and the lines are still very faint on the thumbnail. Nicole Sharp (talk) 13:57, 3 April 2023 (UTC)

Adjustable Compass Example

Adjustable Compass Example should be updated. Currently it uses a declination of 0 degrees, which is the same as a compass that is not adjustable. Ericjster (talk) 22:29, 17 September 2023 (UTC)